Multi table query of MySQL - multi table relationship and related exercises

1. Multi table relation

MySQL The relationship between multiple tables can be summarized as bit ： one-on-one 、 One to many or many to one 、 Many to many .

1.1 One to many foreign key constraints

1.1.1 The creation of

1.1.2 Insertion of foreign key constraints 、 Delete operation

Insert ： Insert data that conforms to the format randomly in the main table , The secondary table needs to rely on the primary key column of the primary table .
Delete ： When the data of the master table is dependent on the slave table , Both master and slave tables have primary keys , The primary table cannot delete the related primary key , Otherwise, you can delete ; The data from the table can be deleted at will .

1.1.3 Delete foreign key constraint key

1.2 Many to many foreign key constraints

1.3 Multi-table query

1.3.1 Cross link query

The content source of Cartesian product ：

1.3.2 Internal connection query

1.3.3 External connection query

1.3.4 Multi table joint query

1.3.5 Self association query

2. Case study

Create table ：

--  establish test1 database 
CREATE DATABASE test1;
--  Choose to use test1 database 
USE test1
--  Create department table 
CREATE TABLE dept(
deptno INT PRIMARY KEY, --  Department number 
dname VARCHAR(20),   --  Department name 
loc VARCHAR(20) --  Department Address 
)

INSERT INTO dept VALUES (10,'accounting','new york');
INSERT INTO dept VALUES (20,'research','dallas');
INSERT INTO dept VALUES (30,'sales','chicago');
INSERT INTO dept VALUES (40,'operations','boston');

--  Create an employee table 
CREATE TABLE emp(
empno INT PRIMARY KEY, --  Employee number 
ename VARCHAR(10),  --  Employee name 
job VARCHAR(9), --  Staff work 
mgr INT,  --  Employee index leader number 
hiredate date, --  Entry time 
sal DOUBLE,  --  Wages 
comm DOUBLE, --  Bonus 
deptno INT  --  Corresponding dept Table foreign key 
);

--  Add the main foreign key relationship between department and employee 
ALTER TABLE emp ADD CONSTRAINT FOREIGN KEY emp(deptno) REFERENCES dept (deptno);


INSERT INTO emp VALUES (7369,'smith','clerk',7902,'1980-12-17',800,null,20);
INSERT INTO emp VALUES (7499,'allen','salesman',7698,'1981-02-20',1600,300,30);
INSERT INTO emp VALUES (7521,'ward','salesman',7698,'1981-02-22',1250,500,30);
INSERT INTO emp VALUES (7566,'jones','manager',7839,'1981-04-02',2975,null,20);
INSERT INTO emp VALUES (7654,'marthin','salesman',7698,'1981-09-28',1250,1400,30);
INSERT INTO emp VALUES (7698,'blake','manager',7839,'1981-05-01',2850,null,30);
INSERT INTO emp VALUES (7782,'clark','manager',7839,'1981-06-09',2450,null,10);
INSERT INTO emp VALUES (7788,'scott','analyst',7566,'1981-07-03',5000,null,20);
INSERT INTO emp VALUES (7839,'king','president',null,'1981-11-17',5000,null,10);
INSERT INTO emp VALUES (7844,'turner','salesman',7698,'1981-09-08',1500,0,30);
INSERT INTO emp VALUES (7876,'adams','clerk',7788,'1987-07-13',1100,null,20);
INSERT INTO emp VALUES (7900,'james','clerk',7698,'1981-12-03',950,null,30);
INSERT INTO emp VALUES (7902,'ford','analyst',7566,'1981-12-03',3000,null,20);
INSERT INTO emp VALUES (7934,'miller','clerk',7782,'1981-01-23',1300,null,10);

--  Create a pay scale 
CREATE TABLE salgrade(
grade INT, --  Grade 
losal DOUBLE, --  minimum wage 
hisal double  --  Maximum wage 
);

INSERT INTO salgrade VALUES (1,700,1200);
INSERT INTO salgrade VALUES (2,1201,1400);
INSERT INTO salgrade VALUES (3,1401,2000);
INSERT INTO salgrade VALUES (4,2001,3000);
INSERT INTO salgrade VALUES (5,3001,9999);

practice ：

--  practice 
-- 1、 Return the name of the Department that owns the employee 、 Department number 
SELECT DISTINCT a.dname,a.deptno FROM dept a join emp b on a.deptno=b.deptno;
-- 2、 The salary level is redundant smith Staff level 
SELECT * FROM emp WHERE sal > (SELECT sal FROM emp WHERE ename = 'smith');
-- 3、 Returns the names of the employee and the manager 
SELECT a.ename,b.ename FROM emp a join emp b on a.empno=b.mgr;
-- 4、 Return the name of the employee and his / her manager whose employment date is earlier than that of his / her manager .
SELECT a.ename,a.hiredate,b.ename,b.hiredate FROM emp a join emp b on a.empno=b.mgr and a.hiredate > b.hiredate;
-- 5、 Return the name of the employee and the name of his / her department .
SELECT a.ename,b.dname FROM emp a join dept b on a.deptno = b.deptno;
-- 6、 Return to work clerk The name of the employee working and the name of the Department .
SELECT a.ename,a.job,b.dname FROM emp a join dept b on a.deptno=b.deptno and job = 'clerk';
-- 7.  Returns the department number and its minimum wage 
SELECT deptno,MIN(sal) FROM emp GROUP BY deptno;
-- 8.  Return the names of all employees in the sales department .
SELECT a.ename,b.dname FROM emp a join dept b on a.deptno=b.deptno and b.dname='sales';
-- 9.  Return to employees who earn more than the average wage 
SELECT ename,sal FROM emp WHERE sal > (SELECT AVG(sal) FROM emp);
-- 10、 Return to scott Employees who do the same job .
SELECT * FROM emp WHERE job = (SELECT job FROM emp WHERE ename = 'scott');
-- 11、 Return above 30 Employee information about the salary level of department employees ;
SELECT * FROM emp WHERE sal > ALL(SELECT sal FROM emp WHERE deptno=30);
-- 12、 Return to the employee's job and the minimum wage for the job ;
SELECT job,MIN(sal) FROM emp GROUP BY job;
-- 13、 Calculate the employee's annual salary , And in order of annual salary 
SELECT ename,(sal * 12 + ifnull(comm,0)) as yeas_sal FROM emp ORDER BY yeas_sal DESC;
-- 14、 Return the salary of the employee whose salary is at the fourth level 
SELECT * FROM emp 
WHERE  sal BETWEEN  (SELECT losal FROM salgrade WHERE grade = 4) AND (SELECT hisal FROM salgrade WHERE grade = 4);

-- 15、 Return the name of the employee whose salary is level II 、 Department location 
--  Here, three tables are combined into Cartesian product , Then use the condition to delete 
SELECT 
* 
FROM dept a 
JOIN emp b on a.deptno=b.deptno
JOIN salgrade c on grade = 2 and b.sal >= c.losal and  b.sal <= c.hisal;

--------------------
SELECT 
* 
FROM dept a , emp b ,salgrade c
WHERE a.deptno=b.deptno
AND  grade = 2 and b.sal >= c.losal and  b.sal <= c.hisal;

3. Reference resources

https://www.bilibili.com/video/BV1iF411z7Pu?p=82&spm_id_from=pageDriver