MySQL GROUP BY 的问题

拿

employee

mysql> SELECT * FROM   employees GROUP  BY gender;
ERROR 1055 (42000): Expression #1 of SELECT list is not in GROUP BY clause and contains nonaggregated column 'employees.employees.emp_no' which is not functionally dependent on columns in GROUP BY clause; this is incompatible with sql_mode=only_full_group_by

其中

employee

mysql> DESCRIBE employees;
+------------+---------------+------+-----+---------+-------+
| Field      | Type          | Null | Key | Default | Extra |
+------------+---------------+------+-----+---------+-------+
| emp_no     | int(11)       | NO   | PRI | NULL    |       |
| birth_date | date          | NO   |     | NULL    |       |
| first_name | varchar(14)   | NO   |     | NULL    |       |
| last_name  | varchar(16)   | NO   |     | NULL    |       |
| gender     | enum('M','F') | NO   |     | NULL    |       |
| hire_date  | date          | NO   |     | NULL    |       |
+------------+---------------+------+-----+---------+-------+
6 rows in set (0.01 sec)

原因

SQL 标准中不允许 SELECT 列表，HAVING 条件语句，或 ORDER BY 语句中出现 GROUP BY 中未列表的可聚合列。而 MySQL 中有一个状态 ONLY_FULL_GROUP_BY 来标识是否遵从这一标准，默认为开启状态。

所以这样的语句是不可以的，

# 🚨
SELECT gender,
last_name
FROM   employees
GROUP  BY gender

将

last_name

# ✅
SELECT gender,
FROM   employees
GROUP  BY gender

# ✅
SELECT gender,
last_name
FROM   employees
GROUP  BY gender,
last_name

但这样的修改查询出来就可能就不是想要的结果了。

解决

三种方式来解决。

关闭 ONLY_FULL_GROUP_BY

可以选择关掉 MySQL 的 ONLY_FULL_GROUP_BY 模式。

有两种方式，通过昨晚设置

sql_mode

首先查看变更前的

sql_mode

mysql> SELECT @@sql_mode;
+-----------------------------------------------------------------------------------------------------------------------+
| @@sql_mode                                                                                                            |
+-----------------------------------------------------------------------------------------------------------------------+
| ONLY_FULL_GROUP_BY,STRICT_TRANS_TABLES,NO_ZERO_IN_DATE,NO_ZERO_DATE,ERROR_FOR_DIVISION_BY_ZERO,NO_ENGINE_SUBSTITUTION |
+-----------------------------------------------------------------------------------------------------------------------+
1 row in set (0.00 sec)

通过以下脚本关闭 ：

SET SESSION sql_mode=(SELECT REPLACE(@@sql_mode,'ONLY_FULL_GROUP_BY,',''));

再次查询

@@sql_mode

mysql> SELECT @@sql_mode;
+----------------------------------------------------------------------------------------------------+
| @@sql_mode                                                                                         |
+----------------------------------------------------------------------------------------------------+
| STRICT_TRANS_TABLES,NO_ZERO_IN_DATE,NO_ZERO_DATE,ERROR_FOR_DIVISION_BY_ZERO,NO_ENGINE_SUBSTITUTION |
+----------------------------------------------------------------------------------------------------+
1 row in set (0.00 sec)

第二种是找到 MySQL 配置文件修改并保存。

MySQL 的配置文件名为

my.cnf

$ mysql --help | grep cnf
order of preference, my.cnf, $MYSQL_TCP_PORT,
/etc/my.cnf /etc/mysql/my.cnf /usr/local/etc/my.cnf ~/.my.cnf

找到后编辑并保存，重启 MySQL 后生效。

[mysqld]
-sql_mode=ONLY_FULL_GROUP_BY,STRICT_TRANS_TABLES,NO_ZERO_IN_DATE,NO_ZERO_DATE,ERROR_FOR_DIVISION_BY_ZERO,NO_ENGINE_SUBSTITUTION
+sql_mode=STRICT_TRANS_TABLES,NO_ZERO_IN_DATE,NO_ZERO_DATE,ERROR_FOR_DIVISION_BY_ZERO,NO_ENGINE_SUBSTITUTION

如果文件中没有

sql_mode

因为

ONLY_FULL_GROUP_BY

ANY_VALUE()

还可以通过 ANY_VALUE() 来改造查询语句以避免报错。

使用

ANY_VALUE()

SELECT gender,
-       last_name
+       ANY_VALUE(last_name)
FROM   employees
GROUP  BY gender

添加列间的依赖

像这个示例中，

# 🚨
SELECT gender,
last_name
FROM   employees
GROUP  BY gender

假如我们让

gender

last_name

gender

gender

last_name

GROUP BY

总结

一般 GROUP BY 会与另外的聚合函数配合使用，比如 COUNT(), SUM() 等。查询所有列无差别地进行 GROUP BY 的情况并不是正常的使用姿势。

相关资源

• 12.20.3 MySQL Handling of GROUP BY
• ONLY_FULL_GROUP_BY
• 12.21 Miscellaneous Functions
• 12.20.1 Aggregate (GROUP BY) Function Descriptions