SQL CROSS JOIN Tutorial With Example | SQL Join Types

SQL CROSS JOIN Tutorial With Example | SQL Join Types is today’s topic. A JOIN clause is used to combine rows from two or more tables, based on a related column between them. SQL Cross Joins are used to join table having no condition in which all the records of the first table comes with all the records of the second table. This type of Join is also called a Cartesian product. Unlike an INNER JOIN or LEFT JOIN, the cross join does not establish the relationship between the joined tables. When each row of the first table is combined with each row from the second table, known as Cartesian join or cross join.

SQL CROSS JOIN Tutorial

The CROSS JOIN joined every row from the first table (T1) with every row from the second table (T2).

In other words, the cross join returns the Cartesian product of rows from both the tables. The number of rows in a Cartesian product is the product of the number of rows in each involved tables.

Let’s say we have two tables A and B.

The following statements perform the cross join and produce a cartesian product of the rows from the A and B tables.

SELECT *
FROM A JOIN B;

SELECT *
FROM A
INNER JOIN B;

SELECT *
FROM A
CROSS JOIN B;

SELECT * 
FROM A, B;

Suppose, the A table has N rows, and B table has M rows, then the CROSS JOIN of these two tables will produce the result set that contains N x M rows.

Imagine that if you have a third table C with the K rows, the result of a CROSS JOIN clause of these three tables will contain N x M x K rows, which may be huge. That is why, you need to be very careful when using a CROSS JOIN clause.

You use an INNER JOIN and LEFT JOIN clauses more often than a CROSS JOIN clause. However, you will find the CROSS JOIN clause very useful in some usecases.

Similarity Between INNER JOIN and CROSS JOIN

If where a condition is used with CROSS JOIN, then it will behave like an INNER JOIN.

NOTE:

If Where Condition is not used with CROSS JOIN, then it will behave like a cartesian product.

Syntax of Cross Join

SELECT COLUMNS_NAME FROM TABLE_1 CROSS JOIN TABLE_2;

1. In the above syntax, COLUMNS_NAME is the name of the columns which the user wants to join.
2. Table_1 and Table_2 are the name of the Tables.

As you can see above that in Table 1, one row will be matched with all the rows of Table 2.

SHOWING CROSS JOIN WITHOUT WHERE CONDITION

See the following query.

Select * from STUDENT CROSS JOIN COURSE;

Output:

You can also obtain this result without using CROSS JOIN Keyword.

See the following query.

Select * from STUDENT, COURSE;

Other Examples:

User can also select optional columns; not all columns are required for Cross Joining.

See the following query.

Select student.roll_no,student.name,course.course 
from student 
CROSS JOIN Course;

See the output.

SHOWING CROSS JOIN USING WHERE CONDITION

See the table below.

Table_1: Employee

Table_2: Department

See the following query.

Select * from Employee 
CROSS JOIN Department 
where Employee.emp_id = Department.emp_id;

The output is following.

As mentioned above, the CROSS JOIN is similar to INNER JOIN if used with where condition.

Additional Information

Suppose there are m rows in Table 1 and n rows in Table 2

Then the total number of rows formed will be (m*n)

In the above Example, there are four rows in Student and three rows in Course tables.

Therefore, the total number of rows formed is 12.

This is also known as cardinality number.

Finally, SQL CROSS JOIN Tutorial With Example | SQL Join Types example is over.

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