我要评论第一步：数据准备
mysql> select * from my_student; +----+------------+-----------+--------+--------+ | id | number | name | sex | addr | +----+------------+-----------+--------+--------+ | 1 | itcast0001 | jim | female | 北京 | | 2 | itcast0002 | hanmeimei | female | 上海 | | 3 | itcast0003 | kate | female | null | | 4 | itcast0004 | tom | male | null | | 5 | itcast0005 | lintao | male | null | | 6 | itcast0006 | 张越 | 女 | null | +----+------------+-----------+--------+--------+ 6 rows in set (0.00 sec)
第二步：数据验证
1.distinct单字段去重
mysql> select distinct addr from my_student; +--------+ | addr | +--------+ | 北京 | | 上海 | | null | +--------+ 3 rows in set (0.00 sec)
2.count和distinct配合使用
1).count(distinct col) 计算该列除 null 之外的不重复行数。
mysql> select count(distinct addr) from my_student; ① +----------------------+ | count(distinct addr) | +----------------------+ | 2 | +----------------------+ 1 row in set (0.00 sec) -- 把上述sql改写成 select count(1) from (select distinct col from ...) a; mysql> select count(1) from (select distinct addr from my_student) a; ② +----------+ | count(1) | +----------+ | 3 | +----------+ 1 row in set (0.00 sec)
注意比较①和②的结果,是不同的.
2).count(distinct col1, col2) 如果其中一列全为 null，那么即使另一列有不同的值，也返回为 0。
mysql> select distinct sex,addr from my_student; +--------+--------+ | sex | addr | +--------+--------+ | female | 北京 | | female | 上海 | | female | null | | male | null | | 女 | null | +--------+--------+ 5 rows in set (0.00 sec) mysql> select count(distinct sex,addr) from my_student; ③ +--------------------------+ | count(distinct sex,addr) | +--------------------------+ | 2 | +--------------------------+ 1 row in set (0.00 sec) mysql> select count(1) from (select distinct sex,addr from my_student) a; ④ +----------+ | count(1) | +----------+ | 5 | +----------+ 1 row in set (0.00 sec)
注意比较③和④的结果,也是不同的.
3.上述同样的操作在oracle数据库验证
1).针对上述1)的场景在oracle数据库中验证后,count(distinct col)也是会计算该列除 null 之外的不重复行数,同mysql一样
2).针对上述2)的场景,count(distinct col1, col2)的写法在oracle数据库中不支持.
总结
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