group by 是在where 之后运行
在写单表查询语法的时候 应该把group by 写在 where 之后
执行顺序
1.先找到表 from 库.表名
2.按照where 约束条件 过滤你想要的记录
3.group by 进行分组
4.分完组以后 再进行相应的查询
分组查询:GROUP BY
一 什么是分组？为什么要分组？
分类一定要找大家都有一样的属性
#1、首先明确一点：分组发生在where之后，即分组是基于where之后得到的记录而进行的 #2、分组指的是：将所有记录按照某个相同字段进行归类，比如针对员工信息表的职位分组，或者按照性别进行分组等 #3、为何要分组呢？
取每个部门的最高工资
取每个部门的员工数
取男人数和女人数 小窍门：‘每’这个字后面的字段，就是我们分组的依据 #4、大前提：
可以按照任意字段分组，但是分组完毕后，比如group by post，只能查看post字段，如果想查看组内信息，需要借助于聚合函数
mysql> select * from employee;
+----+------------+--------+-----+------------+-----------+--------------+------------+--------+-----------+
| id | name | sex | age | hire_date | post | post_comment | salary | office | depart_id |
+----+------------+--------+-----+------------+-----------+--------------+------------+--------+-----------+
| 1 | alex | male | 78 | 2015-03-02 | teacher | NULL | 1000000.31 | 401 | 1 |
| 2 | yuanhao | male | 73 | 2014-07-01 | teacher | NULL | 3500.00 | 401 | 1 |
| 3 | liwenzhou | male | 28 | 2012-11-01 | teacher | NULL | 2100.00 | 401 | 1 |
| 4 | jingliyang | female | 18 | 2011-02-11 | teacher | NULL | 9000.00 | 401 | 1 |
| 5 | jinxin | male | 18 | 1900-03-01 | teacher | NULL | 30000.00 | 401 | 1 |
| 6 | 成龙 | male | 48 | 2010-11-11 | teacher | NULL | 10000.00 | 401 | 1 |
| 7 | 歪歪 | female | 48 | 2015-03-11 | sale | NULL | 3000.13 | 402 | 2 |
| 8 | 丫丫 | female | 38 | 2010-11-01 | sale | NULL | 2000.35 | 402 | 2 |
| 9 | 丁丁 | female | 18 | 2011-03-12 | sale | NULL | 1000.37 | 402 | 2 |
| 10 | 星星 | female | 18 | 2016-05-13 | sale | NULL | 3000.29 | 402 | 2 |
| 11 | 格格 | female | 28 | 2017-01-27 | sale | NULL | 4000.33 | 402 | 2 |
| 12 | 张野 | male | 28 | 2016-03-11 | operation | NULL | 10000.13 | 403 | 3 |
| 13 | 程咬金 | male | 18 | 1997-03-12 | operation | NULL | 20000.00 | 403 | 3 |
| 14 | 程咬银 | female | 18 | 2013-03-11 | operation | NULL | 19000.00 | 403 | 3 |
| 15 | 程咬铜 | male | 18 | 2015-04-11 | operation | NULL | 18000.00 | 403 | 3 |
| 16 | 程咬铁 | female | 18 | 2014-05-12 | operation | NULL | 17000.00 | 403 | 3 |
+----+------------+--------+-----+------------+-----------+--------------+------------+--------+-----------+
16 rows in set (0.00 sec)
这样写 分完组以后 只能取分组字段post
select post from employee group by post
#由于没有设置ONLY_FULL_GROUP_BY,于是也可以有结果，默认都是组内的第一条记录，但其实这是没有意义的mysql> select * from employee group by post;
+----+--------+--------+-----+------------+-----------+--------------+------------+--------+-----------+
| id | name | sex | age | hire_date | post | post_comment | salary | office | depart_id |
+----+--------+--------+-----+------------+-----------+--------------+------------+--------+-----------+
| 12 | 张野 | male | 28 | 2016-03-11 | operation | NULL | 10000.13 | 403 | 3 |
| 7 | 歪歪 | female | 48 | 2015-03-11 | sale | NULL | 3000.13 | 402 | 2 |
| 1 | alex | male | 78 | 2015-03-02 | teacher | NULL | 1000000.31 | 401 | 1 |
+----+--------+--------+-----+------------+-----------+--------------+------------+--------+-----------+
3 rows in set (0.06 sec)
ONLY_FULL_GROUP_BY
查看现在sql_mode 模式
mysql> select @@global.sql_mode;
+--------------------------------------------+
| @@global.sql_mode |
+--------------------------------------------+
| STRICT_TRANS_TABLES,NO_ENGINE_SUBSTITUTION |
+--------------------------------------------+
1 row in set (0.00 sec)
设置mysql严格模式
"ONLY_FULL_GROUP_BY" 只能取分组的字段 以及每个组聚合函数结果
mysql> set global sql_mode="ONLY_FULL_GROUP_BY,STRICT_TRANS_TABLES,NO_ENGINE_SUBSTITUTION ";
Query OK, 0 rows affected (0.00 sec) mysql> select @@global.sql_mode;
+---------------------------------------------------------------+
| @@global.sql_mode |
+---------------------------------------------------------------+
| ONLY_FULL_GROUP_BY,STRICT_TRANS_TABLES,NO_ENGINE_SUBSTITUTION |
+---------------------------------------------------------------+
1 row in set (0.00 sec)
退出一下mysql 再重新登mysql 查看
mysql> quit #设置成功后，一定要退出，然后重新登录方可生效
Bye
报错 已经是严格模式了 只能取分组的字段
mysql> select * from employee group by post;
ERROR 1055 (42000): 'company.employee.id' isn't in GROUP BY
mysql> select post from employee group by post;
+-----------+
| post |
+-----------+
| operation |
| sale |
| teacher |
+-----------+
3 rows in set (0.00 sec)
想统计每个部门的相关数据 用到聚合函数
把某一个组里面的一推人统计出结果 以组为单位统计
#查看MySQL 5.7默认的sql_mode如下：
mysql> select @@global.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 #！！！注意
ONLY_FULL_GROUP_BY的语义就是确定select target list中的所有列的值都是明确语义，
简单的说来，在ONLY_FULL_GROUP_BY模式下，target list中的值要么是来自于聚集函数的结果，要么是来自于group by list中的表达式的值。 #设置sql_mole如下操作(我们可以去掉ONLY_FULL_GROUP_BY模式)：
mysql> set global sql_mode='STRICT_TRANS_TABLES,NO_ZERO_IN_DATE,NO_ZERO_DATE,ERROR_FOR_DIVISION_BY_ZERO,NO_AUTO_CREATE_USER,NO_ENGINE_SUBSTITUTION'; ！！！SQL_MODE设置！！！