MYSQL select DISTINCT values from two columns?

To select distinct values in two columns, you can use least() and greatest() function from MySQL.

Let us create a table with two columns −

mysql> create table SelectDistinctTwoColumns
   −> (
   −> StudentId int,
   −> EmployeeId int
   −> );
Query OK, 0 rows affected (0.60 sec)

Now you can insert records in the table. The query to insert records is as follows −

mysql> insert into SelectDistinctTwoColumns values(100,101);
Query OK, 1 row affected (0.39 sec)

mysql> insert into SelectDistinctTwoColumns values(102,103);
Query OK, 1 row affected (0.13 sec)

mysql> insert into SelectDistinctTwoColumns values(104,105);
Query OK, 1 row affected (0.18 sec)

mysql> insert into SelectDistinctTwoColumns values(100,101);
Query OK, 1 row affected (0.14 sec)

mysql> insert into SelectDistinctTwoColumns values(102,103);
Query OK, 1 row affected (0.12 sec)

mysql> insert into SelectDistinctTwoColumns values(106,107);
Query OK, 1 row affected (0.36 sec)

mysql> insert into SelectDistinctTwoColumns values(104,105);
Query OK, 1 row affected (0.17 sec)

mysql> insert into SelectDistinctTwoColumns values(105,104);
Query OK, 1 row affected (0.35 sec)

Display all records from table with the help of select statement. The query is as follows −

mysql> select *from SelectDistinctTwoColumns;

The following is the output −

+-----------+------------+
| StudentId | EmployeeId |
+-----------+------------+
| 100       | 101        |
| 102       | 103        |
| 104       | 105        |
| 100       | 101        |
| 102       | 103        |
| 106       | 107        |
| 104       | 105        |
| 105       | 104        |
+-----------+------------+
8 rows in set (0.00 sec)

Look at the above output. Some duplicate value in both the columns can be seen. Here is the query that selects distinct value from columns −

mysql> select distinct least(StudentId, EmployeeId) as FirstColumn,
   −> greatest(StudentId, EmployeeId) as SecondColumn from SelectDistinctTwoColumns;

The following is the output −

+-------------+--------------+
| FirstColumn | SecondColumn |
+-------------+--------------+
|         100 |          101 |
|         102 |          103 |
|         104 |          105 |
|         106 |          107 |
+-------------+--------------+
4 rows in set (0.00 sec)