MySQL 用DISTINCT,GROUP BY 去重的用法和区别
DISTINCT,GROUP BY 都有去重的效果，他们区别在哪呢？
建一个表demo_list
| id | name |
| 1 | alan |
| 2 | betty |
| 3 | cat |
| 4 | dade |
| 5 | alan |
| 6 | betty |
现在查出id，name去重
GROUP BY
1 SELECT id
2 FROM demo_list
3 GROUP BY name;
结果：
| id |
| 1 |
| 2 |
| 3 |
| 4 |
DISTINCT
1 SELECT id,COUNT(DISTINCT name)
2 FROM demo_list
3 GROUP BY name;
结果：
| id |
COUNT(DISTINCT name)
|
| 1 | 1 |
| 2 | 1 |
| 3 | 1 |
| 4 | 1 |
两种方法的区别：
group by是按指定的列分组查询的，是一种聚合查询，很多时候是为了做统计用，
distinct 是查询出来以后再把重复的去掉
性能上 group 比 distinct 要好很多
