子查问

1.概念：

查问中嵌套查问，称为子查问。

-- 查问工资最高的员工信息-- 1.查问最高员工的工资是多少 SELECT MAX(salary) FROM emp;-- 2.查问员工信息，并且工资等于9000的SELECT * FROM emp WHERE emp.`salary`= 9000;-- 一条sql就实现这个操作SELECT * FROM emp WHERE emp.`salary`= (SELECT MAX(salary) FROM emp);a+b=5c+a+bc+ (a+b)

2.子查问的不同状况

1.子查问的后果是单行单列的

-- 查问员工工资小于平均工资的人SELECT  * FROM emp WHERE emp.`salary` < (SELECT AVG(salary) FROM emp);

2.多行单列

-- 查问‘财务’和'市场'部的所有的员工信息SELECT id FROM dept WHERE NAME='财务部' OR NAME = '市场部';SELECT * FROM emp WHERE dept_id=3 OR dept_id=2;-- 子查问SELECT * FROM emp WHERE dept_id IN (SELECT id FROM dept WHERE NAME='财务部' OR NAME = '市场部');

3.多行多列

-- 查问员工入职日期是2011-11-11日之后的员工信息和部门信息-- 子查问SELECT * FROM dept t1,(SELECT * FROM emp WHERE emp.`join_date` > '2011-11-11') t2 WHERE t1.`id` = t2.dept_id;SELECT * FROM emp WHERE emp.`join_date` > '2011-11-11';-- 一般的内连贯SELECT * FROM emp t1,dept t2 WHERE t1.`dept_id` = t2.`id` AND t1.`join_date` > '2011-11-11';