MySQL 分组
MySQL About 825 words分组 GROUP BY
- 查询男女生总数
select gender as 性别 ,count(*) from user group by gender;
+------+----------+
| 性别 | count(*) |
+------+----------+
| NULL | 1 |
| | 2 |
| | 8 |
+------+----------+
3 rows in set (0.02 sec)
- 查询各国家人数 HAVING
select home as 国家,count(*) from user group by home;
+------+----------+
| 国家 | count(*) |
+------+----------+
| NULL | 1 |
| 吴 | 4 |
| 蜀 | 3 |
| 魏 | 3 |
+------+----------+
4 rows in set (0.02 sec)
- 分组后的数据筛选
select home,count(*) from user group by home having home='蜀';
select home, count(*) from user where home = '蜀';
+------+----------+
| home | count(*) |
+------+----------+
| 蜀 | 3 |
+------+----------+
1 row in set (0.00 sec)
对比 where 与 having
where是对from后面指定的表进行数据筛选,属于对原始数据的筛选
having是对group by的结果进行筛选
Views: 2,227 · Posted: 2019-04-07
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