【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 don (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 don 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 eleft 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 departmenton 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,salarygradewhere 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.idand 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.studentnoand course.name = student_course.courseno