mysql 数据操作单表查询 having 过滤

SELECT 字段1,字段2... FROM 库名.表名
                  WHERE 条件
                  GROUP BY field
                  HAVING 筛选
                  ORDER BY field
                  LIMIT 限制条数

1.首先找到表库.表名

2. 经过where 过滤条件找到符合条件记录

3.按照分组归类只剩下组这个单位

4.剩下只能取组名和聚合函数，having过滤只有分组和聚合函数

HAVING与WHERE不一样的地方在于

执行优先级从高到低：where > group by > having 
#1. Where 发生在分组group by之前，因而Where中可以有任意字段，但是where绝对不能使用聚合函数。

#2. Having发生在分组group by之后，因而Having中可以使用分组的字段，无法直接取到其他字段,可以使用聚合函数

having 过滤是在分组之后进行

having 一定配合聚合函数使用

例如：

报错

分组后无法取到其他字段

因为 id_info 是别名，是运行在having之后的 distinct 字段在执行 having id_info id_info就不存在， id_info在后面执行所以这里报错

mysql> select post,count(id) as id_info from employee group by post having id_info >5;
ERROR 1463 (42000): Non-grouping field 'id_info' is used in HAVING clause

mysql> select * from employee having id > 5  ;
ERROR 1463 (42000): Non-grouping field 'id' is used in HAVING clause

正确使用

mysql> select post,count(id)  from employee group by post ;
+-----------+-----------+
| post      | count(id) |
+-----------+-----------+
| operation |         5 |
| sale      |         5 |
| teacher   |         6 |
+-----------+-----------+
3 rows in set (0.00 sec)

mysql> select post,count(id)  from employee group by post having count(id) >5 ;
+---------+-----------+
| post    | count(id) |
+---------+-----------+
| teacher |         6 |
+---------+-----------+
1 row in set (0.00 sec)

取职位名以及每个职位里的员工数并且把每个职位里员工数大于5 的职位取出来

mysql> select post,group_concat(id)  from employee group by post ;
+-----------+------------------+
| post      | group_concat(id) |
+-----------+------------------+
| operation | 16,15,14,13,12   |
| sale      | 11,10,9,8,7      |
| teacher   | 6,5,4,3,2,1      |
+-----------+------------------+
3 rows in set (0.00 sec)


mysql> select post,group_concat(id)  from employee group by post having count(id) >5 ;
+---------+------------------+
| post    | group_concat(id) |
+---------+------------------+
| teacher | 6,5,4,3,2,1      |
+---------+------------------+
1 row in set (0.00 sec)