【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