本文主要是介绍Day75（子查询），希望对大家解决编程问题提供一定的参考价值，需要的开发者们随着小编来一起学习吧！
子查询
#进阶7：子查询
/*
含义：
出现在其他语句中的select语句，称为子查询或内查询
外部的查询语句，称为主查询或外查询
分类：
按子查询出现的位置：select后面：仅仅支持标量子查询from后面：支持表子查询where或having后面：★标量子查询（单行） √列子查询 （多行） √行子查询exists后面（相关子查询）表子查询
按结果集的行列数不同：标量子查询（结果集只有一行一列）列子查询（结果集只有一列多行）行子查询（结果集有一行多列）表子查询（结果集一般为多行多列）
*/
#一、where或having后面
/*
1、标量子查询（单行子查询）
2、列子查询（多行子查询）
3、行子查询（多列多行）
特点：
①子查询放在小括号内
②子查询一般放在条件的右侧
③标量子查询，一般搭配着单行操作符使用
> < >= <= = <>
列子查询，一般搭配着多行操作符使用
in、any/some、all
④子查询的执行优先于主查询执行，主查询的条件用到了子查询的结果
*/
#1.标量子查询★
#案例1：谁的工资比 Abel 高?
SELECT last_name,salary
FROM employees
WHERE salary > (SELECT salary FROM employees
WHERE last_name = 'Abel');
#案例2：返回job_id与141号员工相同，salary比143号员工多的员工 姓名，job_id 和工资
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
);
#案例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
);
#案例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 department_id,MIN(salary)
FROM employees
GROUP BY department_id
HAVING MIN(salary)>(
SELECT MIN(salary)
FROM employees
WHERE department_id = 50
);
#2.列子查询（多行子查询）★
#案例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 ( #<>ALL 不在
SELECT d.department_id
FROM employees e
INNER JOIN departments d
ON d.`department_id`=e.`department_id`
WHERE location_id IN(1400,1700)
);
#案例2：返回其它工种中比job_id为‘IT_PROG’工种任一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';
#或
SELECT employee_id,last_name,job_id,salary
FROM employees
WHERE salary<(
SELECT MAX(salary)
FROM employees
WHERE job_id = 'IT_PROG'
)
AND job_id <> 'IT_PROG';
#案例3：返回其它部门中比job_id为‘IT_PROG’部门所有工资都低的员工的员工号、
# 姓名、job_id 以及salary
SELECT employee_id,last_name,job_id,salary
FROM employees
WHERE salary<ALL(
SELECT DISTINCT salary
FROM employees
WHERE job_id = 'IT_PROG'
)
AND job_id<>'IT_PROG';
#3、行子查询（结果集一行多列或多行多列）
#案例：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,salary)=(
SELECT MIN(employee_id),MAX(salary)
FROM employees
);
#或：
SELECT *
FROM employees
WHERE employee_id=(
SELECT MIN(employee_id)
FROM employees
)
AND salary=(
SELECT MAX(salary)
FROM employees
);
#二、select后面
/*
仅仅支持标量子查询
*/
#案例：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;
#案例2：查询员工号=102的部门名
SELECT (
SELECT d.department_name
FROM departments d
INNER JOIN employees e
ON e.`department_id`=d.`department_id`
WHERE e.employee_id=102) 部门名;
#三、from后面
/*
将子查询结果充当一张表，要求必须起别名
*/
#案例：查询每个部门的平均工资的工资等级
SELECT avg_dep.*,grade_level
FROM(
SELECT AVG(salary) avgsal,department_id
FROM employees e
GROUP BY department_id
) avg_dep
INNER JOIN job_grades
ON avg_dep.avgsal BETWEEN lowest_sal AND highest_sal;
#四、exists后面（相关子查询）
/*
语法：
exists(完整的查询语句)
结果：
1或0
*/
SELECT EXISTS(SELECT employee_id FROM employees WHERE salary=300000);
#案例1：查询有员工的部门名
#in
SELECT department_name
FROM departments d
WHERE d.`department_id` IN(
SELECT department_id FROM
employees
);
#exists
SELECT d.department_name
FROM departments d
WHERE EXISTS(
SELECT *
FROM employees e
WHERE e.`department_id`=d.`department_id`
);
#案例2：查询没有女朋友的男神信息
#in
SELECT bo.*
FROM boys bo
WHERE bo.`id` NOT IN(
SELECT b.`boyfriend_id`
FROM beauty b);
#exists
SELECT bo.*
FROM boys bo
WHERE NOT EXISTS(
SELECT b.boyfriend_id
FROM beauty b
WHERE b.`boyfriend_id`=bo.`id`
);
# 练习
#1.查询和Zlotkey相同部门的员工姓名和工资
SELECT last_name,salary
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.查询各部门中工资比本部门平均工资高的员工的员工号, 姓名和工资
SELECT e.employee_id,e.last_name,e.salary
FROM employees e
INNER JOIN (
SELECT AVG(salary) ag,department_id
FROM employees
GROUP BY department_id
) avg_dep
ON avg_dep.department_id = e.department_id
WHERE e.salary>avg_dep.ag;
#4.查询和姓名中包含字母u的员工在相同部门的员工的员工号和姓名
SELECT employee_id,last_name
FROM employees
WHERE department_id IN(
SELECT department_id
FROM employees
WHERE last_name LIKE '%u%'
);
#5. 查询在部门的location_id为1700的部门工作的员工的员工号
SELECT employee_id
FROM employees
WHERE department_id IN(
SELECT department_id
FROM departments
WHERE location_id = 1700);
#或：
SELECT employee_id
FROM employees
WHERE department_id =ANY(
SELECT department_id
FROM departments
WHERE location_id = 1700);
#6.查询管理者是King的员工姓名和工资
SELECT last_name,salary
FROM employees
WHERE manager_id IN(
SELECT 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和last_name显示为一列，列名为 姓.名
SELECT CONCAT(first_name,last_name) '姓.名'
FROM employees
WHERE salary=(
SELECT MAX(salary)
FROM employees
);
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