Node.js and MySQL is one of the necessary binding needed for any web application. MySQL is one of the most popular open source database in world and efficient as well. Almost every popular programming language like Java or PHP provides driver to access and perform operations with MySQL.

In this tutorial i am trying to cover code for learning and code for production. So if you know this already and looking for ready made code for production. Click here to jump there directly.

Introduction:

Node.js is rich with number of popular packages registered at package registry called NPM. Most of them are not so reliable to use for production but there are some on which we can rely upon. For MySQL there is one popular driver called node-mysql.

In this tutorial i am going to cover following points related to Node.js and MySQL.

• Sample code to get started.
• Code for Production.
• Testing concurrent users.

Sample code to get started.

Project directory:

Install dependencies using

Here is sample code which connects to Database and perform SQL query.

Make sure you have started MySQL on default port and changed the parameter in above code then run this code using

Code for production :

Above code is just for learning purpose and not for production payload. In production scenario is different, there may be thousands of concurrent users which turns into tons of MySQL queries. Above code won’t run for concurrent users and here is a proof. Let’s modify our code little bit and add Express routes in that, here it is.

Install siege in your system. I use this command to install it in Ubuntu.

then run our node and server and following command.

Assuming you are running Node server on Port 3000.
Here is the output.

In above code, we are allowing it to run for standalone connection i.e one connection at a time but reality is bit different. You may get 100 or 1000 connection at one particular time and if your server is not powerful enough to serve those request then at least it should put them in queue.

Pool connection in MySQL :

Connection Pooling is mechanism to maintain cache of database connection so that connection can be reused after releasing it. In Node mysql, we can use pooling directly to handle multiple connection and reuse the connection. Let’s write same code with pooling and check whether it can handle multiple connection or not.

Run the app using

and fire 10 concurrent users for 1 minute using siege by using this command.

Here is output.

**UPDATE**

You can directly use pool.query() which internally will acquire connection and release it when query is executed. In my personal code review experience, majority of the developers often forget to release the acquired connection which in turns creates bottleneck and database load.

Refer the code snippet below:

I have used this function in production environment with heavy payload and it works like charm.

Final comments :

Siege is really powerful tool for testing server under pressure. We have created 100 connection limit in code, so you might be wondering that after 100 concurrent connection code will break. Well let me answer it via code. Fire 1000 concurrent user for 1 minute and let’s see how our code reacts.

If your MySQL server is configured to handle such traffic at one socket then it will run and our code will manage the scheduling of concurrent connection. It will serve 100 connection a time but rest 900 will be in queue. So code will not break.

Conclusion :

MySQL is one of widely used database engine in world and with Node it really works very well. Node-mysql pooling and event based debugging is really powerful and easy to code.

Further reading:

• Node-mysql Github
• Pro MERN Stack: Full Stack Web App Development with Mongo, Express, React, and Node

Let's create a table that always contains a fixed amount of rows, using MariaDB 10.3.

The post Circular tables in MariaDB appeared first on Federico Razzoli.

Open source barriers

Right now, it's too hard to contribute to open source.

This leads to a few things that are bad:

1. People who have good ideas don't contribute
2. Open source maintainers are overworked
3. There's less useful open source software than there could be

We want to improve this, so:

We're building a community for web developers to build & share open source modules that work on any website.

There should be a place where you don't need anything "extra" to work on open source: no git, webpack, gems, repos, terminal commands, or non-code tools. If you know the code itself, you should be able to contribute directly without learning special commands.

We're working our way toward this with the first ever online code editor that compiles modules to work on any website:

More than a playground

There are a lot of code "playgrounds" out there for building demos or proofs of concept, but that can be pretty limiting. With the new approach, you can click

And that brings up code to add the actual module to your website. Not a sandboxed or iframe version -- the actual, working module. The code looks like this:

<html>
<head>

  <!-- Paste right before your document </head> -->
  <script id="Anymod-script">
    (function (m,o,d,u,l,a,r,i,z,e) {
      u[m]={Project:o,rq:[],Opts:r,ready:function(j){u[m].rq.push(j)}};function j(s){return encodeURIComponent(btoa(s))};z=l.getElementById(m+'-'+a);r=u.location;
      e=[d+'/page/'+o+'/'+j(r.pathname)+'/'+j(r.host)+'?t='+Date.now(),d];e.map(function(w){i=l.createElement(a);i.defer=1;i.src=w;z.parentNode.insertBefore(i,z);});
    })('Anymod','855EM8','https://cdn.anymod.com/v2',window,document,'script',{ toolkit: true, tips: true, priority: 3 });
  </script>

</head>
<body>

  <!-- Paste where you want the module: Animate CSS grid -->
  <div id="anymod-lllddn"></div>

</body>
</html>

A world of possibilities

Our hope is that an open platform for normal web developers to find, modify, and use modules will help us all to create a huge ecosystem of free, open-source software that works on any website.

Right now there is a library of hundreds of "verified" modules that are ready to use, with thousands more that have been created by developers.

From pre-styled page sections to forms, modular CMS, galleries, team pages, ecommerce, and more, there are lots of ready-to-use modules already available.

Join the community

You can find, clone, and use modules on the platform, and we are building even more tools to help with collaboration.

Soon you will be able to see which modules are looking for contributors, make changes in the browser, and have your updates merged into the original. Both the original author and you will then be listed on the module, and you'll be able to see where it's being used.

Our hope is that we can make open source participation a real option for all web developers by lowering the barriers to contribution.

If you're interested in collaborating, using cool modules, or simply claiming your username, we'd love to have you!

https://anymod.com

Please join Percona’s Bug Analyst, Lalit Choudhary as he presents his talk MySQL 8.0 Architecture and Enhancement on Tuesday, April 9th, 2019 at 6:00 AM PDT (UTC-7) / 9:00 AM EDT (UTC-4).

Register Now

The release of MySQL 8.0 offers much more to users compared to previous releases. There are major changes in architecture as well as adding differentiating features, and improvements to manage the database more efficiently.

In our talk we will go through, MySQL 8.0 architecture:
* On Disk
* In-memory

Examples and use cases will be shared showing the new features introduced in MySQL 8.0.

Register for MySQL 8.0 Architecture and Enhancement to learn more.

We have covered session management in ExpressJs using global variable technique which of course will not work in case of shared server or concurrent execution of http requests which is most familiar production scenario.

Codeforgeek readers requested to provide solution for these issue and the optimal one is to use external session storage which is not dependent on application requests, answer is Redis cause this is the light weight and easy to use NoSQL database.

In this tutorial i am going to explain how to design and code session oriented express web applications by using Redis as external session storage.

To get familiar with Session handling in ExpressJS I recommend to read our first article here.

Getting started with Redis :

If you have already installed Redis please go to next section. For those of you who are not familiar with Redis here is a little introduction.

Redis is key value pair cache and store. it is also referred as data structure server cause keys can contain List, Hash, sets, sorted sets etc.

Redis is fast cause it work in memory data set i.e it by default stores data in your memory than disk and if you from CS background you very well know CRUD operation on memory is way faster than disk, so is Redis.

if you restart Redis or shut it down you may lose all data unless you enable option to dump those data in hard disk. Be careful !

Installation:

1 : On mac

On Mac if you have brew install then just open up your terminal and type

Make sure you have command line tools installed cause it need GCC to compile it.

If you don’t have brew then please install it. It’s awesome !

2 : On ubuntu

Run following command on Ubuntu and rest will be done

3 : On Windows

Well Redis does not support Windows ! Hard luck.

Basic REDIS command :

I am going to mention those command which i need to go with this tutorial. For detailed information please visit the nice demo built by awesome Redis team to make you pro in Redis.

1 : Starting Redis server.

Run this command on terminal.

2 : Open Redis CLI too.

Run this command on terminal.

3 : List all Keys.
Run this command on terminal.

4 : Retrieve information regarding particular key.
Run this command on terminal.

Once you have installed Redis, by running first command you should something like this.

Express session with Redis

To add support of Redis you have to use Redis client and connect-redis. Create express-session and pass it to connect-redis object as parameter. This will initialize it.

Then in session middle ware, pass the Redis store information such as host, port and other required parameters.

Here is sample express code with Redis support. Have a look.

Notice the code where we are initiating the session. Have a look.

If Redis server running, then this is default configuration. Once you have configured it. Store your session key in the way we were doing in previous example.

Fetch the information from redis session key.

Our project:

To demonstrate this i have developed web application which will allow you to register and login and post some status. It’s simple but it will demonstrate you how to handle session using external storage.

Create project folder and copy these code to package.json.

Our package.json :

Install dependencies by running following command.

Our database:

Once completed let’s design simple database to support our application. Here is the diagram.

Database is simple and straight, please create database in MySQL and run following DDL queries.

Our Server code

Server file contains application routes, database support and Redis session support. We will first create connect to database and initialize Redis then according to Routes particular action will happen.

Explanation:

When user provides login credentials then we are checking it against our database. If it’s successful then we are setting database response in our Redis key store. This is where Session has been started.

Now as soon as User go to homepage, we are validating the session key and if it is there, then retrieving the user id from it to fire further MySQL queries.

When user click on Logout, we are calling req.session.destroy() function which in turn deletes the key from Redis and ends the session.

Views ( Index.html and Home.html )

Here is our home page code.

Here is how it looks.

Here is how it looks.

How to run:

Download code from Github and extract the file. Make sure you have installed Redis and created database.

If your database name and MySQL password is different then please update it on bin/index.js file.

Once done type npm start on terminal to start the project.

Now open your browser and go to localhost:3000 to view the app. Register with your email id and password and then Login with same. After login, have a look to Redis using commands mentioned above.

Now logout from System and check for same key. It should not be there.

This is it. Express session with Redis is working.

Conclusion:

Redis is one of the popular key storage database system. Using it to store session is not only beneficial in production environment but also it helps to improve performance of system.

MySQL InnoDB Cluster was introduced in MySQL version 5.7 and consists of three parts – Group Replication, MySQL Shell and MySQL Router. MySQL InnoDB Cluster provides a complete high availability solution for MySQL. In this post, I am going to explain how to setup a three-node cluster using the MySQL Shell.

To begin, I am going to install three instances of the MySQL database and MySQL Shell (both version 8.0.15) on three separate virtual machines with the IP addresses of 192.168.1.161, 192.168.1.162 and 192.168.1.163. I will do most of the work on 192.168.1.161 via the MySQL Shell.

Let’s get started

From a terminal window on 192.168.1.161, I will open the MySQL Shell using the command: mysqlsh. MySQL Shell has three modes – SQL, Javascript and Python. For this post, I will be using the SQL and JavaScript modes. Once you are in the MySQL Shell console, here are the commands to switch between the modes:

SQL mode - \sql
JavaScript Mode - \js
Python mode - \py

Each mode is highlighted with a different color. Here is a screenshot to show you what each mode looks like:

When you first open MySQL Shell, you aren’t connected to a database. I will start Shell (via mysqlsh), connect to the local instance, and then switch to SQL mode.

MySQL Shell 8.0.15-commercial

Copyright (c) 2016, 2019, Oracle and/or its affiliates. All rights reserved.
Oracle is a registered trademark of Oracle Corporation and/or its affiliates.
Other names may be trademarks of their respective owners.


Type '\help' or '\?' for help; '\quit' to exit.

Creating a session to 'root@localhost:3306'
Please provide the password for 'root@localhost:3306': 
Save password for 'root@localhost:3306'? [Y]es/[N]o/Ne[v]er (default No): N
Fetching schema names for autocompletion... Press ^C to stop.
Your MySQL connection id is 12
Server version: 8.0.15-commercial MySQL Enterprise Server - Commercial
No default schema selected; type \use  to set one.

Switching to SQL mode... Commands end with ;

I am starting with three fresh installations of MySQL. I shouldn’t have any databases or tables other than the default ones. And, I want to double-check to make sure there haven’t been any transactions already executed. I can do this with the SHOW MASTER STATUS\G command:

*************************** 1. row ***************************
             File: binlog.000001
         Position: 151
     Binlog_Do_DB: 
 Binlog_Ignore_DB: 
Executed_Gtid_Set: 
1 row in set (0.0003 sec)

From an OS command prompt, you can take a look at the binary log file binlog.000001, which is located inside your MySQL data directory. And the binlog.index file contains a list of the active binary logs, which in this case, is only binlog.000001.

Since this is a new installation, I should only see the four default MySQL databases, and the four default MySQL users:

+--------------------+
| Database           |
+--------------------+
| information_schema |
| mysql              |
| performance_schema |
| sys                |
+--------------------+
4 rows in set (0.0013 sec)

+------------------+-----------+
| user             | host      |
+------------------+-----------+
| mysql.infoschema | localhost |
| mysql.session    | localhost |
| mysql.sys        | localhost |
| root             | localhost |
+------------------+-----------+
4 rows in set (0.0004 sec)

When you use MySQL Shell to build your InnoDB Cluster, you can use pre-built administration commands to configure and manage the cluster. Before adding an instance to the cluster, I can check to see if the instance is suitable for InnoDB Cluster by running the dba.checkInstanceConfiguration command. To use the commands, I will switch to JavaScript mode:

Please provide the password for 'root@localhost:3306': 
Save password for 'root@localhost:3306'? [Y]es/[N]o/Ne[v]er (default No): Y
Validating local MySQL instance listening at port 3306 for use in an InnoDB cluster...

This instance reports its own address as MacVM163.local
Clients and other cluster members will communicate with it through this address by default. If this is not correct, the report_host MySQL system variable should be changed.

Checking whether existing tables comply with Group Replication requirements...
No incompatible tables detected

Checking instance configuration...

Some configuration options need to be fixed:
+--------------------------+---------------+----------------+--------------------------------------------------+
| Variable                 | Current Value | Required Value | Note                                             |
+--------------------------+---------------+----------------+--------------------------------------------------+
| binlog_checksum          | CRC32         | NONE           | Update the server variable                       |
| enforce_gtid_consistency | OFF           | ON             | Update read-only variable and restart the server |
| gtid_mode                | OFF           | ON             | Update read-only variable and restart the server |
| server_id                | 1             |     | Update read-only variable and restart the server |
+--------------------------+---------------+----------------+--------------------------------------------------+

Some variables need to be changed, but cannot be done dynamically on the server.
Please use the dba.configureInstance() command to repair these issues.

{
    "config_errors": [
        {
            "action": "server_update", 
            "current": "CRC32", 
            "option": "binlog_checksum", 
            "required": "NONE"
        }, 
        {
            "action": "restart", 
            "current": "OFF", 
            "option": "enforce_gtid_consistency", 
            "required": "ON"
        }, 
        {
            "action": "restart", 
            "current": "OFF", 
            "option": "gtid_mode", 
            "required": "ON"
        }, 
        {
            "action": "restart", 
            "current": "1", 
            "option": "server_id", 
            "required": ""
        }
    ], 
    "status": "error"
}

From the above output (under the header “Some configuration options need to be fixed:“), I can see I need to add four variables and their correct values to the MySQL configuration file – and I will need to reboot MySQL:

I added the following to the MySQL configuration file (my.cnf) on all three instances under the [mysqld] header – and I restarted all three instances of MySQL:

# MySQL Configuration File
[mysqld]
binlog_checksum=NONE
enforce_gtid_consistency=ON
gtid_mode=ON
server_id=161  # change for each server

After the reboot, I will need to re-launch mysqlsh and/or reconnect to the database. To exit Shell, use the /exit command. I can then run dba.checkInstanceConfiguration again to see if the instance is ready for InnoDB Cluster. (Remember to switch to JavaScript mode (\js) if you aren’t already in JavaScript mode)

Please provide the password for 'root@localhost:3306': 
Save password for 'root@localhost:3306'? [Y]es/[N]o/Ne[v]er (default No): N
Validating local MySQL instance listening at port 3306 for use in an InnoDB cluster...

This instance reports its own address as MacVM161.local
Clients and other cluster members will communicate with it through this address by default. If this is not correct, the report_host MySQL system variable should be changed.

Checking whether existing tables comply with Group Replication requirements...
No incompatible tables detected

Checking instance configuration...
Instance configuration is compatible with InnoDB cluster

The instance 'localhost:3306' is valid for InnoDB cluster usage.

{
    "status": "ok"
}

The above status line of “status”: “ok”, confirms this instance is ready for InnoDB Cluster.

I can configure the first server by using the dba.configureInstance command. You have an option to use root to manage the cluster, or you can choose option #2 which will create a new administrator account for cluster. I decided to choose option #2, which does make the install a little more complicated.

Please provide the password for 'root@localhost:3306': 
Save password for 'root@localhost:3306'? [Y]es/[N]o/Ne[v]er (default No): N
Configuring local MySQL instance listening at port 3306 for use in an InnoDB cluster...

This instance reports its own address as MacVM161.local
Clients and other cluster members will communicate with it through this address by default. If this is not correct, the report_host MySQL system variable should be changed.

WARNING: User 'root' can only connect from localhost.
If you need to manage this instance while connected from other hosts, new account(s) with the proper source address specification must be created.

1) Create remotely usable account for 'root' with same grants and password
2) Create a new admin account for InnoDB cluster with minimal required grants
3) Ignore and continue
4) Cancel

Please select an option [1]: 2
Please provide an account name (e.g: icroot@%) to have it created with the necessary
privileges or leave empty and press Enter to cancel.
Account Name: cluster_adm

The instance 'localhost:3306' is valid for InnoDB cluster usage.

Cluster admin user 'cluster_adm'@'%' created.
The instance 'localhost:3306' is already ready for InnoDB cluster usage.

As I am doing these installs, I like to check the master status as I go along, so I will do that again. First I need to switch to SQL mode.

Switching to SQL mode... Commands end with ;

*************************** 1. row ***************************
             File: binlog.000002
         Position: 151
     Binlog_Do_DB: 
 Binlog_Ignore_DB: 
Executed_Gtid_Set: 
1 row in set (0.0002 sec)

The master status hasn’t changed, so nothing was written to the binary log.

One immediate problem from not using the default root user is you aren’t given the option to assign a password for the cluster_adm user. I can check this by querying the mysql.user table:

*************************** 1. row ***************************
+-------------+------+-----------------------+
| user        | host | authentication_string |
+-------------+------+-----------------------+
| cluster_adm | %    |                       |
+-------------+------+-----------------------+

Since there isn’t a password for cluster_adm, I will go ahead and set it. But – I don’t want to write this to the binary log, as then it would get replicated to the other servers once I start the cluster. I will need to create this user on the other two servers. I can suppress writing to the binary log with SET SQL_LOG_BIN=0; and then turn it back on with SET SQL_LOG_BIN=1;. (Don’t forget to substitute the value of new_password for your actual password)

SET SQL_LOG_BIN=0;
ALTER USER 'cluster_adm'@'%' IDENTIFIED BY 'new_password';
FLUSH PRIVILEGES;
SET SQL_LOG_BIN=1;

I can check again to see if the cluster_adm user now has a password:

+-------------+------+------------------------------------------------------------------------+
| user        | host | authentication_string                                                  |
+-------------+------+------------------------------------------------------------------------+
| cluster_adm | %    | $A$005$',q+B<%=JJ|Mz.WH!XXX/iQ4rvG/3DzX/UharambelivesYp1oODqtNZk25     | 
+-------------+------+------------------------------------------------------------------------+

Since I need to create this user along with the appropriate grants (permissions) on the other two MySQL instances, I can just look at the grants which were given to the cluster_adm user when the dba.configureInstance program created the user:

+------------------------------------------------------------------------------------------------------------------------------------------------+
| Grants for cluster_adm@%                                                                                                                       |
+------------------------------------------------------------------------------------------------------------------------------------------------+
| GRANT RELOAD, SHUTDOWN, PROCESS, FILE, SUPER, REPLICATION SLAVE, REPLICATION CLIENT, CREATE USER ON *.* TO `cluster_adm`@`%` WITH GRANT OPTION |
| GRANT SELECT, INSERT, UPDATE, DELETE ON `mysql`.* TO `cluster_adm`@`%` WITH GRANT OPTION                                                       |
| GRANT SELECT ON `sys`.* TO `cluster_adm`@`%` WITH GRANT OPTION                                                                                 |
| GRANT ALL PRIVILEGES ON `mysql_innodb_cluster_metadata`.* TO `cluster_adm`@`%` WITH GRANT OPTION                                               |
| GRANT SELECT ON `performance_schema`.`replication_applier_configuration` TO `cluster_adm`@`%` WITH GRANT OPTION                                |
| GRANT SELECT ON `performance_schema`.`replication_applier_status_by_coordinator` TO `cluster_adm`@`%` WITH GRANT OPTION                        |
| GRANT SELECT ON `performance_schema`.`replication_applier_status_by_worker` TO `cluster_adm`@`%` WITH GRANT OPTION                             |
| GRANT SELECT ON `performance_schema`.`replication_applier_status` TO `cluster_adm`@`%` WITH GRANT OPTION                                       |
| GRANT SELECT ON `performance_schema`.`replication_connection_configuration` TO `cluster_adm`@`%` WITH GRANT OPTION                             |
| GRANT SELECT ON `performance_schema`.`replication_connection_status` TO `cluster_adm`@`%` WITH GRANT OPTION                                    |
| GRANT SELECT ON `performance_schema`.`replication_group_member_stats` TO `cluster_adm`@`%` WITH GRANT OPTION                                   |
| GRANT SELECT ON `performance_schema`.`replication_group_members` TO `cluster_adm`@`%` WITH GRANT OPTION                                        |
| GRANT SELECT ON `performance_schema`.`threads` TO `cluster_adm`@`%` WITH GRANT OPTION                                                          |
+------------------------------------------------------------------------------------------------------------------------------------------------+
13 rows in set (0.0086 sec)

SET SQL_LOG_BIN=0;
CREATE USER 'cluster_adm'@'%' IDENTIFIED BY 'new_password';
GRANT RELOAD, SHUTDOWN, PROCESS, FILE, SUPER, REPLICATION SLAVE, REPLICATION CLIENT, CREATE USER ON *.* TO `cluster_adm`@`%` WITH GRANT OPTION;
GRANT SELECT, INSERT, UPDATE, DELETE ON `mysql`.* TO `cluster_adm`@`%` WITH GRANT OPTION                                                      ;
GRANT SELECT ON `sys`.* TO `cluster_adm`@`%` WITH GRANT OPTION                                                                                ;
GRANT ALL PRIVILEGES ON `mysql_innodb_cluster_metadata`.* TO `cluster_adm`@`%` WITH GRANT OPTION                                              ;
GRANT SELECT ON `performance_schema`.`replication_applier_configuration` TO `cluster_adm`@`%` WITH GRANT OPTION                               ;
GRANT SELECT ON `performance_schema`.`replication_applier_status_by_coordinator` TO `cluster_adm`@`%` WITH GRANT OPTION                       ;
GRANT SELECT ON `performance_schema`.`replication_applier_status_by_worker` TO `cluster_adm`@`%` WITH GRANT OPTION                            ;
GRANT SELECT ON `performance_schema`.`replication_applier_status` TO `cluster_adm`@`%` WITH GRANT OPTION                                      ;
GRANT SELECT ON `performance_schema`.`replication_connection_configuration` TO `cluster_adm`@`%` WITH GRANT OPTION                            ;
GRANT SELECT ON `performance_schema`.`replication_connection_status` TO `cluster_adm`@`%` WITH GRANT OPTION                                   ;
GRANT SELECT ON `performance_schema`.`replication_group_member_stats` TO `cluster_adm`@`%` WITH GRANT OPTION                                  ;
GRANT SELECT ON `performance_schema`.`replication_group_members` TO `cluster_adm`@`%` WITH GRANT OPTION                                       ;
GRANT SELECT ON `performance_schema`.`threads` TO `cluster_adm`@`%` WITH GRANT OPTION                                                         ;
FLUSH PRIVILEGES;
SET SQL_LOG_BIN=1;

Again - I will check the master status to make sure there haven't been any transactions yet.

*************************** 1. row ***************************
             File: binlog.000003
         Position: 151
     Binlog_Do_DB: 
 Binlog_Ignore_DB: 
Executed_Gtid_Set: 
1 row in set (0.0009 sec)

InnoDB Cluster uses Global Transaction Identifiers (GTIDs). "A global transaction identifier (GTID) is a unique identifier created and associated with each transaction committed on the server of origin (the master). This identifier is unique not only to the server on which it originated, but is unique across all servers in a given replication topology." Source

Each server in the three-node cluster will have its own GTIDs, which is composed of a Universal Unique Identifier (UUID), a colon (:) and an incremental number. To see the UUID for each server, this value is stored in the MySQL data directory in the auto.cnf file:

 # cat auto.cnf
[auto]
server-uuid=ae1a6186-5672-11e9-99b4-80e6004d84ae

Therefore, each of these three servers will have their own UUID being used in the GTIDs. The cluster itself will have a separate UUID being used in its own GTID, and this UUID is generated when the cluster is created. I have the following UUID's for the three servers:

Now I am ready to create the cluster. From the first server (192.168.1.161), I will switch to JavaScript mode, and then I will want to re-connect as the cluster_adm user.

Switching to JavaScript mode...

Creating a session to 'cluster_adm@192.168.1.161:3306'
Please provide the password for 'cluster_adm@192.168.1.161:3306': 
Save password for 'cluster_adm@192.168.1.161:3306'? [Y]es/[N]o/Ne[v]er (default No): N
Fetching schema names for autocompletion... Press ^C to stop.
Closing old connection...
Your MySQL connection id is 19
Server version: 8.0.15-commercial MySQL Enterprise Server - Commercial
No default schema selected; type \use  to set one.

I can create the cluster using the dba.createCluster command:

 MySQL  192.168.1.161:3306 ssl  JS > dba.createCluster("myCluster");
A new InnoDB cluster will be created on instance 'cluster_adm@192.168.1.161:3306'.

Validating instance at 192.168.1.161:3306...

This instance reports its own address as MacVM161.local

Instance configuration is suitable.
Creating InnoDB cluster 'myCluster' on 'cluster_adm@192.168.1.161:3306'...
Adding Seed Instance...

Cluster successfully created. Use Cluster.addInstance() to add MySQL instances.
At least 3 instances are needed for the cluster to be able to withstand up to
one server failure.

The InnoDB Cluster was successfully created. Now when I do a SHOW MASTER STATUS\G, I will see some values under the Executed_Gtid_Set section: (I will need to switch back to the SQL mode)

Switching to SQL mode... Commands end with ;

*************************** 1. row ***************************
             File: binlog.000001
         Position: 12406
     Binlog_Do_DB: 
 Binlog_Ignore_DB: 
Executed_Gtid_Set: ae1a6186-5672-11e9-99b4-80e6004d84ae:1-12,
c446e75c-5674-11e9-bf50-753fdb914192:1-2
1 row in set (0.0003 sec)

Under the Executed_Gtid_Set section, I have two sets of GTID's. One is for the 192.168.1.161 server (ae1a6186-5672-11e9-99b4-80e6004d84ae), and the other is for the cluster (c446e75c-5674-11e9-bf50-753fdb914192).

Executed_Gtid_Set: ae1a6186-5672-11e9-99b4-80e6004d84ae:1-12,
c446e75c-5674-11e9-bf50-753fdb914192:1-2

The Executed_Gtid_Set shows which transactions have been applied to this server (192.168.1.161). There have been 12 transactions for the server (ae1a6186-5672-11e9-99b4-80e6004d84ae:1-12) and two transactions for the cluster (c446e75c-5674-11e9-bf50-753fdb914192:1-2). The GTID's for the cluster should be the same on each server, as we add them to the server. Also, once you add servers to the cluster, the read-only servers will be changed to SUPER_READ_ONLY to prevent write-transactions from being applied to the read-only servers in a single-primary mode cluster. If you want to view the transactions which were executed, you can use the mysqlbinlog utility.

I can reference my earlier table of the UUID's for all of the servers, and I can now add the UUID for the cluster (in red:

I can check the status of the cluster. (after switching back to JavaScript mode)

Switching to JavaScript mode...

{
    "clusterName": "myCluster", 
    "defaultReplicaSet": {
        "name": "default", 
        "primary": "192.168.1.161:3306", 
        "ssl": "REQUIRED", 
        "status": "OK_NO_TOLERANCE", 
        "statusText": "Cluster is NOT tolerant to any failures.", 
        "topology": {
            "192.168.1.161:3306": {
                "address": "192.168.1.161:3306", 
                "mode": "R/W", 
                "readReplicas": {}, 
                "role": "HA", 
                "status": "ONLINE"
            }
        }, 
        "topologyMode": "Single-Primary"
    }, 
    "groupInformationSourceMember": "MacVM161.local:3306"
}

Under the topology section above, we can see server 192.168.1.161 is in the cluster and has the status of ONLINE.

I am now ready to add 192.168.1.162 and 192.168.1.163 to the cluster. But first I need to make sure each server is ready to be added to the cluster using the dba.checkInstanceConfiguration command. I can run these commands from any one of the servers, but I am going to run this from the first server - 192.168.1.161. Prior to this, I went ahead and made the same changes to the MySQL configuration file (my.cnf or my.ini) as I did on 192.168.1.161.

Please provide the password for 'cluster_adm@192.168.1.162:3306': 
Save password for 'cluster_adm@192.168.1.162:3306'? [Y]es/[N]o/Ne[v]er (default No): 
Validating MySQL instance at 192.168.1.162:3306 for use in an InnoDB cluster...

This instance reports its own address as MacVM162.local
Clients and other cluster members will communicate with it through this address by default. If this is not correct, the report_host MySQL system variable should be changed.

Checking whether existing tables comply with Group Replication requirements...
No incompatible tables detected

Checking instance configuration...
Instance configuration is compatible with InnoDB cluster

The instance '192.168.1.162:3306' is valid for InnoDB cluster usage.

{
    "status": "ok"
}

Please provide the password for 'cluster_adm@192.168.1.163:3306': 
Save password for 'cluster_adm@192.168.1.163:3306'? [Y]es/[N]o/Ne[v]er (default No): 
Validating MySQL instance at 192.168.1.163:3306 for use in an InnoDB cluster...

This instance reports its own address as MacVM162.local
Clients and other cluster members will communicate with it through this address by default. If this is not correct, the report_host MySQL system variable should be changed.

Checking whether existing tables comply with Group Replication requirements...
No incompatible tables detected

Checking instance configuration...
Instance configuration is compatible with InnoDB cluster

The instance '192.168.1.163:3306' is valid for InnoDB cluster usage.

{
    "status": "ok"
}

Both servers are ready to go, as indicated by the "status": "ok".

Now I can add the other two servers:

A new instance will be added to the InnoDB cluster. Depending on the amount of
data on the cluster this might take from a few seconds to several hours.

Adding instance to the cluster ...

Please provide the password for 'cluster_adm@192.168.1.162:3306': 
Save password for 'cluster_adm@192.168.1.162:3306'? [Y]es/[N]o/Ne[v]er (default No): 
Validating instance at 192.168.1.162:3306...

This instance reports its own address as MacVM162.local

Instance configuration is suitable.
The instance 'cluster_adm@192.168.1.162:3306' was successfully added to the cluster.

A new instance will be added to the InnoDB cluster. Depending on the amount of
data on the cluster this might take from a few seconds to several hours.

Adding instance to the cluster ...

Please provide the password for 'cluster_adm@192.168.1.163:3306': 
Save password for 'cluster_adm@192.168.1.163:3306'? [Y]es/[N]o/Ne[v]er (default No): 
Validating instance at 192.168.1.163:3306...

This instance reports its own address as MacVM162.local

Instance configuration is suitable.
The instance 'cluster_adm@192.168.1.163:3306' was successfully added to the cluster.

The SHOW MASTER STATUS\G should now be the same on all three servers. I can also check the status of the cluster, to see if all three nodes are ONLINE.

{
    "clusterName": "myCluster", 
    "defaultReplicaSet": {
        "name": "default", 
        "primary": "192.168.1.161:3306", 
        "ssl": "REQUIRED", 
        "status": "OK", 
        "statusText": "Cluster is ONLINE and can tolerate up to ONE failure.", 
        "topology": {
            "192.168.1.161:3306": {
                "address": "192.168.1.161:3306", 
                "mode": "R/W", 
                "readReplicas": {}, 
                "role": "HA", 
                "status": "ONLINE"
            }, 
            "192.168.1.162:3306": {
                "address": "192.168.1.162:3306", 
                "mode": "R/O", 
                "readReplicas": {}, 
                "role": "HA", 
                "status": "ONLINE"
            }, 
            "192.168.1.163:3306": {
                "address": "192.168.1.163:3306", 
                "mode": "R/O", 
                "readReplicas": {}, 
                "role": "HA", 
                "status": "ONLINE"
            }
        }, 
        "topologyMode": "Single-Primary"
    }, 
    "groupInformationSourceMember": "MacVM161.local:3306"
}

All three nodes have the status of ONLINE, so the cluster is up and ready to use.

	Tony Darnell is a Principal Sales Consultant for MySQL, a division of Oracle, Inc. MySQL is the world's most popular open-source database program. Tony may be reached at info [at] ScriptingMySQL.com and on LinkedIn.
	Tony is the author of Twenty Forty-Four: The League of Patriots  Visit http://2044thebook.com for more information.
	Tony is the editor/illustrator for NASA Graphics Standards Manual Remastered Edition  Visit https://amzn.to/2oPFLI0 for more information.

In MySQL, historically GROUP BY was used to provide sorting as well. If a query specified GROUP BY, the result was sorted as if ORDER BY was present in the query.

mysql-5.7> CREATE TABLE t (id INTEGER,  cnt INTEGER);
Query OK, 0 rows affected (0.03 sec)

mysql-5.7> INSERT INTO t VALUES (4,1),(3,2),(1,4),(2,2),(1,1),(1,5),(2,6),(2,1),(1,3),(3,4),(4,5),(3,6);
Query OK, 12 rows affected (0.02 sec)
Records: 12  Duplicates: 0  Warnings: 0

mysql-5.7> SELECT id, SUM(cnt) FROM t GROUP BY id;
+------+----------+
| id   | SUM(cnt) |
+------+----------+
|    1 |       13 |
|    2 |        9 |
|    3 |       12 |
|    4 |        6 |
+------+----------+
4 rows in set (0.00 sec)

MySQL here implicitly sorts the results from GROUP BY (i.e.…

In my previous post, some time ago, I wrote about the new cluster feature of ProxySQL. For that post, we were working with three nodes, now we’ll work with even more! If you’ve installed one ProxySQL per application instance and would like to work up to more, then this post is for you. If this is new to you, though, read my earlier post first for more context.

Check the image below to understand the structure of “one ProxySQL per application”. This means you have ProxySQL installed, and your application (Java, PHP, Apache server etc) in the same VM (virtual machine).

Having taken a look at that you probably have a few questions, such as:

• What happens if you have 20 nodes synced and you now need to add 100 or more nodes?
• How can I sync the new nodes without introducing errors?

Don’t be scared, it’s a simple process.

Remember there are only four tables which can be synced over the cluster. These tables are:

• mysql_query_rules
• mysql_servers
• mysql_users
• proxysql_servers

From a new proxysql cluster installation

After a fresh installation all those four tables are empty. That means if we configure it as a new node in a cluster, all rows in those tables will be copied instantly.

Generally the installation is straightforward.

• Download the latest proxysql package from https://www.percona.com/downloads/proxysql/
• Install ProxySQL using rpm or dpkg depending on your OS
• Install the MySQL client package
• Start the proxysql service

Now ProxySQL is up and all the configuration are in default settings

Connect to the ProxySQL console:

mysql -uadmin -padmin -h127.0.0.1 -P6032 --prompt='\u (\d)>'

Now there are two steps remaining:

• Configure global_variables table
• Configure proxysql_servers table

How to configure the global_variables table

Below is an example showing the minimal parameters to set – you can change the username and password according to your needs.

You can copy and paste the username and passwords from the current cluster and monitoring process by running the next command in a node of the current cluster:

select * from global_variables where variable_name in ('admin-admin_credentials', 'admin-cluster_password', 'mysql-monitor_password', 'admin-cluster_username', 'mysql-monitor_username');

You can update the parameters of the current node by using this as a template:

update global_variables set variable_value='<REPLACE-HERE>' where variable_name='admin-admin_credentials';
update global_variables set variable_value='<REPLACE-HERE>' where variable_name='admin-cluster_username';
update global_variables set variable_value='<REPLACE-HERE>' where variable_name='admin-cluster_password';
update global_variables set variable_value='<REPLACE-HERE>' where variable_name='mysql-monitor_username';
update global_variables set variable_value='<REPLACE-HERE>' where variable_name='mysql-monitor_password';
update global_variables set variable_value=1000 where variable_name='admin-cluster_check_interval_ms';
update global_variables set variable_value=10 where variable_name='admin-cluster_check_status_frequency';
update global_variables set variable_value='true' where variable_name='admin-cluster_mysql_query_rules_save_to_disk';
update global_variables set variable_value='true' where variable_name='admin-cluster_mysql_servers_save_to_disk';
update global_variables set variable_value='true' where variable_name='admin-cluster_mysql_users_save_to_disk';
update global_variables set variable_value='true' where variable_name='admin-cluster_proxysql_servers_save_to_disk';
update global_variables set variable_value=3 where variable_name='admin-cluster_mysql_query_rules_diffs_before_sync';
update global_variables set variable_value=3 where variable_name='admin-cluster_mysql_servers_diffs_before_sync';
update global_variables set variable_value=3 where variable_name='admin-cluster_mysql_users_diffs_before_sync';
update global_variables set variable_value=3 where variable_name='admin-cluster_proxysql_servers_diffs_before_sync';
load admin variables to RUNTIME;
save admin variables to disk;

Configure “proxysql_servers” table

At this point you need keep in mind that you will need to INSERT into this table “all” the IPs from the other ProxySQL nodes.

Why so? Because this table will have a new epoch time and this process will overwrite the rest of the nodes listed by its last table update.

In our example, let’s assume the IP of the new node is 10.0.1.3 (i.e. node 3, below)

INSERT INTO proxysql_servers (hostname,port,weight,comment) VALUES ('10.0.1.1',6032,0,'p1');
INSERT INTO proxysql_servers (hostname,port,weight,comment) VALUES ('10.0.1.2',6032,0,'p2');
INSERT INTO proxysql_servers (hostname,port,weight,comment) VALUES ('10.0.1.3',6032,0,'p3');
LOAD PROXYSQL SERVERS TO RUNTIME;
SAVE PROXYSQL SERVERS TO DISK;

If you already have many ProxySQL servers in the cluster, you can run mysqldump as this will help speed up this process.

If that’s the case, you need to find the most up to date node and use mysqldump to export the data from the proxysql_servers table.

How would you find this node? There are a few stats tables in ProxySQL, and in this case we can use two of these to help identify the right node.

SELECT stats_proxysql_servers_checksums.hostname, stats_proxysql_servers_metrics.Uptime_s, stats_proxysql_servers_checksums.port, stats_proxysql_servers_checksums.name, stats_proxysql_servers_checksums.version, FROM_UNIXTIME(stats_proxysql_servers_checksums.epoch) epoch, stats_proxysql_servers_checksums.checksum, stats_proxysql_servers_checksums.diff_check FROM stats_proxysql_servers_metrics JOIN stats_proxysql_servers_checksums ON stats_proxysql_servers_checksums.hostname = stats_proxysql_servers_metrics.hostname WHERE stats_proxysql_servers_metrics.Uptime_s > 0 ORDER BY epoch DESC

Here’s an example output

+------------+----------+------+-------------------+---------+---------------------+--------------------+------------+
| hostname   | Uptime_s | port | name              | version | epoch               | checksum           | diff_check |
+------------+----------+------+-------------------+---------+---------------------+--------------------+------------+
| 10.0.1.1   | 1190     | 6032 | mysql_users       | 2       | 2019-04-04 12:04:21 | 0xDB07AC7A298E1690 | 0          |
| 10.0.1.2   | 2210     | 6032 | mysql_users       | 2       | 2019-04-04 12:04:18 | 0xDB07AC7A298E1690 | 0          |
| 10.0.1.1   | 1190     | 6032 | mysql_query_rules | 1       | 2019-04-04 12:00:07 | 0xBC63D734643857A5 | 0          |
| 10.0.1.1   | 1190     | 6032 | mysql_servers     | 1       | 2019-04-04 12:00:07 | 0x0000000000000000 | 0          |
| 10.0.1.1   | 1190     | 6032 | proxysql_servers  | 1       | 2019-04-04 12:00:07 | 0x233638C097DE6190 | 0          |
| 10.0.1.2   | 2210     | 6032 | mysql_query_rules | 1       | 2019-04-04 11:43:13 | 0xBC63D734643857A5 | 0          |
| 10.0.1.2   | 2210     | 6032 | mysql_servers     | 1       | 2019-04-04 11:43:13 | 0x0000000000000000 | 0          |
| 10.0.1.2   | 2210     | 6032 | proxysql_servers  | 1       | 2019-04-04 11:43:13 | 0x233638C097DE6190 | 0          |
| 10.0.1.2   | 2210     | 6032 | admin_variables   | 0       | 1970-01-01 00:00:00 |                    | 0          |
| 10.0.1.2   | 2210     | 6032 | mysql_variables   | 0       | 1970-01-01 00:00:00 |                    | 0          |
| 10.0.1.1   | 1190     | 6032 | admin_variables   | 0       | 1970-01-01 00:00:00 |                    | 0          |
| 10.0.1.1   | 1190     | 6032 | mysql_variables   | 0       | 1970-01-01 00:00:00 |                    | 0          |
+------------+----------+------+-------------------+---------+---------------------+--------------------+------------+

For each table, we can see different versions, each related to table changes.  In this case, we need to look for the latest epoch time for the table “proxysql_servers“. In this example , above, we can see that the server with the IP address of  10.0.1.1 is the latest version.  We can now run the next command to get a backup of all the IP data from the current cluster

mysqldump --host=127.0.0.1 --port=6032 --skip-opt --no-create-info --no-tablespaces --skip-triggers --skip-events main proxysql_servers > proxysql_servers.sql

Now, copy the output to the new node and import into proxysql_servers table. Here’s an example: the data has been exported to the file proxysql_servers.sql which we’ll now load to the new node:

source proxysql_servers.sql
LOAD PROXYSQL SERVERS TO RUNTIME;
SAVE PROXYSQL SERVERS TO DISK;

You can run the next SELECTs to verify that the new node has the data from the current cluster, and in doing so ensure that the nodes are in sync as expected:

select * from mysql_query_rules;
select * from mysql_servers;
select * from mysql_users ;
select * from proxysql_servers;

How can we check if there are errors in the synchronization process?

Run the next command to fetch data from the table stats_proxysql_servers_checksums:

SELECT hostname, port, name, version, FROM_UNIXTIME(epoch) epoch, checksum, diff_check FROM stats_proxysql_servers_checksums  ORDER BY epoch;

Using our example, here’s the data saved in stats_proxysql_servers_checksums for the proxysql_servers table

admin ((none))>SELECT hostname, port, name, version, FROM_UNIXTIME(epoch) epoch, checksum, diff_check FROM stats_proxysql_servers_checksums  ORDER BY epoch;
+------------+------+-------------------+---------+---------------------+--------------------+------------+---------------------+
| hostname   | port | name              | version | epoch               | checksum           | diff_check | DATETIME('NOW')     |
+------------+------+-------------------+---------+---------------------+--------------------+------------+---------------------+
...
| 10.0.1.1   | 6032 | proxysql_servers  | 2       | 2019-03-25 13:36:17 | 0xC7D7443B96FC2A94 | 92         | 2019-03-25 13:38:31 |
| 10.0.1.2   | 6032 | proxysql_servers  | 2       | 2019-03-25 13:36:34 | 0xC7D7443B96FC2A94 | 92         | 2019-03-25 13:38:31 |
...
| 10.0.1.3   | 6032 | proxysql_servers  | 2       | 2019-03-25 13:37:00 | 0x233638C097DE6190 | 0          | 2019-03-25 13:38:31 |
...
+------------+------+-------------------+---------+---------------------+--------------------+------------+---------------------+

As we can see in the column “diff” there are some differences in the checksum of the other nodes: our new node (IP 10.0.1.3) has the checksum 0x233638C097DE6190 where as nodes 1 and 2 both show 0xC7D7443B96FC2A94

This indicates that these nodes have different data. Is this correct? Well, yes, in this case, since in our example the nodes 1 and 2 were created on the current ProxySQL cluster

How do you fix this?

Well, you connect to the node 1 and INSERT the data for node 3 (the new node).

This change will propagate the changes over the current cluster – node 2 and node 3 – and will update the table proxysql_servers.

INSERT INTO proxysql_servers (hostname,port,weight,comment) VALUES ('10.0.1.3',6032,0,'p3');
LOAD PROXYSQL SERVERS TO RUNTIME;

What happens if the new nodes already exist in the proxysql_servers table of the current cluster?

You can add the new ProxySQL node IPs to the current cluster before you start installing and configuring the new nodes.

In fact, this works fine and without any issues. The steps I went through, above, are the same, but when you check for differences in the table stats_proxysql_servers_checksums you should find there are none, and that the “diff” column should be displayed as 0.

How does ProxySQL monitor other ProxySQL nodes to sync locally?

Here’s a link to the explanation from the ProxySQL wiki page https://github.com/sysown/proxysql/wiki/ProxySQL-Cluster

Proxies monitor each other, so they immediately recognize that when a checksum of a configuration changes, the configuration has changed. It’s possible that the remote peer’s configuration and its own configuration were changed at the same time, or within a short period of time. So the proxy must also check its own status.

When proxies find differences:

• If its own version is 1 , find the peer with version > 1 and with the highest epoch, and sync immediately
• If its own version is greater than 1, starts counting for how many checks they differ
  • when the number of checks in which they differ is greater than cluster__name___diffs_before_sync and cluster__name__diffs_before_sync itself is greater than 0, find the peer with version > 1 and with the highest epoch, and sync immediately.

Note: it is possible that a difference is detected against one node, but the sync is performed against a different node. Since ProxySQL bases the sync on the node with the highest epoch, it is expected that all the nodes will converge.

To perform the syncing process:

These next steps run automatically when other nodes detect a change: this is an example for the table mysql_users

The same connection that’s used to perform the health check is used to execute a series of SELECTstatements in the form of SELECT _list_of_columns_ FROM runtime_module

SELECT username, password, active, use_ssl, default_hostgroup, default_schema, schema_locked, transaction_persistent, fast_forward, backend, frontend, max_connections FROM runtime_mysql_users;

In the node where it detected a difference in the checksum, it will run the next statements automatically:

DELETE FROM mysql_servers;
INSERT INTO mysql_users(username, password, active, use_ssl, default_hostgroup, default_schema, schema_locked, transaction_persistent, fast_forward, backend, frontend, max_connections) VALUES ('user_app','secret_password', 1, 0, 10, '', 0, 1, 0, 0, 1, 1000);
LOAD MYSQL USERS TO RUNTIME;
SAVE MYSQL USERS TO DISK;

Be careful:

After you’ve added the new ProxySQL node to the cluster, all four tables will sync immediately using the node with the highest epoch time as the basis for sync. Any changes to the tables previously added to the cluster, will propagate and overwrite the cluster

Summary

ProxySQL cluster is a powerful tool, and it’s being used more and more in production environments. If you haven’t tried it before, I’d recommend that you start testing ProxySQL cluster with a view to improving your application’s service. Hope you found this post helpful!

SQL Where Clause Example | SQL Where Query Tutorial is today’s topic. The WHERE clause is used to filter the database records. The WHERE clause is used to extract only those records that fulfill the specified condition. The SQL WHERE clause is used to specify the condition while fetching the data from a single table or by joining the multiple tables. If a given condition is satisfied, then only it returns the specific value from the table. You should use a WHERE clause to filter the records and fetching the necessary records.

SQL Where Clause Example

The WHERE clause is not only used in the SELECT statement; it is also used in an UPDATE, DELETE statement. The syntax of a SELECT statement with a WHERE clause is following.

SELECT column1, column2, columnN 
FROM table_name
WHERE [condition]

“SELECT column1, column2, column3 FROM tableName” is the standard SELECT statement.

“WHERE” is the keyword that restricts the select query result set and “condition” is a filter to be applied to the results. The filter can be the range, single value or subquery.

Now, we will head for the example, but before that, you might check out my Create table in SQL and How to insert data in database posts.

If you have followed that tutorial then in that tutorial, I have created a table and inserted some records.

Now, we fetch the records based on the WHERE clause.

Okay, now first let’s fetch all records.

For that, I need to write the following query.

SELECT * FROM Apps

See the following output.

SQL WHERE Query

SQL requires single quotes around text values. However, the numeric fields should not be enclosed in the quotes.

Okay, now let’s use the WHERE query to filter the record. Let’s fetch the record by CreatorName column.

Write the following query.

Select * from Apps
Where CreatorName = 'Krunal'

The output is following.

Let’s fetch the row by filtering AppCategory.

Select * from Apps
Where AppCategory = 'Investment'

The output is following.

You can specify the condition using the comparison or logical operators like >, <, =, LIKE, NOT, etc.

SQL Where Statement with Logical Operator

We can use the Logical operator with the Where clause to filter out the data by comparing.

Let’s say we fetch the data whose AppPrice is greater than 70$. See the following query.

Select * from Apps
Where AppPrice > 70

The output is following.

Operators in The WHERE Clause

The following operators can be used in a WHERE clause.

WHERE clause combined with AND LOGICAL Operator

The WHERE clause when used with the AND operator, is only executed if All the filter criteria specified are met. See the following query.

Select CreatorName from Apps
Where AppPrice > 70 AND AppName = 'Moneycontrol'

So, we are fetching the only CreatorName in which the AppPrice is > 70 and AppName = MoneyControl. See the below output.

WHERE clause combined with IN Keyword

The WHERE clause when used together with an IN keyword only affects the rows whose values match the list of values provided in the IN keyword. IN helps to reduce the number of OR clauses you may have to use. See the following query.

Select * from Apps
Where AppPrice IN (50, 60, 110)

In the above query, we are fetching those records whose AppPrice is either 50, 60, or 110. If AppPrice matches in the database, then it will give us that row in the output. 

In our case, only 50 and 60 matches, 110 is not there in the database. So, two rows will be returned.

WHERE clause combined with OR LOGICAL Operator

Select * from Apps
Where AppPrice = 50 OR AppCategory = 'Fashion'

Select * from Apps
Where AppPrice NOT IN(40, 50, 60)

The post SQL Where Clause Example | SQL Where Query Tutorial appeared first on AppDividend.