SQL basic syntax II

1、 Multi-table query - Internal connection query

• Inner join queries are part of the data where two tables intersect ( Data associated with primary and foreign keys )

--  Explicit inner connection 
SELECT  Name  FROM  Table name 1 [INNER] JOIN  Table name 2 ON  Conditions ;

--  example ：
--  Query user name , Age and order number 
SELECT
	u.`name`,	--  full name 
	u.`age`,	--  Age 
	o.`number`	--  The order no. 
FROM
	USER u          --  User table 
JOIN
	order o     --  The order sheet 
ON 
	u.`id` = o.`uid`;


--  Implicit inner join 
SELECT  Name  FROM  Table name 1, Table name 2 WHERE  Conditions ;

--  Query user name , Age . And order number 
SELECT
	u.`name`,	--  full name 
	u.`age`,	--  Age 
	o.`number`	--  The order no. 
FROM
	USER u,		--  User table 
	order o     --  The order sheet 
WHERE
	u.`id`=o.`uid`;

2、 Multi-table query - External connection query

• The left outer join
  • Query principle
    • Query all the data in the left table , And the left and right tables have the intersection part of the data

--  Standard grammar  OUTER It can be omitted 
SELECT  Name  FROM  Table name 1 LEFT [OUTER] JOIN  Table name 2 ON  Conditions ;


--  Query all user information , And the corresponding order information of the user 
SELECT
	u.`name`,	--  full name 
	u.`age`,	--  Age 
	o.`number`	--  The order no. 
FROM
	USER u          --  User table 
LEFT JOIN
	order o     --  The order sheet 
ON
	u.`id`=o.`uid`;

• Right connection
  • Query principle
    • Query all the data in the right table , And the left and right tables have the intersection part of the data

--  Basic grammar  OUTER It can be omitted 
SELECT  Name  FROM  Table name 1 RIGHT [OUTER] JOIN  Table name 2 ON  Conditions ;

--  Query all order information , And the user information of the order 
SELECT
	u.`name`,	--  full name 
	u.`age`,	--  Age 
	o.`number`	--  The order no. 
FROM
	USER u          --  User table 
RIGHT JOIN
	order o     --  The order sheet 
ON
	u.`id`=o.`uid`;

3、 Multi-table query - Subquery

• Subquery Introduction
  • The query statement is nested in the query statement . We call nested queries subqueries ！
• Subquery - The result is a single row, single column
  • As a condition , Use operators to judge ！

--  Standard grammar 
SELECT  Name  FROM  Table name  WHERE  Name =(SELECT  Aggregate functions ( Name ) FROM  Table name  [WHERE  Conditions ]);

--  Query the name of the oldest user 
SELECT NAME,age FROM USER WHERE age = (SELECT MAX(age) FROM USER);

• Subquery - The result is multi row, single column

  • As a condition , Use the operator in or not in Judge ！

--  Standard grammar 
SELECT  Name  FROM  Table name  WHERE  Name  [NOT] IN (SELECT  Name  FROM  Table name  [WHERE  Conditions ]); 

--  Check the order information of Zhang San and Li Si 
SELECT number,uid FROM order
WHERE uid IN (SELECT id FROM USER WHERE NAME=' Zhang San ' OR NAME=' Li Si ');

• Subquery - The result is multi row, multi column

  • It can be used as a virtual table to participate in the query ！

--  Standard grammar 
SELECT  Name  FROM  Table name  [ Alias ],(SELECT  Name  FROM  Table name  [WHERE  Conditions ]) [ Alias ] [WHERE  Conditions ];

--  Query the order table id Greater than 4 Order information and user information of 
SELECT * FROM USER u,(SELECT * FROM order WHERE id>4) o WHERE u.id=o.uid;

4、 Multi-table query - Self association query

• Introduction to self association query
  • There are data associations in the same table . You can query the same table multiple times ！

--  Create an employee table 
CREATE TABLE employee(
	id INT PRIMARY KEY AUTO_INCREMENT,
	NAME VARCHAR(20),
	mgr INT,
	salary DOUBLE
);
--  Add data 
INSERT INTO employee VALUES (1001,' The Monkey King ',1005,9000.00),
(1002,' Pig eight quit ',1005,8000.00),
(1003,' The sand monk ',1005,8500.00),
(1004,' Small white dragon ',1005,7900.00),
(1005,' Tang's monk ',NULL,15000.00),
(1006,' Wusong ',1009,7600.00),
(1007,' Li Kui ',1009,7400.00),
(1008,' Lin Chong ',1009,8100.00),
(1009,' Song Jiang ',NULL,16000.00);

--  Check the names of all employees and their immediate superiors , Employees without superiors also need to inquire 
/*  analysis ：  Employee name  employee surface   The name of the immediate superior  employee surface   Conditions ：employee.mgr = employee.id  Query all the data in the left table , And the left and right tables intersect some data , Use left outer connection  */
SELECT
	t1.name,	--  Employee name 
	t1.mgr,		--  Superior number 
	t2.id,		--  Employee number 
	t2.name     --  Employee name 
FROM
	employee t1  --  The employee table 
LEFT OUTER JOIN
	employee t2  --  The employee table 
ON
	t1.mgr = t2.id;