MySQL learns the relationship between Day2 Sorting Query / aggregation function / grouping query / paging query / constraint / multiple tables

One 、DQL Query statement

1.1 Sort query

SELECT * FROM student ORDER BY math; --  The default is ascending  ASC
SELECT * FROM student ORDER BY math DESC; --  Descending 

--  Rank according to math scores , If the math scores are the same   They are ranked according to their English scores 
SELECT * FROM student ORDER BY math ASC,english ASC;

1.2 Aggregate functions

SELECT COUNT(NAME) FROM student;
SELECT COUNT(IFNULL (english,0))   The number of  FROM student;

SELECT MIN(math) FROM student;
SELECT MAX(english) FROM student;
SELECT SUM(english) FROM student;
SELECT AVG(math) FROM student;

1.3 Group query

--  Group by sex , Check the men separately 、 The average score of female students 
SELECT sex,AVG(math)FROM student GROUP BY sex;
--  Group by sex , Check the men separately 、 Number of female students 
SELECT sex,AVG(math),COUNT(id)FROM student GROUP BY sex;

--  Group by sex , Check the men separately 、 The average score of female students , The score is below 70 Points do not participate in grouping 
SELECT sex,AVG(math),COUNT(id) FROM student WHERE math > 70 GROUP BY sex;

--  Group by sex , Check the men separately 、 The average score of female students , The score is below 70 Points do not participate in grouping , After grouping, the number of people should be greater than 2 personal 
SELECT sex,AVG(math),COUNT(id) FROM student WHERE math > 70 GROUP BY sex HAVING COUNT(id)>2;

SELECT sex,AVG(math),COUNT(id)   The number of  FROM student WHERE math > 70 GROUP BY sex HAVING  The number of >2;

1.4 Paging query

--  Three records per page 
SELECT *FROM student LIMIT 0,3;  --  first page 

SELECT * FROM student LIMIT 3,3; --  The second page 
SELECT * FROM student LIMIT 6,3; --  The third page 
--  The formula ：  Index started  = （ The current page number  -1） *  Number of entries per page 

Two 、 constraint

2.1 summary

constraint ： Limit the data in the table , Make sure the data is correct 、 Effectiveness and integrity

2.2 Non empty constraint

--  Create a table and add a non empty constraint 
CREATE TABLE stu(
	id INT,
	NAME VARCHAR(20) NOT NULL
);

--  Delete name Non empty constraint of 
ALTER TABLE stu MODIFY NAME VARCHAR(20);


--  Add a non empty constraint after creating the table 
ALTER TABLE stu MODIFY NAME VARCHAR(20) NOT NULL;
SELECT * FROM stu;

2.2 Unique constraint

--  Create a table   Add unique constraints 
CREATE TABLE stu2(
	id INT,
	phone_number VARCHAR(20) UNIQUE --  Added a unique constraint 
);
--  Be careful mysql in , There can be multiple column values limited by unique constraints null

--  Delete unique constraint 
ALTER TABLE stu2 DROP INDEX phone_number ;

--  Add a unique constraint after creating the table 
ALTER 	TABLE stu2 MODIFY phone_number VARCHAR(20) UNIQUE;


SELECT	* FROM stu2;

2.3 Primary key constraint

CREATE TABLE stu1(
	id INT PRIMARY KEY, --  to id Add primary key constraint 
	NAME VARCHAR(20)
);

SELECT *FROM stu1;

--  Delete gradually 
ALTER TABLE stu1 DROP PRIMARY KEY;

--  Add the primary key after creating the table 
ALTER TABLE stu1 MODIFY id INT PRIMARY KEY;

2.3 Primary key constraint — Automatic growth

CREATE TABLE stu2(
	id INT PRIMARY KEY AUTO_INCREMENT, --  to id Add primary key constraint and automatic growth 
	NAME VARCHAR(20)
);

--  Delete auto growth 
ALTER TABLE stu2 MODIFY id INT;
--  Add autogrow 
ALTER TABLE stu2 MODIFY id INT AUTO_INCREMENT;


SELECT *FROM stu2;

INSERT INTO stu2 VALUES(NULL,'bbb'); --  Automatic growth is only related to the previous line 

2.4 Foreign key constraints

--  Solution ： Divide into two tables 
--  Create department table （id,dep_name,dep_location）
--  a party , Main table 

CREATE TABLE department(
	id INT PRIMARY KEY AUTO_INCREMENT,
	dep_name VARCHAR(20),
	dep_location VARCHAR(20)
);
--  Create an employee table （id,name,age,dep_id）
--  In many ways , From the table 
CREATE TABLE employee(
	id INT PRIMARY KEY AUTO_INCREMENT,
	NAME VARCHAR(20),
	age INT,
	dep_id INT, --  The foreign key corresponds to the primary key of the primary table 
	CONSTRAINT emp_dept_fk FOREIGN KEY (dep_id) REFERENCES department(id)

);
--  Add two departments 
INSERT INTO department VALUES (NULL,' R & D department ',' Guangzhou '),(NULL,' The sales department ',' Shenzhen ');
--  Add employees ,dep_id  Indicates the employee's Department 

INSERT INTO employee (NAME,age,dep_id) VALUES (' Zhang San ',20,1);
INSERT INTO employee (NAME,age,dep_id) VALUES (' Li Si ',21,1);
INSERT INTO employee (NAME,age,dep_id) VALUES (' Wang Wu ',20,1);
INSERT INTO employee (NAME,age,dep_id) VALUES (' Lao Wang ',20,2);
INSERT INTO employee (NAME,age,dep_id) VALUES (' King ',22,2);
INSERT INTO employee (NAME,age,dep_id) VALUES (' Xiao Wang ',18,2);


SELECT * FROM employee;

--  Delete foreign key 
ALTER TABLE employee DROP FOREIGN KEY emp_dept_fk;

--  Add foreign keys 
ALTER TABLE employee ADD CONSTRAINT emp_dept_fk FOREIGN KEY (dep_id) REFERENCES department(id)

2.4 Foreign key constraints – Cascade operation

-- ON UPDATE CASCADE  update cascade 
-- ON DELETE CASCADE  cascading deletion 
--  Delete foreign key 
ALTER TABLE employee DROP FOREIGN KEY emp_dept_fk;
--  Add foreign keys   Set cascading updates 
ALTER TABLE employee ADD CONSTRAINT emp_dept_fk FOREIGN KEY 
(dep_id) REFERENCES department(id) ON UPDATE CASCADE;

--  Add foreign keys   Set cascading updates   cascading deletion 
ALTER TABLE employee ADD CONSTRAINT emp_dept_fk FOREIGN KEY 
(dep_id) REFERENCES department(id) ON UPDATE CASCADE ON DELETE CASCADE;

3、 ... and 、 Relationships between multiple tables

3.1 Introduction to multi table relationship

3.2 One to many relationship implementation

3.3 Many to many relationship implementation

3.4 One to one relationship

3.5 Multi table relation cases

--  Create a travel route classification table  tab_category
-- cid  Classification of tourist routes , Automatic growth 
-- cname  Tourism route classification name is not empty , only , character string  100
CREATE TABLE tab_category(
	cid INT PRIMARY KEY AUTO_INCREMENT,
	cname VARCHAR(100) NOT NULL UNIQUE 
);
  Create travel itineraries tab_route
/* rid  Tourist routes , Automatic growth  rname  The name of tourist route is not empty , only , character string 100 price  Price  rdate  Practice on shelves , The date type  cid  Foreign keys , Classification  */
 CREATE TABLE tab_route(
	rid INT PRIMARY KEY AUTO_INCREMENT,
	rname VARCHAR(100) NOT NULL UNIQUE,
	price DOUBLE,
	rdate DATE,
	cid INT,
	FOREIGN KEY (cid) REFERENCES tab_category(cid)
 );
 
 /*  Create a user table tab_user uid  With the head of the household key , Self growth  username  User name length 100, only , Non empty  password  Password length 30, Non empty  name  Real name length 100 birthday  Birthday  sex  Gender  telephone  cell-phone number , character string 11 email  mailbox , String length 100 */
CREATE TABLE tab_user(
	uid INT PRIMARY KEY AUTO_INCREMENT,
	username VARCHAR(100) NOT NULL UNIQUE,
	PASSWORD VARCHAR(30) NOT NULL,
	NAME VARCHAR(100),
	birthday DATE,
	sex CHAR(1) DEFAULT ' male ',
	telephone VARCHAR(11),
	email VARCHAR(100)
);
 
 /*  Create a collection table  tab_favorite rid  Travel routes id, Foreign keys  date  Collection practice  uid  user id, Foreign keys  rid  and uid Can't repeat , Set the composite key , The same user cannot collect the same line twice  */
 CREATE TABLE tab_favorite(
	 rid INT,--  line id
	 DATE DATETIME,
	 uid INT, --  user id
	 --  Create composite keys 
	 PRIMARY KEY(rid,uid), --  Combined the primary key 
	 FOREIGN KEY (rid) REFERENCES tab_route(rid),
	 FOREIGN KEY (uid) REFERENCES tab_user(uid)
);


 

Four 、 normal form

4.1 Paradigm overview

4.2 Detailed explanation of three paradigms

The problem is :
1. There is very serious data redundancy ( repeat ): full name 、 Department name 、 Department of Wang Ren
2. There is a problem adding data : When adding a new department and Dean , Illegal data
3. There is a problem with data deletion : Zhang Wuji graduated , Delete data , The data of the system - Delete... Together .
After using the second paradigm to solve the problem

Following the third paradigm , All the problems have been solved

5、 ... and 、 Restore and backup of database

5.1 Command line / Graphical tool backup