SQL ———————————————— in exits

select A.* from Person B where A.id in (select B.id from You B where   B.id = A.id )


select A.* from Person B where exits (select 1 from You B where   B.id = A.id )

1. in()适合B表比A表数据小的情况

2. exists()适合B表比A表数据大的情况

当A表数据与B表数据一样大时,in与exists效率差不多,可任选一个使用.