EXISTS和IN

SQL> select * from student;

       SNO SNAME          SCLASS
---------- ---------- ----------
      1001 Tom                 1
      1002 Lucy                3
      1003 Lily
      1004 Jerry               3

SQL> select * from class;

       CNO CNAME
---------- ----------
         1 计算机1班
         2 化学1班
         3 物理1班

SQL> select sname,sno from student where sclass in (select cno from class);

SNAME             SNO
---------- ----------
Tom              1001
Lucy             1002
Jerry            1004

SQL> select sname,sno from student where exists (select * from class where cno = sclass);

SNAME             SNO
---------- ----------
Tom              1001
Lucy             1002
Jerry            1004

当数据量大的时候，使用EXISTS比IN高