SQL>select * from usertable; USERID、USERNAME ______ 1 user1 2 null 3 user3 4 null 5 user5 6 user6 SQL>select*from usergrade; USERID、USERNAME、GRADE ______ 1 user190 2 null 80 7 user780 8 user890 执行语句: (1)select count(*)from usergrade where username not in(select username from usertable); (2)select count(*)from usergrade g where not exists(select null from usertable t where t. userid=g.userid and t.username=g.usemame);