目录
子查询
#进阶八、子查询
/*
出现在其他语句中的select语句，那么select语句称为子查询/内查询，外面的其他语句称为主查询/外查询。
子查询不一定只出现在select语句的内部，在以后的学习中，子查询有可能出现在insert、update、delete语句中。
分类：
按子查询出现的位置进行分类：
1、select后面(用的最少)
要求：子查询的结果为单行单列（标量子查询）
2、from后面
要求：子查询的结果可以为多行多列（表子查询）
3、where、having后面(用的最多)
要求：子查询的结果必须为单列（列子查询、标量子查询）
4、exists后面
要求：（相关子查询）
特点：
1、当子查询在where、having的后面时，要求必须放在条件的右侧；
2、子查询一般放在小括号中；
3、子查询的执行优先于主查询
4、标量子查询对应单行操作符，>、<、>=、<=、=、<>
5、列子查询对应多行操作符，any/some、in、all
*/
#举例
SELECT first_name
FROM employees
WHERE department_id IN (
SELECT department_id
FROM departments
WHERE location_id = 1700
);
-- 一、where、having后面
#1、标量子查询
#谁的工资比Abel高
SELECT e.*
FROM employees e
WHERE salary > (
SELECT salary
FROM employees
WHERE last_name = 'Abel'
);
#返回job_id为141号员工相同，salary比143号员工多的员工姓名、工资、job_id
SELECT e.`last_name`, e.`salary`, e.`job_id`
FROM employees e
WHERE e.`job_id` = (SELECT job_id FROM employees WHERE employee_id = 141) AND
e.`salary` > (SELECT salary FROM employees WHERE employee_id = 143);
#返回工资最少的员工信息last_name、job_id和salary
SELECT last_name, job_id, salary
FROM employees
WHERE salary = (
SELECT MIN(salary) FROM employees
);
#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 department_id 部门编号, MIN(salary) 最低工资
FROM employees
GROUP BY department_id
HAVING 最低工资 > (
SELECT MIN(salary) FROM employees
WHERE department_id = 50
)
ORDER BY 最低工资 ASC;
#2、列子查询（单列多行）
/*
列子查询需要搭配多行操作符来执行
IN / NOT IN: 等于 / 不等于列表中的任意一个
ANY / SOME : 和子查询返回的某一个值进行比较
ALL：和子查询返回的所有值进行比较
*/
#返回location_id是1400或者1700的部门中的所有员工姓名
SELECT last_name 姓名
FROM employees
WHERE department_id IN (
SELECT department_id
FROM departments
WHERE location_id IN (1400, 1700)
);
#返回其他工种中比job_id为'IT_PROG'工种任意员工工资低的员工的：员工号、姓名、job_id、以及salary
SELECT employee_id, last_name, job_id, salary
FROM employees
WHERE salary < ANY (
SELECT DISTINCT salary
FROM employees
WHERE job_id = 'IT_PROG'
) AND job_id <> 'IT_PROG';
#返回其他工种中比job_id为'IT_PROG'工种所有员工工资低的员工：员工号、姓名、job_id以及salary
SELECT employee_id, last_name, job_id, salary
FROM employees
WHERE salary < ALL(
SELECT salary
FROM employees
WHERE job_id = 'IT_PROG'
) AND job_id <> 'IT_PROG';
-- 二、select后面
#查询每个部门的员工个数
SELECT d.`department_id`, d.`department_name`, (
SELECT COUNT(*)
FROM employees e
WHERE e.`department_id` = d.`department_id`
) 个数
FROM departments d;
#查询员工号=102的部门名
SELECT employee_id, (
SELECT department_name
FROM departments d
WHERE d.department_id = e.`department_id`
) 部门名
FROM employees e
WHERE e.`employee_id` = 102;
-- 三、from后面
#将子查询的结果集当做一张表
#查询每个部门的平均工资等级
SELECT ag_dep.*, j.grade_level
FROM (
SELECT AVG(salary) 平均工资, department_id FROM employees GROUP BY department_id
) ag_dep INNER JOIN job_grades j
ON 平均工资 BETWEEN j.`lowest_sal` AND j.`highest_sal`;
-- 四、exists后面（相关子查询）
#指子查询有没有结果,有没有值，返回1/0.
SELECT EXISTS(SELECT employee_id FROM employees);
#1
SELECT EXISTS(SELECT salary FROM employees WHERE salary > 30000);
#0
/*
语法：
exists(子查询)
最终结果只有两种情况：1 / 0
*/
#查询有员工的部门名
SELECT DISTINCT department_name
FROM departments d
LEFT JOIN employees e
ON d.`department_id` = e.`department_id`
WHERE e.`employee_id` IS NOT NULL;
或者
SELECT department_name
FROM departments d
WHERE EXISTS (
SELECT *
FROM employees e
WHERE e.`department_id` = d.`department_id`
);
或者
SELECT department_name
FROM departments d
WHERE d.department_id IN (
SELECT department_id
FROM employees
);
-- 五、子查询的练习
#1、查询和Zlotkey相同部门的员工姓名和工资
SELECT last_name, salary, department_id
FROM employees
WHERE department_id = (
SELECT department_id
FROM employees
WHERE last_name = 'Zlotkey'
);
#2、查询工资比公司平均工资高的员工的员工号、姓名和工资
SELECT employee_id, last_name, salary
FROM employees
WHERE salary > (
SELECT AVG(salary)
FROM employees
);
#3、查询各部门中比本部门平均工资高的员工的员工号、姓名和工资
SELECT employee_id, last_name, salary, e.`department_id`
FROM (
SELECT AVG(salary) ag, department_id
FROM employees
GROUP BY department_id
) temp JOIN employees e
ON temp.department_id = e.`department_id`
WHERE e.`salary` > temp.ag;
#4、查询和姓名中包含字母u的员工在同一部门的员工号和姓名
SELECT employee_id, last_name
FROM employees
WHERE department_id IN (
SELECT DISTINCT department_id
FROM employees
WHERE last_name LIKE '%u%'
);
#5、查询在部门的location_id=1700的部门工作的员工的员工号(IN或者=ANY)
SELECT employee_id
FROM employees
WHERE department_id IN (
SELECT DISTINCT department_id
FROM departments
WHERE location_id = 1700
);
#6、查询管理者是King的员工姓名和工资(姓名为k_ing的管理者可能有多个，当你不确定是=还是in，那么就用in)
SELECT last_name, salary
FROM employees
WHERE manager_id IN (
SELECT DISTINCT employee_id
FROM employees
WHERE last_name = 'K_ing'
);
#7、查询工资最高的员工的姓名，要求first_name，last_name显示为一列，列名为姓名
SELECT CONCAT(first_name, last_name) AS 姓名
FROM employees
WHERE salary = (
SELECT MAX(salary)
FROM employees
);