MySQL Select语句DISTINCT对于多列？
要了解多列的MySQLselect语句DISTINCT，让我们看一个示例并创建一个表。创建表的查询如下
mysql> create table selectDistinctDemo -> ( -> InstructorId int NOT NULL AUTO_INCREMENT PRIMARY KEY, -> StudentId int, -> TechnicalSubject varchar(100) -> );
使用insert命令在表中插入一些记录。查询如下
mysql> insert into selectDistinctDemo(StudentId,TechnicalSubject) values(121,'Java'); mysql> insert into selectDistinctDemo(StudentId,TechnicalSubject) values(121,'MongoDB'); mysql> insert into selectDistinctDemo(StudentId,TechnicalSubject) values(121,'MySQL'); mysql> insert into selectDistinctDemo(StudentId,TechnicalSubject) values(298,'Python'); mysql> insert into selectDistinctDemo(StudentId,TechnicalSubject) values(298,'SQL Server'); mysql> insert into selectDistinctDemo(StudentId,TechnicalSubject) values(397,'C#');
使用select语句显示表中的所有记录。查询如下
mysql> select *from selectDistinctDemo;
以下是输出
+--------------+-----------+------------------+ | 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)
这是对多列使用选择语句DISTINCT的查询
mysql> select InstructorId,StudentId,TechnicalSubject from selectDistinctDemo -> where InstructorId IN -> ( -> select max(InstructorId) from selectDistinctDemo -> group by StudentId -> ) -> order by InstructorId desc;
以下是输出
+--------------+-----------+------------------+ | InstructorId | StudentId | TechnicalSubject | +--------------+-----------+------------------+ | 6 | 397 | C# | | 5 | 298 | SQL Server | | 3 | 121 | MySQL | +--------------+-----------+------------------+ 3 rows in set (0.10 sec)