SQL UNION Operator Tutorial With Example | Union Clause in SQL

SQL UNION Operator Tutorial With Example | Union Clause in SQL is today’s topic. SQL UNION operator is used to combine a result sets of 2 or more SELECT statements. It removes the duplicate rows between the several SELECT statements.

What is the use of Union Operator

A UNION operator is used to combine a result-set of two or more SELECT statements.

1. Each SELECT statement within a UNION must have the same number of columns selected.
2. It should have the same number of columns expressions and same datatypes written with the same order.
3. The columns in each SELECT statement should also be in the same order.
4. The result set obtained by Union clause excluding the duplicate rows between the various results set.

In this article, we’ll review:

1. UNION and UNION all and how they work.
2. We discuss the rules for using UNION vs. UNION ALL.
3. SQL Operator Syntax.
4. How to use the simple SQL UNION clause in the select statement.
5. How to use the SQL UNION with the queries that have a WHERE clause.

Syntax of SQL Union Clause

See the following syntax.

Select columns from table_name where condition
UNION
Select columns from table_name where condition;

The UNION operator selects only the distinct values by default. If we want to allow the duplicate values, we can use the UNION ALL operator. See the following syntax.

Select columns from table_name where a condition 
UNION ALL
Select columns from table_name where condition;

PARAMETERS

The columns are the name of the columns that you want to display.

Table_name is the name of the tables.

Where Conditions is optional. It is the conditions that must be met for the records to be selected.

SQL UNION Operator Tutorial With Example

Let’s see the example. See the following SQL Table. You can use ALTER TABLE query if you want to change the table.

Consider the table.

Employee

Department

See the following query.

Select Emp_id FROM Employee 
UNION 
Select Emp_id FROM Department;

Explanation

The UNION operator removed all the duplicates that were generated by both the select statements. You can see that Emp_Id 301 and 401 were displayed once.

If you do not wish to remove the duplicate values, try using the UNION ALL operator.

UNION ALL CLAUSE in SQL

The UNION ALL operator is used to combine the results of the two SELECT statements, which includes duplicate rows as well.

The rules that apply to the UNION ALL clause is entirely the same as that of UNION clause as previously discussed.

Now, see the following query

Select Emp_id FROM Employee 
UNION  ALL 
Select Emp_id FROM Department;

See the output.

Explanation

As you can see that Emp_id 301 and 401 was displayed twice. This happened because we used UNION ALL operator who didn’t remove any duplicates.

Example: (Different Field Names)

Previously what you saw was the columns which were selected was of the same name now we are going to deal with queries having different column names, but they need to have same datatypes.

See the following query.

Select Emp_id from Employee 
UNION 
Select Dept_no from Department;

See the following output.

You can also use UNION ALL in the above query. See the below example.

Select Emp_id from Employee 
UNION ALL 
Select Dept_no from Department;

Use of Union with where clause

Consider the following tables.

CUSTOMER

ORDERS

See the following query.

Select Id, city from customer 
where city = ”Patna” 
UNION 
select O_ID, city from orders where city = ”Patna”;

Also, see the below query.

Select Id, city from customer 
where city = ”Patna” 
UNION 
select cust_id, city from orders where city = ”Patna”;

See the output.

Another UNION EXAMPLE in SQL

We can also alias the column name using AS keyword for changing the column name for temporary results.

Let’s see the query.

Select Id AS Customer_ID, city  from customer
where city = ”Patna” 
UNION 
select cust_id AS Customer_ID, city from orders where city = ”Patna”;

See the following output.

There are two other clauses which are like the union clause

1. Intersect clause: It is used to combine two select statements but returns only rows only from the select statements that are identical to a row in the second SELECT statement.
2. Except clause: It is used to returns rows from first select statements that are not returned by the second select statement.

If you want to compare the two dates and return a count of the field based on the date values. For example, I have a date field in a table called the last updated date. I have to check if the trunc(last_updated_date >= trunc(sysdate-13).

Since you are using a COUNT function which is an aggregate function, we would recommend using a UNION operator.

Summary

When determining whether to use the UNION vs. UNION ALL, there are a few points to be considered. Use UNION ALL if you know that the result sets from the multiple queries don’t overlap or generate the duplicates and remember, if you need to use the parentheses, you can do that. You can also pivot and transform the output.

Just make sure that your data types match up and if you need to do an ORDER BY, do it at the end after you’ve run all those set queries. By nature, when it comes to UNION vs. UNION ALL, UNION ALL is faster than UNION; as UNION operators incur additional overhead to eliminate duplicates.

Finally, SQL UNION Operator Tutorial With Example is over.

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