子查询
说明:当一个查询语句中又嵌套了另一个完整的select语句,则被嵌套的select语句称为子查询或内查询,外面的select语句称为主查询或外查询
分类:
按子查询出现的位置进行分类:
|--select后面 要求:子查询的结果为单行单列(标量子查询)
|--from后面 要求:子查询的结果可以为多行多列
|--where或having后面 要求:子查询的结果必须为单列
|--单行子查询
|--多行子查询
|--exists后面 要求:子查询结果必须为单列(相关子查询)
1. where和having后面
|--标量子查询(单行子查询,查询出来的结果为一行一列)
|--列子查询(多行子查询)
|--行子查询(多列多行)
特点:
1. 子查询放在条件中,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. 单行子查询对应了 单行操作符:> < >= <= = <>
多行子查询对应了 多行操作符:any/some all in
1.1 单行子查询
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salary>①的员工信息
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
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job_id=① and salary>②的信息
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
①查询最低工资
SELECT MIN(salary)
FROM employees
②查询salary=①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查询各部门的最低工资,筛选看哪个部门的最低工资>①
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 多行子查询
多行操作符:
① in:判断某字段是否在指定列表内
x in(10,30,50)
② any/some:判断某字段的值是否满足其中任意一个
x>any(10,30,50)
可以使用x>min()替换
x=any(10,30,50)
可以使用x in(10,30,50)替换
③ all:判断某字段的值是否满足里面所有的
x >all(10,30,50)
可以使用x >max()替换
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
SELECT department_id
FROM departments
WHERE location_id IN(1400,1700)
②查询department_id = ①的姓名
SELECT last_name
FROM employees
WHERE department_id IN(
SELECT DISTINCT department_id
FROM departments
WHERE location_id IN(1400,1700)
);
题目:返回其它部门中比job_id为‘IT_PROG’部门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’部门的工资
SELECT DISTINCT salary
FROM employees
WHERE job_id = 'IT_PROG'
②查询其他部门的工资<任意一个①的结果
SELECT employee_id,last_name,job_id,salary
FROM employees
WHERE salary<ANY(
SELECT DISTINCT salary
FROM employees
WHERE job_id = 'IT_PROG'
);
等价于
SELECT employee_id,last_name,job_id,salary
FROM employees
WHERE salary<(
SELECT MAX(salary)
FROM employees
WHERE 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
①查询job_id为‘IT_PROG’部门的工资
SELECT DISTINCT salary
FROM employees
WHERE job_id = 'IT_PROG'
②查询其他部门的工资<所有①的结果
SELECT employee_id,last_name,job_id,salary
FROM employees
WHERE salary<ALL(
SELECT DISTINCT salary
FROM employees
WHERE job_id = 'IT_PROG'
);
等价于
SELECT employee_id,last_name,job_id,salary
FROM employees
WHERE salary<(
SELECT MIN(salary)
FROM employees
WHERE job_id = 'IT_PROG'
);
2. 放在select后面
案例:查询部门编号是50的员工个数
3. 放在from后面
把结果集充当一个表来用
案例:查询每个部门的平均工资的工资级别
①查询每个部门的平均工资
SELECT AVG(salary),department_id
FROM employees
GROUP BY department_id
②将①和sal_grade两表连接查询
SELECT dep_ag.department_id,dep_ag.ag,g.grade
FROM sal_grade g
JOIN (
SELECT AVG(salary) ag,department_id
FROM employees
GROUP BY department_id
) dep_ag ON dep_ag.ag BETWEEN g.min_salary AND g.max_salary;
4. 放在exists后面
结果是boolean只有1/0两种
案例1 :查询有无名字叫“张三丰”的员工信息
SELECT EXISTS(
SELECT *
FROM employees
WHERE last_name = 'Abel'
) 有无Abel;
案例2:查询没有女朋友的男神信息
USE girls;
SELECT bo.*
FROM boys bo
WHERE bo.`id` NOT IN(
SELECT boyfriend_id
FROM beauty b
)
SELECT bo.*
FROM boys bo
WHERE NOT EXISTS(
SELECT boyfriend_id
FROM beauty b
WHERE bo.id = b.boyfriend_id
);