多表查询案例

-- 部门表
CREATE TABLE dept(
id int PRIMARY KEY PRIMARY KEY,
dname varchar(50),
loc VARCHAR(50)
);

-- 职务表，职务名称，职务描述
CREATE TABLE job(
id int PRIMARY KEY,
jname VARCHAR(20),
description VARCHAR(50)
);

-- 员工表
create table emp(
id int PRIMARY KEY,
ename VARCHAR(50),
job_id INT,-- 职务id
mgr INT,-- 上级领导
joindate DATE,-- 入职日期
salary DECIMAL(7,2),-- 工资
bonus DECIMAL(7,2),-- 奖金
dept_id int,-- 所在部门编号
CONSTRAINT emp_jobid_ref_job_id_fk FOREIGN KEY (job_id) REFERENCES job(id),
CONSTRAINT emp_deptid_ref_dept_id_fk FOREIGN KEY (dept_id) REFERENCES dept(id)
);

-- 工资等级表
create table salarygrade(
grade int PRIMARY KEY,-- 级别
losalary int,-- 最低工资
hisalary int-- 最高工资
);

-- 添加4个部门
INSERT into dept(id,dname,loc) VALUES
(10,'教研部','北京'),
(20,'学工部','上海'),
(30,'销售部','广州'),
(40,'财务部','深圳');

-- 添加4个职务
INSERT into job(id,jname,description) VALUES
(1,'董事长','管理整个公司，接单'),
(2,'经理','管理部门员工'),
(3,'销售员','向客人推销产品'),
(4,'文员','使用办公软件');

-- 添加员工
INSERT into emp(id,ename,job_id,mgr,joindate,salary,bonus,dept_id) VALUES
(1001,'孙悟空',4,1004,'2000-12-17','8000.00',NULL,20),
(1002,'卢俊义',3,1006,'2001-02-20','16000.00','3000.00',30),
(1003,'林冲',3,1006,'2001-02-22','12500.00','5000.00',30),
(1004,'唐僧',2,1009,'2001-04-02','29750.00',NULL,20),
(1005,'李逵',4,1006,'2001-09-28','12500.00','14000.00',30),
(1006,'宋江',2,1009,'2001-05-01','28500.00',NULL,30),
(1007,'刘备',2,1009,'2001-09-01','24500.00',NULL,10),
(1008,'猪八戒',4,1004,'2007-04-19','30000.00',NULL,20),
(1009,'罗贯中',1,NULL,'2001-11-17','50000.00',NULL,10),
(1010,'吴用',3,1006,'2001-09-08','15000.00','0.00',30),
(1011,'沙僧',4,1004,'2007-05-23','11000.00',NULL,20),
(1012,'李逵',4,1006,'2001-12-03','9500.00',NULL,30),
(1013,'小白龙',4,1004,'2001-12-03','30000.00',NULL,20),
(1014,'关羽',4,1007,'2002-01-23','13000.00',NULL,10);

-- 添加5个工资等级
INSERT into salarygrade(grade,losalary,hisalary) VALUES
(1,7000,12000),
(2,12010,14000),
(3,14010,20000),
(4,20010,30000),
(5,30010,99990);

-- 1.查询所有员工信息。查询员工编号，员工姓名，工资，职务名称，职务描述
-- 隐式内连接写法
SELECT
    emp.id,
    emp.ename,
    emp.salary,
    job.jname,
    job.description
FROM
    emp,
    job
WHERE
    emp.job_id = job.id;

-- 显示内连接写法
SELECT
    emp.id,
    emp.ename,
    emp.salary,
    job.jname,
    job.description
FROM
    emp
INNER JOIN job ON emp.job_id = job.id;

-- 2.查询员工编号，员工姓名，工资，职务名称，职务描述，部门名称，部门位置
-- 隐式内连接写法
SELECT
    emp.id,
    emp.ename,
    emp.salary,
    job.jname,
    job.description,
    dept.dname,
    dept.loc
FROM
    emp,
    job,
    dept
WHERE
    emp.job_id = job.id
AND emp.dept_id = dept_id;

-- 显示内连接写法
SELECT
    emp.id,
    emp.ename,
    emp.salary,
    job.jname,
    job.description,
    dept.dname,
    dept.loc
FROM
    emp
INNER JOIN job ON emp.job_id = job.id
INNER JOIN dept ON emp.dept_id = dept_id;

-- 3.查询员工姓名，工资，工资等级
SELECT
    emp.ename,
    emp.salary,
    t1.grade
FROM
    emp,
    salarygrade t1
WHERE
    emp.salary >= t1.losalary
AND emp.salary <= t1.hisalary;

-- 4.查询员工姓名，工资，职务名称，职务描述，部门名称，部门位置，工资等级
SELECT
    emp.ename,
    emp.salary,
    job.jname,
    job.description,
    dept.dname,
    dept.loc,
    t1.grade
FROM
    emp
INNER JOIN job ON emp.job_id = job_id
INNER JOIN dept ON emp.dept_id = dept.id
INNER JOIN salarygrade t1 ON emp.salary BETWEEN t1.losalary
AND t1.hisalary;

-- 5.查询出部门编号、部门名称、部门位置、部门人数
/*
部门人数：在emp表中按照dept_id进行分组，然后count(*) 统计数量
使用子查询，让部门表和分组后的表进行内连接
*/
SELECT
    *
FROM
    dept;

SELECT
    dept_id,
    count(*)
FROM
    emp
GROUP BY
    dept_id;

-- 把它看做成一个虚拟表
-- 虚拟表的id为dept_id 虚拟表名起 别名为 t1  虚拟表count（*） 起别名为 count
SELECT
    dept.id,
    dept.dname,
    dept.loc,
    t1.count
FROM
    dept,
    (
        SELECT
            dept_id,
            count(*) count
        FROM
            emp
        GROUP BY
            dept_id
    ) t1
WHERE
    dept.id = t1.dept_id;