小知识，大挑战！本文正在参与“程序员必备小知识”创作活动。
前言
最近复习了下Mysql，感觉写好SQL语句不是一件容易的事，还是要多进行练习才能写好SQL语句 于是在网上看了视频，跟着写了下来~ 流程和思路都写在源码中
练习
表的结构
表的内容
emp表
dept表
salarygrade表
job表
源码
-- 多表查询练习
-- 部门表
create TABLE dept (
id INT PRIMARY KEY,
dname VARCHAR(255),
loc VARCHAR(255)
);
-- 添加四个部门
INSERT INTO dept 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");
-- 职务表
CREATE TABLE job (
id INT PRIMARY KEY,
jname VARCHAR(25),
description VARCHAR(90)
)
-- 添加4个职务
INSERT INTO job (id,jname,description) VALUES
(1,"董事长","管理公司，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office")
--员工表
CREATE TABLE emp (
id INT PRIMARY KEY, -- 员工id
ename VARCHAR (50), -- 员工姓名
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)
);
-- 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),
(100,"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);
-- 工资等级表
CREATE TABLE salarygrade (
grade int PRIMARY KEY,
losalary INT,
hisalary INT
);
-- 添加5个工资等级
INSERT INTO salarygrade VALUES
(1,7000,12000),
(2,12010,14000),
(3,14010,20000),
(4,20010,30000),
(5,30010,99990)
-- 需求:
-- 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
/*
1.查询两张表
2.条件是emp.job_id = job.id
*/
SELECT
t1.id,-- 员工编号
t1.ename,-- 员工姓名
t1.salary,-- 工资
t2.jname,-- 职务名称
t2.description -- 植物描述
FROM
emp t1,
job t2
WHERE
t1.job_id = t2.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
/*
1.查询三张表
2.条件是emp.job_id = job.id and emp.dept_id = dept.id
*/
SELECT
t1.id,-- 员工编号
t1.ename,-- 员工姓名
t1.salary,-- 工资
t2.jname,-- 职务名称
t2.description,-- 植物描述
t3.dname,-- 部门名称
t3.loc -- 部门位置
FROM
emp t1,
job t2,
dept t3
WHERE
t1.job_id = t2.id
AND t1.dept_id = t3.id
-- 3.查询员工姓名，工资，工资等级
/*
1.查询两张表emp表和salarygrade
2.条件是emp.salary >= salarygrade.losalary and emp.salary <= salarygrade.hisalary(表之间没有关联不能用等值来进行判断了)
3.简化上一步emp.salary BETWEEN salarygrade.losalary and salarygrade.hisalary
*/
SELECT
t1.ename,
t1.salary,
t2.grade
FROM
emp t1,
salarygrade t2
WHERE
t1.salary BETWEEN t2.losalary
AND t2.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
/*
1.员工名称，工资emp，职务名称，职务描述job,部门名称，部门位置dept，工资等级salarygrade
2.条件emp.job_id = job.id and emp.dept_id = dept.id and emp.salary BETWEEN salarygrade.losalary and salarygrade.hisalary
*/
SELECT
t1.ename,
t1.salary,
t2.jname,
t2.description,
t3.dname,
t3.loc,
t4.grade
FROM
emp t1,
job t2,
dept t3,
salarygrade t4
WHERE
t1.job_id = t2.id
AND t1.dept_id = t3.id
AND t1.salary BETWEEN t4.losalary
AND t4.hisalary
-- 5.查询出部门编号、部门名称、部门位置、部门人数
/*
1.部门编号、部门名称、部门位置dept,部门人数 emp表
2.使用分组查询，按照emp.dept_id完成分组，查询count（id）
3.使用子查询将第二部的查询结果和dept表进行关联查询
*/
SELECT
t1.id,
t1.dname,
t1.loc,
t2.total
FROM
dept t1,
( SELECT dept_id, COUNT( id ) total FROM emp GROUP BY dept_id ) t2
WHERE
t1.id = t2.dept_id
-- 6.查询所有员工的姓名及其直接上级的姓名,没有领导的员工也需要查询
/*
1.姓名 emp，直接上级的姓名 emp
* emp的id和mgr是自关联的
2.条件 emp.id = emp.mgr
3.查询左表的所有数据，和交集数据
*使用左外连接查询
*/
SELECT
t1.ename,
t1.mgr,
t2.id,
t2.ename
FROM
emp t1
LEFT JOIN emp t2 ON t1.mgr = t2.id
