MySQL multi table query introduction classic case

--  Departmental table 
CREATE TABLE dept ( 
	did INT PRIMARY KEY, --  department id
	dname VARCHAR ( 50 ), --  Department name 
	loc VARCHAR ( 50 ) --  Department Address 
) 

--  Job list 
CREATE TABLE job ( 
	jid INT PRIMARY KEY, --  position id
	jname VARCHAR ( 20 ), --  Job title 
	description VARCHAR ( 50 ) --  Job description 
);

--  The employee table 
CREATE TABLE emp (
	eid INT PRIMARY KEY,--  staff id
	ename VARCHAR ( 50 ),--  Employee name 
	job_id INT,--  position id
	mgrid INT,--  Superior leaders id
	joindate DATE,--  Date of entry 
	salary DECIMAL ( 7, 2 ),--  Wages 
	bonus DECIMAL ( 7, 2 ),--  Bonus 
	dept_id INT,--  Department number 
	CONSTRAINT emp_job_id_ref_job_jid FOREIGN KEY ( job_id ) REFERENCES job ( jid ),
	CONSTRAINT emp_dept_id_ref_dept_did FOREIGN KEY ( dept_id ) REFERENCES dept ( did ) 
);

--  Pay scale 
CREATE TABLE salarygrade ( 
	grade INT PRIMARY KEY, --  Level 
	losalary INT, --  minimum wage 
	hisalary INT --  Maximum wage 
);

--  add to 4 Departments 
INSERT INTO dept ( did, dname, loc )
VALUES
	( 10, ' Teaching and Research Department ', ' Beijing ' ),
	( 20, ' Department of science and Engineering ', ' Shanghai ' ),
	( 30, ' The sales department ', ' Guangzhou ' ),
	( 40, ' Finance Department ', ' Shenzhen ' );
	
--  add to 4 Positions 
INSERT INTO job ( jid, jname, description )
VALUES
	( 1, ' Chairman of the board of directors ', ' Managing the whole company , order ' ),
	( 2, ' The manager ', ' Management staff ' ),
	( 3, ' Salesperson ', ' Sell products to customers ' ),
	( 4, ' Clerk ', ' Using office software ' );
	
--  Add employees 
INSERT INTO emp ( eid, ename, job_id, mgrid, joindate, salary, bonus, dept_id )
VALUES
	( 1001, ' The Monkey King ', 4, 1004, '2000-12-17', '8000.00', NULL, 20 ),
	( 1002, ' Jun-yi lu ', 3, 1006, '2001-02-20', '16000.00', '3000.00', 30 ),
	( 1003, ' Lin Chong ', 3, 1006, '2001-02-22', '12500.00', '5000.00', 30 ),
	( 1004, ' Tang's monk ', 2, 1009, '2001-04-02', '29750.00', NULL, 20 ),
	( 1005, ' Li Kui ', 4, 1006, '2001-09-28', '12500.00', '14000.00', 30 ),
	( 1006, ' Song Jiang ', 2, 1009, '2001-05-01', '28500.00', NULL, 30 ),
	( 1007, ' Liu bei ', 2, 1009, '2001-09-01', '24500.00', NULL, 10 ),
	( 1008, ' Pig eight quit ', 4, 1004, '2007-04-19', '30000.00', NULL, 20 ),
	( 1009, ' Luo Guanzhong ', 1, NULL, '2001-11-17', '50000.00', NULL, 10 ),
	( 1010, ' Wu Yong ', 3, 1006, '2001-09-08', '15000.00', '0.00', 30 ),
	( 1011, ' Monk sha ', 4, 1004, '2007-05-23', '11000.00', NULL, 20 ),
	( 1012, ' Li Kui ', 4, 1006, '2001-12-03', '9500.00', NULL, 30 ),
	( 1013, ' Small white dragon ', 4, 1004, '2001-12-03', '30000.00', NULL, 20 ),
	( 1014, ' Guan yu ', 4, 1007, '2002-01-23', '13000.00', NULL, 10 );

--  add to 5 A salary scale 
INSERT INTO salarygrade ( grade, losalary, hisalary )
VALUES
	( 1, 7000, 12000 ),
	( 2, 12010, 14000 ),
	( 3, 14010, 20000 ),
	( 4, 20010, 30000 ),
	( 5, 30010, 99990 );
	
-- 1. Query all employee information . Enquiry of employee number , Employee name , Wages , Job title , Job description 
--  Involving tables ：emp,job
SELECT e.eid,e.ename,e.salary,j.jname,j.description 
	FROM emp e,job j 
		WHERE e.job_id = j.jid;

SELECT e.eid,e.ename,e.salary,j.jname,j.description
	FROM emp e
		JOIN job j
			ON e.job_id = j.jid;
			
-- 2. Enquiry of employee number , Employee name , Wages , Job title , Job description , Department name , Department position 
--  Involving tables ：emp,job,dept
SELECT e.eid,e.ename,e.salary,j.jname,j.description,d.did,d.dname
	FROM emp e,job j,dept d
		WHERE e.job_id = j.jid and e.dept_id = d.did;

SELECT e.eid,e.ename,e.salary,j.jname,j.description,d.did,d.dname
	FROM emp e
		JOIN job j
			ON e.job_id = j.jid
		JOIN dept d
			ON e.dept_id = d.did;

-- 3. Check the name of the employee , Wages , Pay scale 
--  Involving tables ：emp,salarygrade
SELECT e.ename,e.salary,s.grade
	FROM emp e,salarygrade s
		WHERE salary BETWEEN s.losalary AND s.hisalary;
		
SELECT e.ename,e.salary,s.grade
	FROM emp e
		JOIN salarygrade s
			ON salary > s.losalary 
				AND salary < s.hisalary;

-- 4. Check the name of the employee , Wages , Job title , Job description , Department name , Department position , Pay scale 
--  Involving tables ：emp,job,dept,salarygrade
SELECT e.eid,e.ename,e.salary,j.jname,j.description,d.did,d.dname,s.grade
	FROM emp e,job j,dept d,salarygrade s
		WHERE e.job_id = j.jid 
			AND e.dept_id = d.did 
			AND salary BETWEEN s.losalary AND s.hisalary;

SELECT e.eid,e.ename,e.salary,j.jname,j.description,d.did,d.dname,s.grade
	FROM emp e
		JOIN job j
			ON e.job_id = j.jid
	  JOIN dept d
			ON e.dept_id = d.did
		JOIN salarygrade s
			ON salary BETWEEN s.losalary AND s.hisalary;

-- 5. Find out the department number 、 Department name 、 Department position 、 Number of departments 
--  Involving tables ：emp,dept
SELECT * 
	FROM dept;
	
SELECT dept_id, count(*) 
	FROM emp 
		GROUP BY dept_id;	
	
SELECT d.did,d.dname,d.loc,t1.count
	FROM dept d,(SELECT dept_id,count(*) count
						FROM emp
							GROUP BY dept_id
	) t1
		WHERE d.did = t1.dept_id;
	
SELECT d.did,d.dname,d.loc,t1.count
	FROM dept d
		LEFT JOIN (SELECT dept_id,count(*) count
						FROM emp
							GROUP BY dept_id
	  ) t1
			ON d.did = t1.dept_id;