分组查询时，select的字段是否一定要都在group by中

分组查询时，select的字段是否一定要都在group by中?

分组查询关键字group by通常和集合函数（MAX、MIN、COUNT、SUM、AVG）一起使用，它可以对一列或者多列结果集进行分组。例如要统计超市水果的种类，需要用count函数，要统计哪个水果价格最高，要用MAX（）函数。

一般情况下，我们在使用group by的时候，select中的列都要出现在group by中，比如select id,name,age from tuser group by id,name,age，那么我们是不是都要严格按照这种模式来写sql呢？下面我们来一起探索下。

数据准备

创建一张学生表

CREATE TABLE student1 (
  	id int(11) NOT NULL COMMENT '学号',
  	name varchar(60) NOT NULL COMMENT '姓名',
  	birth date NOT NULL COMMENT '出生日期',
  	sex varchar(1) DEFAULT NULL,
  	age int(11) NOT NULL,
  	score int(11) NOT NULL,
  	PRIMARY KEY (id)
);

插入数据

insert into student values(1,'Tom','1998-10-01','男',23,96),(2,'Jim','1997-07-04','男',24,95),(3,'Lily','1999-11-12','女',21,99),(4,'Lilei','1996-09-21','男',25,90),(5,'Lucy','1999-12-02','女',21,93),(6,'Jack','1988-04-27','男',32,89),(7,'Liam','1991-09-08',' 男',28,100);

测试验证

1. select中的列都出现在group by中，通过下面的结果可以看出是可以正常执行的。

mysql> select id,name,score from student where score >95  group by id,name,score;

+----+------+-------+
| id | name | score |
+----+------+-------+
|  1 | Tom  |    96 |
|  3 | Lily |    99 |
|  7 | Liam |   100 |
+----+------+-------+
3 rows in set (0.01 sec)

2. group by中只保留score或者name

mysql> select id,name,score from student where score >95  group by score;
ERROR 1055 (42000): Expression #1 of 
SELECT list is not in GROUP BY clause 
and contains nonaggregated column 
'test.student.id' which is not functionally 
dependent on columns in GROUP BY clause; 
this is incompatible with sql_mode=only_full_group_by
mysql> select id,name,score from student where score >95  group by name;
ERROR 1055 (42000): Expression #1 of 
SELECT list is not in GROUP BY clause 
and contains nonaggregated column 
'test.student.id' which is not functionally 
dependent on columnsin GROUP BY clause; 
this is incompatible with sql_mode=only_full_group_by

3. group by中只保留id

mysql> select id,name,score from student where score >95  group by id;
+----+------+-------+
| id | name | score |
+----+------+-------+
|  1 | Tom  |    96 |
|  3 | Lily |    99 |
|  7 | Liam |   100 |
+----+------+-------+
3 rows in set (0.00 sec)

通过这个实验可以看出group by中只保留id是可以正常执行的，为什么？id字段有什么特殊性呢？

通过表结构可以看出id字段是主键，查询官方文档，有针对主键列的解释。

SELECT name, address, MAX(age) FROM t GROUP BY name;

The query is valid if name is a primary key of t or is a unique NOT NULL column. In such cases,MySQL recognizes that the selected column is functionally dependent on a grouping column. Forexample, if name is a primary key, its value determines the value of address because each group has only one value of the primary key and thus only one row. As a result, there is no randomness in the choice of address value in a group and no need to reject the query.

The query is invalid if name is not a primary key of t or a unique NOT NULL column.

大致的意思是：如果name列是主键或者是唯一的非空列，name上面的查询是有效的。这种情况下，MySQL能够识别出select中的列依赖于group by中的列。比如说，如果name是主键，它的值就决定了address的值，因为每个组只有一个主键值，分组中的每一行都具有唯一性，因此也不需要拒绝这个查询。

4. 验证唯一非空索引

增加name字段的唯一性约束

alter table student add unique(name);mysql> select id,name,score from student where score >95  group by name;
+----+------+-------+
| id | name | score |
+----+------+-------+
|  7 | Liam |   100 |
|  3 | Lily |    99 |
|  1 | Tom  |    96 |
+----+------+-------+
3 rows in set (0.00 sec)

通过上面的例子也验证了，对于有唯一性约束的字段，也可以不用在group by中把select中的字段全部列出来。不过针对主键或者唯一性字段进行分组查询意义并不是很大，因为他们的每一行都是唯一的。

ONLY_FULL_GROUP_BY

我们在上面提到select中的列都出现在group by中，其实在MySQL5.7.5之前是没有此类限制的，5.7.5版本在sql_mode中增加了ONLY_FULL_GROUP_BY参数，用来开启或者关闭针对group by的限制。下面我们在分别开启和关闭ONLY_FULL_GROUP_BY限制的情况下分别进行验证。

查询 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_AUTO_CREATE_USER,
NO_ENGINE_SUBSTITUTION |
+-------------------------------------------------------------------------------------------------------------------------------------------++
1 row in set (0.00 sec)

sql_mode动态去除ONLY_FULL_GROUP_BY限制

mysql> SET @@sql_mode = sys.list_drop(@@sql_mode, 'ONLY_FULL_GROUP_BY');Query OK, 0 rows affected (0.05 sec)

再次执行分组查询

mysql> select id,name,score from student where score >95  group by score;+----+------+-------+| id | name | score |+----+------+-------+|  1 | Tom  |    96 ||  3 | Lily |    99 ||  7 | Liam |   100 |+----+------+-------+3 rows in set (0.00 sec)

sql_mode动态增加ONLY_FULL_GROUP_BY限制

SET @@sql_mode = sys.list_add(@@sql_mode, 'ONLY_FULL_GROUP_BY');
再次执行分组查询

mysql> select id,name,score from student where score >95  group by score;
ERROR 1055 (42000): Expression #1 of 
SELECT list is not in GROUP BY clause 
and contains nonaggregated column 
'test.student.id' which is not functionally 
dependent on columns in GROUP BY clause; 
this is incompatible with sql_mode=only_full_group_by。