Mysql基础(五)
进阶7 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:
slecet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:(重点)
标量子查询:(重点)
列子查询:(重点)
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(结果集中有一行多列)
表子查询(结果集一般为多行多列)
*/
#一:where 或having 后面
#1、表量子查询(单行子查询)
#2、列子查询(多行子查询)
#3、行子查询(多行多列)
/*
特点:
1、子查询方法在小括号内
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
*/
#标量子查询
#案例1、谁的工资比 Abel的高
SELECT * FROM employees
WHERE salary > (
SELECT salary
FROM employees
WHERE last_name = 'Abel'
);
#案例2:
#题目: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
#①查询公司的 最低工资
SELECT MIN(salary)
FROM employees
#②查询last_name,job_id和salary,要求salary=①
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
);
#非法使用标量子查询
SELECT MIN(salary),department_id
FROM employees
GROUP BY department_id
HAVING MIN(salary)>(
SELECT salary
FROM employees
WHERE department_id = 250
);
#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
SELECT *
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
*/
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 department_name
FROM departments d
WHERE EXISTS(
SELECT *
FROM employees e
WHERE d.`department_id`=e.`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 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`
);
进阶 8 分页查询
#进阶八:分页查询(重点)
/*
应用要显示的数据,一页显示不全,需要分页提交SQL请求
语法: SELECT 查询列表
FROM 表
【join type】
【on 连接条件
WHERE 筛选条件
group by 分组字段
having 分组后的筛选
order by 排序的字段】
limit 起始索引,要显示的条目数
起始索引从0开始
特点:
1、limit 语句放在查询语句的最后
2、公式
要显示的页数 page ,每页的条目数 size
SELECT 查询列表
FROM 表
limit (page-1)*size ,size;
*/
#案例1:查询前五条员工信息
SELECT * FROM employees LIMIT 0,5;
#案例2:查询第11条到25条
SELECT * FROM employees LIMIT 10,15;
#案例3:有奖金的员工的信息,并且工资较高的前10名
SELECT * FROM employees
WHERE commission_pct IS NOT null
ORDER BY salary DESC LIMIT 10;