Mysql distinct语法详解

Mysql distinct语法详解

0.背景

• 首先查看测试表的数据

mysql> select * from customers;
+-------------+----------+
| customer_id | city     |
+-------------+----------+
| 163         | hangzhou |
| 9you        | shanghai |
| alibaba     | hangzhou |
| netease     | hangzhou |
| 163         | beijing  |
| alibaba     | shanghai |
| 163         | beijing  |
+-------------+----------+
7 rows in set (0.00 sec)

1.distinct 语法

• ​​distinct a,b​​​ <=> ​​distinct (a,b)​​ 没问题

mysql> select
    -> distinct customer_id
    -> ,city
    -> from customers;
+-------------+----------+
| customer_id | city     |
+-------------+----------+
| 163         | hangzhou |
| 9you        | shanghai |
| alibaba     | hangzhou |
| netease     | hangzhou |
| 163         | beijing  |
| alibaba     | shanghai |
+-------------+----------+
6 rows in set (0.00 sec)

• 但是distinct(customer_id,city)会报错

mysql> select
    -> distinct (customer_id,city)
    -> from customers;
ERROR 1241 (21000): Operand should contain 1 column(s)

• ​​select a, distinct b,​​也会报错

mysql> select
    -> city
    -> ,distinct customer_id
    -> from customers;
ERROR 1064 (42000): You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the right syntax to use near 'distinct customer_id
from customers' at line 3

• ​​distinct a​​没问题

mysql> select
    -> distinct customer_id
    -> from customers;
+-------------+
| customer_id |
+-------------+
| 163         |
| 9you        |
| alibaba     |
| netease     |
+-------------+
4 rows in set (0.00 sec)