1、count distinct 优化
--优化前 select count(distinct id )from tablename --优化后 select count(1) from (select distinct id from tablename)tmp; select count(1) from (select id from tablename group by id)tmp; --注意,如果id 存在空值,那么优化后的两条sql必须写为count(id),否者count会多一条。count(1)和cunt(字段)的区别。