【SQL】多表查询案例

📢本章节主要学习使用SQL多表查询的案例,多表查询基础概念请点击此处。

🎄数据准备

首先我们创建一个新的表也就是薪资等级表，其余两个表(员工表和薪资表)在多表查询章节中已经创建。
然后我么根据这三个表完成下面的12个需求。

create table salaryGrade(
    grade int,
    losal int,
    hisal int
)comment '薪资等级表';

insert into salaryGrade values (1,0,3000),
 (2,3001,5000),
 (3,5001,8000),
 (4,8001,10000),
 (5,10001,15000),
 (6,15001,20000),
 (7,20001,25000),
 (8,25001,30000);

🎄案例

⭐案例1

. 📢 查询员工的姓名、年龄、职位、部门信息（隐式内连接）

select employee.name,employee.age,employee.job,department.name from employee,department
where employee.dept_id = department.id;

⭐案例2

📢 查询年龄小于30 岁的员工的姓名、年龄、职位、部门信息（显式内连接）

select e.name,e.age,e.job,d.name from
employee as e join department as d
    on (e.dept_id = d.id and e.age < 30);

还有一种是在on子句后面加where条件

select e.name,e.age,e.job,d.name from
employee as e join department as d
    on e.dept_id = d.id where e.age > 30;

⭐案例3

📢 查询拥有员工的部门ID 、部门名称

这里有个重点要使用distinct对查出的列进行去重操作。
对于distinct来说是它后面所有的列都完全相同时才会去重。

select distinct department.id, department.name from employee,department
where employee.dept_id = department.id

⭐案例4

📢查询所有年龄大于40岁的员工, 及其归属的部门名称; 如果员工没有分配部门, 也需要展示出

来 ( 外连接 )

select e.*,department.name from (select * from employee where employee.age > 40) as e
    left outer join department on e.dept_id = department.id;

还有一种实现方式更为简单

select employee.*,department.name from employee left outer join department
    on employee.dept_id = department.id where employee.age > 40

⭐案例5

📢查询所有员工的工资等级

select employee.*,salaryGrade.grade from employee,salarygrade
                              where employee.salary between salaryGrade.losal and salaryGrade.hisal;

⭐案例6

📢 查询 " 研发部 " 所有员工的信息及工资等级

首先涉及到3个表,3个表的连接条件至少有两个，先确定连接条件
连接条件:(employee.salary between salaryGrade.losal and salaryGrade.hisal) and department.id = employee.dept_id
查询条件:department.name = '研发部'

select employee.*,salaryGrade.grade from employee ,department,salaryGrade
where  (employee.salary between salaryGrade.losal and salaryGrade.hisal) and department.id = employee.dept_id
and department.name = '研发部'

⭐案例7

📢 "研发部 " 员工的平均工资

select department.name, avg(employee.salary) as '平均工资'
from employee,
     department
where employee.dept_id = department.id
  and department.name = '研发部';

/* 每个部门平均工资 */
select department.name, avg(employee.salary) as '平均工资'
from employee,
     department
where employee.dept_id = department.id
group by employee.dept_id;

⭐案例8

📢查询工资比灭绝高的员工

这是一个典型的标量子查询，因为返回的值只有一个值。

select name from employee where salary > (
select salary from employee where name = '灭绝')

当然这个也可以使用自查询，只不过比子查询要复杂

select a.name from employee as a join employee as b on a.salary > b.salary where  b.name = '灭绝'

⭐案例9

📢 查询比平均薪资高的员工信息

select name from employee where salary > (select avg(salary) from employee);

⭐案例10

📢 查询低于本部门平均工资的员工信息

select * from employee as a where a.salary <
(select avg(b.salary) from employee  as b where b.dept_id = a.dept_id)

还可以使用分组查询+自连接

select a.* from employee as a join
(select employee.dept_id,avg(employee.salary) as avg_salary from employee group by dept_id )
as b on a.dept_id = b.dept_id
where a.salary < b.avg_salary

⭐案例11

📢 查询所有的部门信息 , 并统计部门的员工人数

这里使用的子查询属于select类型。

select a.id, a.name, (select count(*)  from employee as b where b.dept_id = a.id) as '部门人数'
from department as a;

⭐案例12

📢 查询所有学生的选课情况 , 展示出学生名称 , 学号 , 课程名称

首先我们先把三张表的连接条件写出来。
然后再去写表的查询条件。这个案例不需要写查询条件。

select student.name, student_course.studentno, student_course.courseno
from student,
     course,
     student_course
where student.no = student_course.studentno
  and course.name = student_course.courseno

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