子查询：
一、概念：
查询中嵌套查询，成嵌套查询为子查询
二、子查询不同情况
1、子查询的结果是单行单列的：
* 子查询可以作为条件，使用运算符去判断。运算法有：>、>=、<= 、=
举例：
#列出比平均工资低的所以员工信息
SELECT * FROM employees WHERE salary <(SELECT AVG(salary) FROM employees);2、子查询的结果是多行单列的：
* 子查询可以作为条件，使用运算符in来判断
举例：
#查询部分名为Adm和Sal的所有员工信息
SELECT *
FROM
employees em
WHERE
em.department_id
IN
(SELECT
dep.department_id
FROM
departments dep
WHERE
dep.department_name = "Adm" OR dep.department_name = "Sal"
);3、子查询的结果是多行多列的：
*子查询可以作为一张虚拟表参数查询
#采用子查询方式
SELECT
new_em.first_name,dep.`department_name`,new_em.hiredate
FROM
departments dep,
(SELECT * FROM employees em WHERE em.`hiredate` > "2014-03-01") new_em
WHERE
dep.`department_id` = new_em.department_id;
#采用普通内连接方式
SELECT
emp.`first_name`,dep.`department_name`,emp.`hiredate`
FROM
departments dep, employees emp
WHERE
dep.`department_id` = emp.`department_id` AND emp.`hiredate` > "2014-03-01";其它说明：
1、in/not in(可选择值列表) ---等于列表中的任意一个
比如： a in(10,20,30); 可以替换为a = 10 or a = 20 or a=30;
2、any/some ---和子查询返回结果的某个值满足比较关系就返回true
比如：a > any(10,20,30), 可以替换为 a > min(10,20,30);
3、all --- 和子查询中的每一个值都进行比较都满足比较关系时返回true
比如：a > all(10,20,30); 可替换为a > max(10,20,30);
#列出location_id为1400和1700的部门中所有员工的姓名
SELECT last_name
FROM
`employees` e
WHERE
e.department_id IN(
SELECT department_id
FROM
departments
WHERE
location_id IN(1400,1700)
);
#查询其它工种中比job_id为`IT_PROG`的员工中某一员工工资低的员工工号、姓名、和工资
SELECT e.job_id, e.last_name, e.salary 员工工资
FROM
employees e
WHERE
e.salary < ANY(
SELECT salary
FROM
employees
WHERE
job_id = 'IT_PROG'
)
AND
e.job_id != 'IT_PROG' #e.job_id <> 'IT_PROG'
ORDER BY 员工工资 DESC;
#查询其它工种中比job_id为`IT_PROG`的员工中所有员工工资都低的员工工号、姓名、和工资
SELECT e.job_id, e.last_name, e.salary as em_salarys
FROM
employees e
WHERE
e.salary < ALL(
SELECT salary
FROM
employees
WHERE
job_id = 'IT_PROG'
)
AND
e.job_id != 'IT_PROG' /*e.job_id <> 'IT_PROG'*/
ORDER BY em_salarys DESC;#查询每个部的部门信息和部门对应员工个数---使用非子查询方法
SELECT d.*,COUNT(employee_id) 部门人数
FROM
departments d
LEFT JOIN
employees e
ON
d.`department_id` = e.`department_id`
GROUP BY d.`department_id`
ORDER BY 部门人数 DESC;
#查询每个部的部门信息和部门对应员工个数---使用selec后跟子查询
#执行过程：先列出部门信息，然后从列出部门信息第一条到最后一条逐条和子查询进行操作---唯一一个子查询后于主查询进行
SELECT d.* ,(SELECT COUNT(*) FROM `employees` e WHERE d.`department_id` = e.department_id)
FROM
departments d;#查询员工号等于102的部门
SELECT d.department_name
FROM
departments d
WHERE
d.`department_id` = (SELECT e.`department_id` FROM employees e WHERE e.`employee_id` = 102);#查询每个部门的平均工资等级
SELECT j.`grade_level` FROM `jop_grades` j
SELECT AVG(salary) FROM `employees` e GROUP BY e.department_id
SELECT g.`grade_level`,avg_inf.avg_sal
FROM
(SELECT AVG(salary) avg_sal FROM employees GROUP BY department_id) avg_inf #这里子语句相当虚拟出一个新的表单
INNER JOIN
jop_grades g
ON
avg_inf.avg_sal BETWEEN g.lowest_sal AND g.highest_sal#列出各个部门中工资高于本部门平均工资的员工数和部门号
#1、列出各部门员工数和部门号 ---虚拟表一
SELECT e1.department_id, COUNT(*),salary
FROM
employees AS e1
GROUP BY
e1.department_id
#2、列出各个部门平均工资---虚拟表二，为了和表一能够形成关系，表二列也列出department_id
SELECT AVG(salary) avg_sal, department_id
FROM
employees AS e2
GROUP BY
e2.department_id;#合并
SELECT e1.department_id, COUNT(*)
FROM
employees AS e1
INNER JOIN
(SELECT department_id, AVG(salary) avg_sal FROM employees AS e2 GROUP BY e2.department_id) b3
ON
e1.`department_id` = b3.department_id
AND e1.`salary` > b3.avg_sal
GROUP BY
e1.`department_id`三、exists后面（相关子查询）
作用是判断子查询有没有结果的存在。
SELECT EXISTS (SELECT department_id FROM employees);
返回结果是：1
SELECT EXISTS (SELECT department_id FROM employees WHERE department_id = 9999);返回结果是：0
一般exists很少使用，因为都可以使用IN去替代
#查询员工人数不为0的部门名 ---使用exists
SELECT d.department_name
FROM
departments AS d
WHERE EXISTS(SELECT COUNT(*) FROM employees e WHERE d.`department_id` = e.`department_id` )#查询员工人数不为0的部门名 ---使用IN
SELECT d.department_name
FROM
departments AS d
WHERE
d.`department_id`
IN (SELECT department_id FROM employees AS e WHERE e.`department_id` = d.`department_id`);