8、MySQL数据库-子查询

八、子查询

1.where 或 having后面

1.1 标量子查询（单行）

1.谁的工资比Abel高
SELECT *
FROM employees
WHERE salary>(
	SELECT salary
	FROM employees
	WHERE last_name='Abel'
);

2.返回job_id与141号员工相同，salary比143号员工多的员工姓名，
SELECT last_name,job_id,salary
FROM employees
WHERE job_id=(
	SELECT job_id
	FROM employees
	WHERE employee_id=141
)AND salary>(
	SELECT salary
	FROM employees
	WHERE employee_id=143
);

3.返回公司工资最少的员工的last_name,job_id和salary
SELECT last_name,job_id,salary
FROM employees
WHERE salary=(
	SELECT MIN(salary)
	FROM employees
);

4.查询最低工资大于50号部门最低工资的部门id和其最低工资
SELECT MIN(salary),department_id
FROM employees
GROUP BY department_id
HAVING MIN(salary)>(
	SELECT  MIN(salary)
	FROM employees
	WHERE department_id = 50
);

1.2列子查询（多行子查询）

1.返回location_id是1400或1700的部门中的所有员工姓名
SELECT last_name
FROM employees
WHERE department_id  in(
	SELECT DISTINCT department_id
	FROM departments
	WHERE location_id IN(1400,1700)
);

2.返回其它岗位中比job_id为‘IT_PROG’岗位任一工资低的员工的员工号、姓名、job_id 以及salary
（1）SELECT last_name,employee_id,job_id,salary
FROM employees
WHERE salary<ANY(
	SELECT DISTINCT salary
	FROM employees
	WHERE job_id = 'IT_PROG'
) AND job_id<>'IT_PROG';

（2）SELECT last_name,employee_id,job_id,salary
FROM employees
WHERE salary<(
	SELECT MAX(salary)
	FROM employees
	WHERE job_id = 'IT_PROG'
) AND job_id<>'IT_PROG';

3.返回其它岗位中比job_id为‘IT_PROG’岗位所有工资都低的员工的员工号、姓名、job_id 以及salary
（1）SELECT last_name,employee_id,job_id,salary
FROM employees
WHERE salary<ALL(
	SELECT DISTINCT salary
	FROM employees
	WHERE job_id = 'IT_PROG'

) AND job_id<>'IT_PROG';

（2）SELECT last_name,employee_id,job_id,salary
FROM employees
WHERE salary<(
	SELECT MIN( salary)
	FROM employees
	WHERE job_id = 'IT_PROG'

) AND job_id<>'IT_PROG';

1.3行子查询（多列多行）

1.查询员工编号最小并且工资最高的员工信息
SELECT *
FROM employees
WHERE employee_id=(
	SELECT MIN(employee_id)
	FROM employees
)AND salary=(
	SELECT MAX(salary)
	FROM employees
);
行子查询
SELECT * 
FROM employees
WHERE (employee_id,salary)=(
	SELECT MIN(employee_id),MAX(salary)
	FROM employees
);

2.select后面

1.查询每个部门的员工个数
SELECT d.*,(
	SELECT COUNT(*)
	FROM employees e
	WHERE e.department_id = d.`department_id`
 ) 个数
 FROM departments d;

2.查询员工号=102的部门名
SELECT (
	SELECT department_name
	FROM departments d
	INNER JOIN employees e
	ON d.department_id=e.department_id
	WHERE e.employee_id=102
	
) 部门名;

3.查询员工信息，并显示出员工所属的部门名称
#第一种做法，将员工表和部门表连接
select e.ename, d.dname from emp e, dept d where e.deptno=d.deptno;
#第二种做法，在select语句中再次嵌套select语句完成部分名称的查询
select e.ename, (select d.dname from dept d where e.deptno=d.deptno) as dname from emp e;

3、from后面

1.查询每个部门的平均工资的工资等级
SELECT  ag_dep.*,g.`grade_level`
FROM (
	SELECT AVG(salary) ag,department_id
	FROM employees
	GROUP BY department_id
) ag_dep
INNER JOIN job_grades g
ON ag_dep.ag BETWEEN lowest_sal AND highest_sal;

4.exists后面

1.查询有员工的部门名
（1）in
SELECT department_name
FROM departments d
WHERE d.`department_id` IN(
	SELECT department_id
	FROM employees
)
（2）exists
SELECT department_name
FROM departments d
WHERE EXISTS(
	SELECT *
	FROM employees e
	WHERE d.`department_id`=e.`department_id`
);

2.查询没有女朋友的男神信息
（1）in
SELECT department_name
FROM departments d
WHERE d.`department_id` IN(
	SELECT department_id
	FROM employees
)
（2）exists
SELECT department_name
FROM departments d
WHERE EXISTS(
	SELECT *
	FROM employees e
	WHERE d.`department_id`=e.`department_id`
);