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
列子查询，一般搭配着多行操作符使用
in、any/some、all
#1.标量子查询★
#案例1：谁的工资比 Abel 高?
#①查询Abel的工资 SELECT salary FROM employees WHERE last_name = 'Abel' #②查询员工的信息，满足 salary>①结果 SELECT * 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
#①查询141号员工的job_id SELECT job_id FROM employees WHERE employee_id = 141 #②查询143号员工的salary SELECT salary FROM employees WHERE employee_id = 143 #③查询员工的姓名，job_id 和工资，要求job_id=①并且salary>② 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
#①查询50号部门的最低工资 SELECT MIN(salary) FROM employees WHERE department_id = 50 #②查询每个部门的最低工资 SELECT MIN(salary),department_id FROM employees GROUP BY department_id #③ 在②基础上筛选，满足min(salary)>① SELECT MIN(salary),department_id 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
#①查询location_id是1400或1700的部门编号 SELECT DISTINCT department_id FROM departments WHERE location_id IN(1400,1700) #②查询员工姓名，要求部门号是①列表中的某一个 SELECT last_name FROM employees WHERE department_id <>ALL( SELECT DISTINCT department_id FROM departments 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
#①查询job_id为‘IT_PROG’部门任一工资 SELECT DISTINCT salary FROM employees WHERE job_id = 'IT_PROG' #②查询员工号、姓名、job_id 以及salary，salary<(①)的任意一个 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'; #或 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';
#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 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'; #或 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';
#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
ELECT * FROM employees WHERE (employee_id,salary)=( SELECT MIN(employee_id),MAX(salary) FROM employees ); #①查询最小的员工编号 SELECT MIN(employee_id) FROM employees #②查询最高工资 SELECT MAX(salary) FROM employees #③查询员工信息 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 department_name,e.department_id FROM departments d INNER JOIN employees e ON d.department_id=e.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
#①查询每个部门的平均工资 SELECT AVG(salary),department_id FROM employees GROUP BY department_id SELECT * FROM job_grades; #②连接①的结果集和job_grades表，筛选条件平均工资 between lowest_sal and highest_sal 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;
#四、exists后面（相关子查询）
语法：
exists(完整的查询语句)
结果：
1或0
#案例：查询没有女朋友的男神信息
#in SELECT bo.* FROM boys bo WHERE bo.id NOT IN( SELECT boyfriend_id FROM beauty ) #exists SELECT bo.* FROM boys bo WHERE NOT EXISTS( SELECT boyfriend_id FROM beauty b WHERE bo.`id`=b.`boyfriend_id` );
习题测试
#1. 查询和Zlotkey相同部门的员工姓名和工资 #①查询Zlotkey的部门 SELECT department_id FROM employees WHERE last_name = 'Zlotkey' #②查询部门号=①的姓名和工资 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。 #①查询平均工资 SELECT AVG(salary) FROM employees #②查询工资>①的员工号，姓名和工资。 SELECT last_name,employee_id,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 #①查询各部门的平均工资 SELECT AVG(salary),department_id FROM employees GROUP BY department_id #②连接①结果集和employees表，进行筛选 SELECT employee_id,last_name,salary,e.department_id FROM employees e INNER JOIN ( SELECT AVG(salary) ag,department_id FROM employees GROUP BY department_id ) ag_dep ON e.department_id = ag_dep.department_id WHERE salary>ag_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 #①查询姓名中包含字母u的员工的部门 SELECT DISTINCT department_id FROM employees WHERE last_name LIKE '%u%' #②查询部门号=①中的任意一个的员工号和姓名 SELECT last_name,employee_id 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为1700的部门工作的员工的员工号 #①查询location_id为1700的部门 SELECT DISTINCT department_id FROM departments WHERE location_id = 1700 #②查询部门号=①中的任意一个的员工号 SELECT employee_id FROM employees WHERE department_id =ANY( 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 #①查询姓名为king的员工编号 SELECT employee_id FROM employees WHERE last_name = 'K_ing' #②查询哪个员工的manager_id = ① 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 #①查询最高工资 SELECT MAX(salary) FROM employees #②查询工资=①的姓.名 SELECT CONCAT(first_name,last_name) "姓.名" FROM employees WHERE salary=( SELECT MAX(salary) FROM employees );