sql查询 oracle数据库
SQL> select * from usertable;
USERID USERNAME
----------- ----------------
1 user1
2 user2
3 user3
4 user4
5 user5
SQL> select * from usergrade_2;
USERNAME GRADE
---------------- ----------
user9 90
user8 80
user7 80
user2 90
user1 100
user1 80
执行 Select count(*) from usertable t1 where exists
(select 'x' from usergrade_2 t2 where t1.username=t2.username and rownum <=1);
给出你们的答案和解释(关键是你们怎么样理解) 谢谢!!!