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There are several kinds of multi table queries ：
        1、 Merge result set ;UNION、UNION ALL
        2、 Link query
                2.1 Internal connection [INNER] JOIN ON
                2.2 External connection OUTER JOIN ON
                         The left outer join LEFT [OUTER] JOIN
                         Right connection RIGHT [OUTER] JOIN
                         Full outer join （MySQL I won't support it ）FULL JOIN
                2.3 Natural join NATURAL JOIN
        3、 Subquery

01、 Merge result set ：UNION、UNION ALL

（1） effect ： To merge a result set is to combine two select The query results of the statement are combined ！
（2） There are two ways to merge result sets ：
UNION： Remove duplicate records
UNION ALL： Do not remove duplicate records
（3） The two results of the merger ： Number of columns 、 The column type must be the same .
Premise ： Create data table t1 and t2：
# Create data table t1 and t2
#(1) Create data table t1, And insert data 
CREATE TABLE t1(
`a` INT,
`b` VARCHAR(10)
);
INSERT INTO t1 VALUES(1,'a'),(2,'b'),(3,'c'),(4,'d');
DELETE FROM t1 

#(1) Create data table t2, And insert data 
DROP TABLE t2;
CREATE TABLE t2(
`c` INT,
`d` VARCHAR(10)
);
INSERT INTO t2 VALUES(4,'d'),(5,'e'),(6,'g');

1、UNION: Remove duplicate records

#(1)UNION: Remove duplicate records 
SELECT * FROM t1
UNION
SELECT * FROM t2

--> final result ：
mysql> SELECT * FROM t1
    -> UNION
    -> SELECT * FROM t2;
+------+------+
| a    | b    |
+------+------+
|    1 | a    |
|    2 | b    |
|    3 | c    |
|    4 | d    |
|    5 | e    |
|    6 | g    |
+------+------+

2、UNION ALL: Do not remove duplicate records

(2)UNION ALL: Do not remove duplicate records 
SELECT * FROM t1
UNION ALL
SELECT * FROM t2

--> final result 
mysql> SELECT * FROM t1
    -> UNION ALL
    -> SELECT * FROM t2;
+------+------+
| a    | b    |
+------+------+
|    1 | a    |
|    2 | b    |
|    3 | c    |
|    4 | d    |
|    4 | d    |
|    5 | e    |
|    6 | g    |
+------+------+

02、 Link query

Join query is to find the product of multiple tables , for example t1 Connect t2, So the result of the query is t1*t2.
Join queries produce Cartesian Products , That is, a lot of error messages will be generated , It needs to be filtered , Filter through primary and foreign key relationships 、

（1） Link query

（2） Internal connection

（3） External connection

（4） Natural join

#2. Link query :
/*
    Join queries produce Cartesian Products , That is, a lot of error messages will be generated , It needs to be filtered , Filter through primary and foreign key relationships 

*/
# Create data table t1、t2, And insert data , Make the following connection query 
# produce t1*t2 data 
SELECT * FROM t1,t2;
# operation emp dept
SELECT * FROM emp,dept;

# Use the master foreign key relationship as a condition to remove useless information 
SELECT * FROM emp,dept WHERE emp.deptno=dept.deptno;
# The above query results will query all columns of the two tables , Maybe you don't need so many columns , Now you can specify the columns to query .
SELECT emp.empno,emp.ename,emp.sal,dept.deptno,dept.loc FROM emp,dept WHERE emp.deptno=dept.deptno;
# Aliasing tables ：
SELECT e.empno,e.ename,e.sal,d.deptno,d.loc FROM emp e,dept d WHERE e.deptno=d.deptno;

#2.1  Internal connection 
/*
     Characteristics of internal connection ： The query results must meet the conditions . For example, we ask emp Insert a record in the table ：
	1015  Zhang San   Cleaner  1009 1999-12-31 80000.00 50
     among deptno by 50, And in the dept Only in the table 10、20、30、40 department , So the above query 
     The result will not appear “ Zhang San ” This record , Because it can't satisfy e.deptno=d.deptno This condition .
*/
# Inner connection standard statement ： The internal connection will not display records that do not meet the conditions 
SELECT * FROM emp e INNER JOIN dept d ON e.deptno=d.deptno;

# Before use emp Add another row of data to the table ：
#2.2  External connection ： Records that do not meet the conditions will be displayed 
#(1) The left outer join ： First, query all the data in the left table , Then go to the right table , The data satisfied in the right table shows , Dissatisfied display NULL
SELECT * FROM emp e LEFT OUTER JOIN dept d ON e.deptno=d.deptno;
#(2) Right connection ： First, query all the data in the right table , Then go to the left table , The data satisfied in the left table shows , Dissatisfied display NULL
SELECT * FROM emp e RIGHT OUTER JOIN dept d ON e.deptno=d.deptno;

#2.3  Natural join ： Automatically find filter conditions to remove useless data in Cartesian product 
# Natural internal connection 
SELECT * FROM emp NATURAL JOIN dept;
# Natural left outer connection 
SELECT * FROM emp NATURAL LEFT JOIN dept; 
# Natural right outer connection 
SELECT * FROM emp NATURAL RIGHT JOIN dept;

03、 Subquery

Subquery ( nested queries )： One select The statement contains another complete select sentence

3.1 Pay more than JONES The employees'

#3.1  Pay more than JONES The employees' 
# First step ： Inquire about JONES The salary of  -->2975
SELECT sal FROM emp WHERE `ename`='JONES';
# The second step ： Query salary greater than 2975 Employee records of 
SELECT * FROM emp WHERE `sal`>2975;
# Merge write 
SELECT * FROM emp WHERE `sal`>(SELECT sal FROM emp WHERE `ename`='JONES');

--> final result ：
mysql> SELECT * FROM emp WHERE `sal`>(SELECT sal FROM emp WHERE `ename`='JONES');
+-------+-------+-----------+------+------------+---------+------+--------+
| empno | ename | job       | mgr  | hiredate   | sal     | comm | deptno |
+-------+-------+-----------+------+------------+---------+------+--------+
|  7788 | SCOTT | ANALYST   | 7566 | 1987-04-19 | 3000.00 | NULL |     20 |
|  7839 | KING  | PRESIDENT | NULL | 1981-11-17 | 5000.00 | NULL |     10 |
|  7902 | FORD  | ANALYST   | 7566 | 1981-12-03 | 3000.00 | NULL |     20 |
+-------+-------+-----------+------+------------+---------+------+--------+

3.2 Query and SCOTT Employees in the same department

#3.2  Query and SCOTT Employees in the same department 
# First step ： Inquire about SCOTT Department number of  -->20
SELECT deptno FROM emp WHERE `ename`='SCOTT';
# The second step ： Inquiry Department No 20 Employee records of 
SELECT * FROM emp WHERE `deptno`=20;
# Merge write 
SELECT * FROM emp WHERE `deptno`=(SELECT deptno FROM emp WHERE `ename`='SCOTT');

--> final result ：
mysql> SELECT * FROM emp WHERE `deptno`=(SELECT deptno FROM emp WHERE `ename`='SCOTT');
+-------+-------+---------+------+------------+---------+------+--------+
| empno | ename | job     | mgr  | hiredate   | sal     | comm | deptno |
+-------+-------+---------+------+------------+---------+------+--------+
|  7369 | SMITH | CLERK   | 7902 | 1980-12-17 |  800.00 | NULL |     20 |
|  7566 | JONES | MANAGER | 7839 | 1981-04-02 | 2975.00 | NULL |     20 |
|  7788 | SCOTT | ANALYST | 7566 | 1987-04-19 | 3000.00 | NULL |     20 |
|  7876 | ADAMS | CLERK   | 7788 | 1987-05-23 | 1100.00 | NULL |     20 |
|  7902 | FORD  | ANALYST | 7566 | 1981-12-03 | 3000.00 | NULL |     20 |
+-------+-------+---------+------+------------+---------+------+--------+

3.3 Pay more than 30 Employee information of department No

#3.3  Pay more than 30 Employee information of department No 
# First step ： Find out first 30 The highest salary in department No -->2850
SELECT MAX(sal) FROM emp WHERE `deptno`=30;
# The second step ： Pay more than 30 Employee information of department No 
SELECT * FROM emp WHERE `sal`>2850;
# Merge write ：
SELECT * FROM emp WHERE `sal`>(SELECT MAX(sal) FROM emp WHERE `deptno`=30);
# Another way of writing 
SELECT * FROM emp WHERE `sal`>ALL(SELECT MAX(sal) FROM emp WHERE `deptno`=30);

--> final result ：
mysql> SELECT * FROM emp WHERE `sal`>(SELECT MAX(sal) FROM emp WHERE `deptno`=30);
+-------+-------+-----------+------+------------+---------+------+--------+
| empno | ename | job       | mgr  | hiredate   | sal     | comm | deptno |
+-------+-------+-----------+------+------------+---------+------+--------+
|  7566 | JONES | MANAGER   | 7839 | 1981-04-02 | 2975.00 | NULL |     20 |
|  7788 | SCOTT | ANALYST   | 7566 | 1987-04-19 | 3000.00 | NULL |     20 |
|  7839 | KING  | PRESIDENT | NULL | 1981-11-17 | 5000.00 | NULL |     10 |
|  7902 | FORD  | ANALYST   | 7566 | 1981-12-03 | 3000.00 | NULL |     20 |
+-------+-------+-----------+------+------------+---------+------+--------+

3.4 Inquire about work and salary and MARTIN（ Martin ） Exactly the same employee information

#3.4  Inquire about work and salary and MARTIN（ Martin ） Exactly the same employee information 
# First step ： Inquire about MARTIN Work and salary  -->
SELECT job,sal FROM emp WHERE `ename`='MARTIN';
# The second step ： Inquire about work and salary and MARTIN（ Martin ） Exactly the same employee information 
SELECT * FROM emp WHERE `job`='SALESMAN' AND `sal`=1250.00;
# Merge write 
SELECT * FROM emp WHERE (job,sal) 
IN(SELECT job,sal FROM emp WHERE `ename`='MARTIN');

--> final result ：
mysql> SELECT * FROM emp WHERE (job,sal)
    -> IN(SELECT job,sal FROM emp WHERE `ename`='MARTIN');
+-------+--------+----------+------+------------+---------+---------+--------+
| empno | ename  | job      | mgr  | hiredate   | sal     | comm    | deptno |
+-------+--------+----------+------+------------+---------+---------+--------+
|  7521 | WARD   | SALESMAN | 7698 | 1981-02-22 | 1250.00 |  500.00 |     30 |
|  7654 | MARTIN | SALESMAN | 7698 | 1981-09-28 | 1250.00 | 1400.00 |     30 |
+-------+--------+----------+------+------------+---------+---------+--------+

3.5 Yes 2 Employee information of more than direct reports

#3.5  Yes 2 Employee information of more than direct reports 
# First step ： First, find out mgr More than twice -->7698 7839
SELECT mgr FROM emp GROUP BY mgr HAVING COUNT(mgr)>2
# The second step ： Yes 2 Employee information of more than direct reports 
SELECT * FROM emp WHERE `empno`=7698 OR `empno`=7839;
# Merge write 
SELECT * FROM emp WHERE empno IN(SELECT mgr FROM emp GROUP BY mgr HAVING COUNT(mgr)>2);

--> final result ：
mysql> SELECT * FROM emp WHERE empno IN(SELECT mgr FROM emp GROUP BY mgr HAVING COUNT(mgr)>2);
+-------+-------+-----------+------+------------+---------+------+--------+
| empno | ename | job       | mgr  | hiredate   | sal     | comm | deptno |
+-------+-------+-----------+------+------------+---------+------+--------+
|  7698 | BLAKE | MANAGER   | 7839 | 1981-05-01 | 2850.00 | NULL |     30 |
|  7839 | KING  | PRESIDENT | NULL | 1981-11-17 | 5000.00 | NULL |     10 |
+-------+-------+-----------+------+------------+---------+------+--------+

3.6 Query the employee number as 7788 The name of the employee 、 Wages 、 Department name 、 Department Address

#3.6  Query the employee number as 7788 The name of the employee 、 Wages 、 Department name 、 Department Address 
SELECT ename,sal,dname,loc FROM emp,dept WHERE emp.deptno=dept.deptno AND empno=7788
# Now? dept In this table, only 3 Column , And this 3 Columns are all we need . If dept This watch has 100 Column , We only need one deptno、dname、loc this 3 Column , So we're in from You can use subqueries later 
SELECT e.ename,e.sal,d.dname,d.loc
FROM emp e,(SELECT dname,loc,deptno FROM dept)d
WHERE e.deptno=d.deptno AND e.empno=7788

--> final result ;
mysql> SELECT e.ename,e.sal,d.dname,d.loc
    -> FROM emp e,(SELECT dname,loc,deptno FROM dept)d
    -> WHERE e.deptno=d.deptno AND e.empno=7788;
+-------+---------+----------+--------+
| ename | sal     | dname    | loc    |
+-------+---------+----------+--------+
| SCOTT | 3000.00 | RESEARCH | DALLAS |
+-------+---------+----------+--------+

Self join : Connect yourself , names

3.7 seek 7369 Employee number 、 full name 、 Manager number and manager name

#3.7  seek 7369 Employee number 、 full name 、 Manager number and manager name 
SELECT empno,ename,mgr,ename FROM emp WHERE empno=7369
SELECT ename FROM emp WHERE empno=7902 # Get the name of the manager according to the obtained manager number 


SELECT e1.empno AS ' Employee number ',e1.ename AS ' Employee name ',e1.mgr ' Employee's manager number ',e2.ename AS ' Employee's manager name ' FROM emp e1,emp e2 
WHERE e1.empno=7369 AND e1.mgr=e2.empno

--> final result ：
mysql> SELECT e1.empno AS ' Employee number ',e1.ename AS ' Employee name ',e1.mgr ' Employee's manager number ',e2.ename AS ' Employee's manager name ' FROM emp e1,emp e2
    -> WHERE e1.empno=7369 AND e1.mgr=e2.empno;
+----------+----------+----------------+----------------+
|  Employee number  |  Employee name  |  Employee's manager number  |  Employee's manager name  |
+----------+----------+----------------+----------------+
|     7369 | SMITH    |           7902 | FORD           |
+----------+----------+----------------+----------------+

3.8 Ask for all information about the highest paid employees in all departments

#3.7.2  Ask for all information about the highest paid employees in all departments 
# Calculate the maximum salary corresponding to each department according to the department number 
SELECT deptno,MAX(sal) FROM emp GROUP BY deptno;
# Not rigorous enough ： All information about the highest paid employees in all departments 
SELECT * FROM emp WHERE sal=5000 OR sal=3000 OR sal=2850 OR sal=1200;
SELECT * FROM emp WHERE sal IN(SELECT MAX(sal) FROM emp GROUP BY deptno)
# The following commands are recommended ： rigorous 
SELECT e1.* FROM emp e1,(SELECT MAX(sal) maxsal,deptno FROM emp GROUP BY deptno)d
WHERE e1.sal=d.maxsal AND e1.deptno=d.deptno;

--> final result 
mysql> SELECT e1.* FROM emp e1,(SELECT MAX(sal) maxsal,deptno FROM emp GROUP BY deptno)d
    -> WHERE e1.sal=d.maxsal AND e1.deptno=d.deptno;;
+-------+-------+-----------+------+------------+---------+------+--------+
| empno | ename | job       | mgr  | hiredate   | sal     | comm | deptno |
+-------+-------+-----------+------+------------+---------+------+--------+
|  7698 | BLAKE | MANAGER   | 7839 | 1981-05-01 | 2850.00 | NULL |     30 |
|  7788 | SCOTT | ANALYST   | 7566 | 1987-04-19 | 3000.00 | NULL |     20 |
|  7839 | KING  | PRESIDENT | NULL | 1981-11-17 | 5000.00 | NULL |     10 |
|  7902 | FORD  | ANALYST   | 7566 | 1981-12-03 | 3000.00 | NULL |     20 |
|  7988 | LISA  |  Procter & Gamble       | 7782 | 2022-07-22 | 1200.00 | NULL |     50 |
+-------+-------+-----------+------+------------+---------+------+--------+

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