MYSQL 从两列中选择不同的值?
mysqlmysqli database
要从两列中选择不同的值，您可以使用 MySQL 中的 least() 和 greatest() 函数。
让我们创建一个包含两列的表 −
mysql> create table SelectDistinctTwoColumns −> ( −> StudentId int, −> EmployeeId int −> ); Query OK, 0 rows affected (0.60 sec)
现在您可以在表中插入记录。插入记录的查询如下 −
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)
使用 select 语句显示表中的所有记录。查询如下 −
mysql> select *from SelectDistinctTwoColumns;
以下是输出 −
+-----------+------------+ | 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)
查看上面的输出。可以看到两列中都有一些重复的值。以下是从列中选择不同值的查询 −
mysql> select distinct least(StudentId, EmployeeId) as FirstColumn, −> greatest(StudentId, EmployeeId) as SecondColumn from SelectDistinctTwoColumns;
以下是输出 −
+-------------+--------------+ | FirstColumn | SecondColumn | +-------------+--------------+ | 100 | 101 | | 102 | 103 | | 104 | 105 | | 106 | 107 | +-------------+--------------+ 4 rows in set (0.00 sec)
