SQL语句总结4之子查询

将一个查询的结果,做为另外一个查询的参数,即是子查询

1.在子查询中使用聚合函数
在老师表中查询所有高于平均年龄的老师

SELECT * FROM teacher
WHERE age > 
(SELECT  AVG(age) from teacher)

2.IN子查询

SELECT * FROM student
WHERE CNO=10
AND SNO IN (SELECT SNO FROM STUDENT WHERE CNO = 11)

3.EXISTS子查询
基本语法如下:

SELECT column_name
FROM   table_name
WHERE  [NOT] EXISTS (subquery)

4.SOME/ALL子查询
待研究

5.UNIQUE子查询
待研究