1、单行子查询
        select ename,deptno,sal
        from emp
        where deptno=(select deptno from dept where loc='NEW YORK')；

     2、多行子查询
        SELECT ename,job,sal
        FROM EMP
        WHERE deptno in ( SELECT deptno FROM dept WHERE dname LIKE 'A%')；

     3、多列子查询
        SELECT deptno,ename,job,sal
        FROM EMP
        WHERE (deptno,sal) IN (SELECT deptno,MAX(sal) FROM EMP GROUP BY deptno)；

     4、内联视图子查询
       (1)SELECT ename,job,sal,rownum
          FROM (SELECT ename,job,sal FROM EMP ORDER BY sal)；
       (2)SELECT ename,job,sal,rownum
          FROM ( SELECT ename,job,sal FROM EMP ORDER BY sal)
          WHERE rownum<=5；
 
     5、在HAVING子句中使用子查询
        SELECT deptno,job,AVG(sal) FROM EMP GROUP BY deptno,job HAVING AVG(sal)>(SELECT sal FROM EMP WHERE ename='MARTIN')；