How to apply CROSS JOIN correctly in MySQL?

MySQLMySQLi Database

The CROSS JOIN in MySQL returns the Cartesian product of rows from the joined tables. Let us see an example by creating a table −

mysql> create table PairDemo
    -> (
    -> UserId int NOT NULL AUTO_INCREMENT PRIMARY KEY,
    -> UserName varchar(20),
    -> UserRelationshipName varchar(20)
    -> );
Query OK, 0 rows affected (0.56 sec)

Example

Insert some records in the table using insert command. The query is as follows −

mysql> insert into PairDemo(UserName,UserRelationshipName) values('John','James');
Query OK, 1 row affected (0.15 sec)
mysql> insert into PairDemo(UserName,UserRelationshipName) values('Carol','James');
Query OK, 1 row affected (0.20 sec)
mysql> insert into PairDemo(UserName,UserRelationshipName) values('Carol','David');
Query OK, 1 row affected (0.25 sec)
mysql> insert into PairDemo(UserName,UserRelationshipName) values('Sam','David');
Query OK, 1 row affected (0.19 sec)

Display all records from the table using a select statement. The query is as follows −

mysql> select *from PairDemo;

Output

+--------+----------+----------------------+
| UserId | UserName | UserRelationshipName |
+--------+----------+----------------------+
|      1 |     John |                James |
|      2 |    Carol |                James |
|      3 |    Carol |                David |
|      4 |      Sam |                David |
+--------+----------+----------------------+
4 rows in set (0.00 sec)

The following is the query to apply CROSS JOIN −

mysql> SELECT U.UserName, UR.UserRelationshipName
    -> FROM (
    -> (SELECT DISTINCT UserName from PairDemo) U
    -> CROSS JOIN
    -> (SELECT DISTINCT UserRelationshipName from PairDemo) UR
    -> ) WHERE NOT EXISTS (
    -> SELECT * FROM PairDemo tbl
    -> WHERE tbl.UserName = U.UserName
    -> AND tbl.UserRelationshipName = UR.UserRelationshipName
    -> ) ;

Here is the output that displays the result of the above query implementing CROSS JOIN −

+----------+----------------------+
| UserName | UserRelationshipName |
+----------+----------------------+
|      Sam |                James |
|     John |                David |
+----------+----------------------+
2 rows in set (0.03 sec)
raja
Published on 19-Mar-2019 08:08:55
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