MySQL Select Statement DISTINCT for Multiple Columns?

To understand the MySQL select statement DISTINCT for multiple columns, let us see an example and create a table. The query to create a table is as follows

mysql> create table selectDistinctDemo
   -> (
   -> InstructorId int NOT NULL AUTO_INCREMENT PRIMARY KEY,
   -> StudentId int,
   -> TechnicalSubject varchar(100)
   -> );
Query OK, 0 rows affected (0.50 sec)

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

mysql> insert into selectDistinctDemo(StudentId,TechnicalSubject) values(121,'Java');
Query OK, 1 row affected (0.15 sec)
mysql> insert into selectDistinctDemo(StudentId,TechnicalSubject) values(121,'MongoDB');
Query OK, 1 row affected (0.16 sec)
mysql> insert into selectDistinctDemo(StudentId,TechnicalSubject) values(121,'MySQL');
Query OK, 1 row affected (0.15 sec)
mysql> insert into selectDistinctDemo(StudentId,TechnicalSubject) values(298,'Python');
Query OK, 1 row affected (0.11 sec)
mysql> insert into selectDistinctDemo(StudentId,TechnicalSubject) values(298,'SQL Server');
Query OK, 1 row affected (0.15 sec)
mysql> insert into selectDistinctDemo(StudentId,TechnicalSubject) values(397,'C#');
Query OK, 1 row affected (0.13 sec)

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

mysql> select *from selectDistinctDemo;

The following is the output

+--------------+-----------+------------------+
| InstructorId | StudentId | TechnicalSubject |
+--------------+-----------+------------------+
| 1            | 121       | Java             |
| 2            | 121       | MongoDB          |
| 3            | 121       | MySQL            |
| 4            | 298       | Python           |
| 5            | 298       | SQL Server       |
| 6            | 397       | C#               |
+--------------+-----------+------------------+
6 rows in set (0.00 sec)

Here is the query to use select statement DISTINCT for multiple columns

mysql> select InstructorId,StudentId,TechnicalSubject from selectDistinctDemo
-> where InstructorId IN
   -> (
   -> select max(InstructorId) from selectDistinctDemo
   -> group by StudentId
   -> )
-> order by InstructorId desc;

The following is the output

+--------------+-----------+------------------+
| InstructorId | StudentId | TechnicalSubject |
+--------------+-----------+------------------+
| 6            | 397       | C#               |
| 5            | 298       | SQL Server       |
| 3            | 121       | MySQL            |
+--------------+-----------+------------------+
3 rows in set (0.10 sec)