编辑代码

create database test; 
use test; 
create table table1( 
  id int,
  pid int 
); 

create table table2( 
    id int 
);
insert into table1 values(1,1); 
insert into table1 values(null,2); 
insert into table2 values(1);
insert into table2 values(2);

select table1.pid 
from table1,table2 
where table1.id in(table2.id);