Question

sql count 两个表

A，B；两个表，请问如何实现用一条sql语句，count数据

$sql ="select COUNT(*) as n from A , B";

Answer 1

楼上麻烦。且效率极低。

select count(表1主键)+(select count(表2主键) from 表2) from 表1

Answer 2

select count(user_id) from (select a.user_id user_id from table_a a union all select b.user_id user_id from table_b);