这是官方手册的一句话：
In MySQL, you can obtain the number of distinct expression combinations that do not contain NULL by giving a list of expressions. In standard SQL, you would have to do a concatenation of all expressions inside COUNT(DISTINCT …).
能解释一下吗？
In MySQL, you can obtain the number of distinct expression combinations that do not contain NULL by giving a list of expressions. In standard SQL, you would have to do a concatenation of all expressions inside COUNT(DISTINCT …).
能解释一下吗？
解决方案
20
就是说在mysql中，可以这么写，里面可以有多个字段count(distinct 多个字段)：
mysql> select count(distinct id,v) from tb_m; +--+ | count(distinct id,v) | +--+ | 1 | +--+ 1 row in set (0.04 sec)
这个是mysql特有的语法，而一般其他的数据库，例如oracle，sql server，只能这么写一个字段count（distinct 一个字段）：
mysql> select count(distinct id) from tb_m; +--+ | count(distinct id) | +--+ | 1 | +--+ 1 row in set (0.12 sec)
20
select count(*) from (select distinct id from tb_m) t
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