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1、 database (database)： It's organized according to the data structure 、 Warehouse for storing and managing data In short A database is a container for storing data

2、DBMS（DataBase Management System）： Database management system Database software
Common database relation software ：Mysql Oracle DB2 mongoDB SQLServer

3、sql： Structured query language SQL Is a standard computer language for accessing and processing databases .

MySQL Is the most popular relational database management system , stay WEB Application aspect MySQL It's the best RDBMS(Relational Database Management System： Relational database management system ) One of the application software .

1.2、mysql The advantages of

Open source
High performance Fast execution
Easy to use

1.3、mysql Common commands

        show databases;            Query the database
       use Library name ; Using a database
       show tables; Query table
       desc Table name ; View table structure
       create table Table name （ Name Column type , Name Column type ...）; Create table

1.4、SQL The classification of languages

        DDL    Data definition language    Used to define a database surface Column, etc. create drop alter etc.
       DML   Data operation language    Add, delete and modify the data in the table
       DQL   Data query language    Query data in table
       DCL   Data control language    Define the number of access permissions

Two 、 operation mysql

2.1、DDL （Data Definition language）

Database schema definition language DDL(Data Definition Language)

2.1.1、 Operating the database （CRUD）

1、C（create） Create database ：

--  Create database 
 create database name;
--  Create a database if the database does not exist 
 create database if not exist name;
--  Create database and specify character set 
create database name character set  Character set name ;

create database zhouhao if not exists character set gbk;

2、R（Retrieve） Inquire about

 show databases; --  Query all databases 
 shou create database name; --  Query the character set of a database & Create statement

3、U（Update） modify

 --  Modify the character set of the database 
ALTER DATABASE zhou CHARACTER SET gbk;
ALTER DATABASE zhou CHARACTER SET utf8;

4、D（Delete） Delete

--  Delete database 
drop database name;
--  Delete if it exists 
drop database if exists name;

5、 Using a database

--  Query the name of the currently used database 
select database();
--  Change the database 
use name;

2.1.2、 Operation data sheet （CRUD）

1、 Create table ：

 /*
  create table tablename( Name   type , Name   type ...)

   Database type ：
    int         integer 
    double         floating-point 
    date          date   Include only mm / DD / yyyy  yyyy-MM-dd
    datetime      date   Mm / DD / yyyy HHM / S  yyyy-MM-dd HH:mm:ss
    timestamp      Time stamp   Mm / DD / yyyy HHM / S   If you do not assign a value to this field in the future or assign a value of null Then the current time is automatically assigned by default 
    varchar       character string  
*/

--  Create a person surface 
CREATE TABLE person(
	id INT PRIMARY KEY AUTO_INCREMENT,
	NAME VARCHAR(55),
	email VARCHAR(20),
	addr VARCHAR(20)
);
DESC person;

2、 Inquire about ：

--  Query all the tables in the database 
show tables;
        
--  Query table structure  
desc tablename;

3、 modify :
Modify the name of the table

Modify the character set of the table

Add columns Modify the column Delete column

--  Modify the name of the table 
ALTER TABLE  Table name  RENAME TO  The new name of the table ;

--  Modify the subject character set 
ALTER TABLE  Table name  CHARACTER SET  Character set ;

--  Add a column 
ALTER TABLE  Table name  ADD  Name   data type ;

--  Modify the column 
ALTER TABLE  Table name  CHANGE  Name   New column names   New data types ;
ALTER TABLE  Table name  MODIFY  Name   New data types 

--  Delete column 
ALTER TABLE  Table name  DROP  Name ;

4、 Delete :

drop table name;
drop table name if exists;

5、 Copy table ：

create table  Table name 1 like  The name of the copied table ;

2.2、DML

Database operation language In short, it is to operate on the data in the table （CRUD）1、 Add data

/*  Adding data in the table uses  insert keyword  
	 grammar ：insert into tablename ( Name 1, Name 2...) values( value 1, value 2...)  
*/
-- 1、 towards person Add a piece of data to the table 
INSERT INTO person (id,NAME,email,addr)VALUES(NULL,' Zhang San ','[email protected]',' hunan ');


-- 2、 Match and add data in order   Only values  however values The data in should match the table fields one by one 
INSERT INTO person VALUES(2," Li Si ","[email protected]"," Guangzhou ");

-- 3、 Match according to column name  
INSERT INTO person(NAME,addr,email) VALUES(" zhang wuji ",' sichuan ','[email protected]');

-- 4、 Add the results of the query to the table  
INSERT INTO student SELECT * FROM stu;
INSERT INTO student(id,NAME,age) SELECT id,NAME,age FROM stu;

2、 Modifying data

--  Modifying data  
UPDATE person SET NAME = 'zhangsan' WHERE id = 1;
--  If there is no condition   Then modify all the data in the table

3、 Delete data

--  Delete data   Use delete keyword 
--  Basic grammar  delete from  Table name  [where Conditions ]  The conditions are optional 

DELETE FROM person WHERE id=1;
--  If there is no condition   Then delete all data in the table 

/*  There are two ways to delete all records ：
    1、delete from  Table name ;  Not recommended   How many pieces of data will be deleted many times   Low efficiency 
    2、truncate table  Table name ;  Delete the table first   Create an identical empty table   Efficient   Only delete   There are two operations to create a table 
*/

2.3、DQL

Data query language

Inquire about keyword select

Common query
- Query a single field
- Query multiple fields
- Query all fields
- names
- duplicate removal
Conditions of the query
- where Conditions
  - Conditional expression
  - Logical expression
- Fuzzy query like Generally used with wildcards
Sort query
- order by Field asc ( Ascending ) desc( Descending )
Aggregate functions
- count Count
- max Maximum
- min minimum value
- avg Average
- sum Sum up
Group query
- group by Field
Paging query limit

Sample code ：

Common query

--  Query field 
SELECT NAME FROM person;

--  Query multiple fields 
SELECT NAME,email FROM person;

--  Query all fields 
SELECT * FROM person;

--  names   Use AS  Or blank space   To express an alias 
SELECT NAME AS  full name  FROM person;
SELECT NAME   full name  FROM person;

--  duplicate removal 
SELECT DISTINCT NAME AS  full name  FROM person;

Conditions of the query

--  Conditions of the query 
SELECT NAME AS  full name  FROM person WHERE id = 2;

/*  Conditional expression filter 
	> < <= >= != (<>) It's not equal to  
*/
SELECT NAME AS  full name  FROM person WHERE id > 1; 

/*  Logical expression filtering 
	and or not
*/	
SELECT NAME AS  full name  FROM person WHERE id > 1 AND id < 10; 
SELECT NAME  full name ,email AS  mailbox  FROM person WHERE id = 2 OR id = 3;

Fuzzy query

/*  Fuzzy query 
	like  Generally used with wildcards 
	betwwen and 
	in
	is null
*/
SELECT NAME FROM person WHERE NAME LIKE '% Four %';



/*  Sort query 
 grammar ：
	select  Query list 
	from  surface 
	[where  filter ]
	order by  Sort list  [asc( Ascending )|desc（ Descending ）
	
*/
SELECT * 
FROM person
WHERE id > 2
ORDER BY id DESC;

Aggregate functions

/*  Aggregate functions 
	1、count   Calculate the number of data   Generally, non empty columns are selected   Primary key   perhaps  count（*） 
	2、max
	3、min
	4、sum
	5、avg  

 Note that when calculating the aggregate function, the column with null The data of  
	 Solution  ：
		1、 Choose not null Columns of values 
		2、 Use ifnull function 
*/
SELECT COUNT(addr) FROM person; -- 5
SELECT COUNT(NAME) FROM person; -- 8

SELECT MAX(english) AS  The highest score in English  FROM student;

SELECT MAX(math) AS  The highest score in math  FROM student;

SELECT SUM(math) FROM student;

SELECT AVG(math) FROM student;

Group query Paging query

/*  Group query 
	group by  Grouping field 
*/
SELECT * FROM person GROUP BY addr;

/*  Paging query 
	limit  The starting search quote   Query conditions per page 
*/		
		
SELECT * FROM person LIMIT 0,2; # first page   One page shows two pieces of data 
SELECT * FROM person LIMIT 2,2; #  The second page 
SELECT * FROM person LIMIT 4,2; #  The third page

3、 ... and 、 constraint

Concept ： Limit the data in the table Ensure the validity of data correctness integrity

classification ： Primary key constraint Non empty constraint Unique constraint Foreign key constraints

1、 Primary key constraint

primary key
meaning ： Non empty and unique
A table can only have one primary key The primary key is the unique identifier in the table

Primary key （PRIMARY KEY） The full title of the title is “ Primary key constraint ”, yes mysql The most frequently used constraint in . In general , For convenience DBMS Find records in the table faster , A primary key will be set in the table .

matters needing attention ：

Only one primary key can be defined for each table .
The primary key value must uniquely identify each row in the table , And cannot be NULL, That is, two rows of data with the same primary key value cannot exist in the table . This is the principle of uniqueness .
A field name can only appear once in the federated primary key field table .
The federated primary key cannot contain unnecessary redundant fields . When a field in the union primary key is deleted , If the primary key composed of the remaining fields still satisfies the uniqueness principle , Then the joint primary key is incorrect . This is the minimization principle .

#  When creating a table, specify the primary key 
CREATE TABLE person (id INT PRIMARY KEY,NAME VARCHAR(20));

#  Delete primary key 
ALTER TABLE person DROP PRIMARY KEY;

#  Add primary key 
ALTER TABLE person MODIFY id INT PRIMARY KEY;

2、 Non empty constraint

not null
Indicates that the value of a specified column is non empty That is, it cannot be null

For fields with non null constraints , If the user does not specify a value when adding data , The database system will report an error . Can pass CREATE TABLE or ALTER TABLE Statements for . Add a keyword after the definition of a column in the table NOT NULL As a determiner , To constrain that the value of the column cannot be empty .

such as , In the user information table , If you don't add a user name , Then this user information is invalid , In this case, you can set a non empty constraint for the user name field .

#  When creating a table, specify a non empty constraint 
CREATE TABLE person (id INT,NAME VARCHAR(20) NOT NULL);
SELECT * FROM person;

#  Create table   Change the column to specify a non empty constraint 
ALTER TABLE person MODIFY id INT NOT NULL;

#  Create table   Delete non empty constraints 
ALTER TABLE person MODIFY id;

3、 Unique constraint

unique

The value of a column cannot be repeated Unique constraint can have null value

Unique constraint （Unique Key） It means that the values of fields in all records cannot be repeated . for example , by id After the uniqueness constraint is added to the field , For each record id Value is the only , There can be no repetition . If one of the records says id The value is ‘1’, Then there is no other record in the table id Value for ‘1’.

Unique constraints are similar to primary key constraints in that they can ensure the uniqueness of columns . The difference is , A unique constraint can have more than one in a table , And the columns with unique constraints are allowed to have null values , But there can only be one null value . The primary key constraint can only have one in a table , And no null value is allowed . such as , In the user information table , To avoid duplicate user names in the table , You can set the user name as a unique constraint .

#  Specify unique constraints when creating tables 
CREATE TABLE person(id INT,phonenumber VARCHAR(20) UNIQUE);

#  Create table   Change the column to specify a unique constraint 
ALTER TABLE person MODIFY id INT UNIQUE;

#  Create table   Delete unique constraint   The syntax of deletion is different from the above   A special 
ALTER TABLE person DROP INDEX id;

4、 Foreign key constraints

foreign key

mysql Of Foreign key constraints （FOREIGN KEY） Is a special field of the table , Often used with primary key constraints . For two tables that have an association , In the associated field The table where the primary key is located is the primary table （ Parent table ）, The table where the foreign key is located is the slave table （ Sub table ）.
Foreign keys are used to establish the relationship between the master table and the slave table , Establish a connection between the data of two tables , Constrains the consistency and integrity of data in two tables .

such as , A fruit stand , Only apples 、 Peach 、 Plum 、 Watermelon etc. 4 Kind of fruit , that , When you come to the fruit stand to buy fruit, you can only choose apples 、 Peach 、 Plum and watermelon , No other fruit can be bought .

When a record is deleted from the main table , The corresponding records from the table must be changed accordingly .
A table can have one or more foreign keys , The foreign key can be null , If not null , Then the value of each foreign key must be equal to a value of the primary key in the primary table .

#  Add foreign keys when creating tables 
CREATE TABLE emp(
    id INT PRIMARY KEY AUTO_INCREMENT,
    NAME VARCHAR(20),
    age INT,
    dept_id INT,
    CONSTRAINT emp_dept_id FOREIGN KEY (dept_id) REFERENCES dept(id)
);

SELECT * FROM emp;

#2.  Delete foreign key 
ALTER TABLE emp DROP FOREIGN KEY emp_dept_id;

#3.  After creating the table , Add foreign keys 
ALTER TABLE emp ADD CONSTRAINT emp_dept_id FOREIGN KEY (dept_id) REFERENCES dept(id);

Foreign key constraint considerations ：

The primary table must already exist in the database , Or the table that is currently being created . In the latter case , Then the master table and the slave table are the same table , Such a table is called a self reference table , This structure is called self referential integrity .
The primary key must be defined for the primary table .
Primary key cannot contain null value , But null values are allowed in foreign keys . in other words , As long as each non null value of the foreign key appears in the specified primary key , The content of this foreign key is correct .
Specify the column name or combination of column names after the table name of the main table . This column or combination of columns must be the primary key or candidate key of the main table .
The number of columns in the foreign key must be the same as the number of columns in the primary key of the primary table .
The data type of the column in the foreign key must be the same as that of the corresponding column in the primary key of the primary table .

5、 Cascade operation

Associated foreign key Master table update The foreign key table will also be updated

#  Add foreign keys to set cascading updates  
ALTER TABLE emp ADD CONSTRAINT emp_dept_id 
FOREIGN KEY (dept_id) REFERENCES dept(id)
ON UPDATE CASCADE  --  Auto update 
ON DELETE CASCADE  --  Automatically delete

Cascade operation means , When you operate the main table , Automatic operation from table

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