MySQL必知必会 -- 分组数据

聚集函数可用来汇总数据，如果我们要对这些数据进行分组处理的话，就需要我们进行数据的分组。

数据分组group by

MariaDB [test]> select * from linux ;
+-------+--------+------+------+-------+------------+
| user  | passwd | sex  | age  | price | math_price |
+-------+--------+------+------+-------+------------+
| user2 | 222    | girl | 23   | 155   | 36         |
| user3 | 333    | boy  | 12   | 78    | 66         |
| user4 | 444    | boy  | 22   | NULL  | NULL       |
| user5 | 555    | boy  | 30   | NULL  | 43         |
| a     | passa  | boy  | 22   | NULL  | NULL       |
| 1     | pass1  | girl | 30   | NULL  | NULL       |
| user1 | 111    | boy  | -18  | 35    | 88         |
| user6 | 666    | girl | 16   | 35    | 43         |
+-------+--------+------+------+-------+------------+
8 rows in set (0.00 sec)

MariaDB [test]> select count(*) from linux;
+----------+
| count(*) |
+----------+
|        8 |
+----------+
1 row in set (0.00 sec)

MariaDB [test]> select count(*),sex from linux group by sex;
+----------+------+
| count(*) | sex  |
+----------+------+
|        5 | boy  |
|        3 | girl |
+----------+------+
2 rows in set (0.00 sec)

这样就统计出了总人数中男女分别的数量

MariaDB [test]> select count(*),sex from linux group by sex with rollup;
+----------+------+
| count(*) | sex  |
+----------+------+
|        5 | boy  |
|        3 | girl |
|        8 | NULL |
+----------+------+
3 rows in set (0.00 sec)

使用WITH ROLLUP关键字，可以得到每个分组以及每个分组汇总级别（针对每个分组）的值。

过滤分组HAVING子句。

WHERE子句并没有分组的概念
学过的所有类型的WHERE子句都可以用HAVING来替代。唯一的差别是WHERE过滤行，而HAVING过滤分组。

MariaDB [test]> select count(*),sex from linux group by sex where conut(*) > 3;
ERROR 1064 (42000): You have an error in your SQL syntax; check the manual that corresponds to your MariaDB server version for the right syntax to use near 'where conut(*) > 3' at line 1

MariaDB [test]> select count(*),sex from linux group by sex having count(*) > 3;
+----------+-----+
| count(*) | sex |
+----------+-----+
|        5 | boy |
+----------+-----+
1 row in set (0.04 sec)

MariaDB [test]> select count(*),sex from linux group by sex having count(*) > 2;
+----------+------+
| count(*) | sex  |
+----------+------+
|        5 | boy  |
|        3 | girl |
+----------+------+
2 rows in set (0.00 sec)

我们可以看到用where进行过滤的时候会报错，但是我们用having的时候则可以过滤，就说明having是用于过滤分组的。

HAVING和WHERE的差别:
WHERE在数据分组前进行过滤，HAVING在数据分组后进行过滤。这是一个重
要的区别，WHERE排除的行不包括在分组中。这可能会改变计算值，从而影响HAVING子句中基于这些值过滤掉的分组。

MariaDB [test]> select count(*),sex from linux where age> 22 group by sex ;
+----------+------+
| count(*) | sex  |
+----------+------+
|        1 | boy  |
|        2 | girl |		/WHERE排除的行不包括在分组中
+----------+------+
2 rows in set (0.00 sec)

MariaDB [test]> select count(*),sex from linux where age> 22 group by sex having count(*)>2;
Empty set (0.00 sec)		

MariaDB [test]> select count(*),sex from linux where age> 22 group by sex having count(*)>1;
+----------+------+
| count(*) | sex  |
+----------+------+
|        2 | girl |
+----------+------+
1 row in set (0.00 sec)

分组和排序

一般在使用GROUP BY子句时，应该也给出ORDER BY子句。这是保证数据正确排序的唯一方法。

MariaDB [test]> select sum(age),sex from linux group by sex having sum(age) >= 68; 
+----------+------+
| sum(age) | sex  |
+----------+------+
|      104 | boy  |
|       69 | girl |
+----------+------+
2 rows in set (0.00 sec)

MariaDB [test]> select sum(age),sex from linux group by sex having sum(age) >= 68 order by sum(age); 
+----------+------+
| sum(age) | sex  |
+----------+------+
|       69 | girl |
|      104 | boy  |
+----------+------+
2 rows in set (0.00 sec)

SELECT子句使用顺序

select _ from _ where _ group by _ habing _ order by _ limit