Mysql database (I)

1, The role of database

（1） Data storage ： A large number of data generated by the program are saved when the program runs and after the program ends .

（2） Data integrity ： Connect data and the structure between data , Connect dependencies between data and programs .

（3） Data reading ： In order to make the written data more convenient to read .

（4） Data security ： Prevent data loss caused by external factors .

（5） structured ： The storage of data in the database can rely on the logic of two-dimensional table structure to store data , You can refer to the original dependency and institutional relationship of data to store data .

（6） independence ： The data stored in the database and the application are independent of each other , They don't influence each other .

（7） participatory ： Multiple users can jointly analyze the data resources in the computer database , The same data can be used by multiple people at the same time , So as to achieve efficient database sharing .

（8） Security ： Password of the user when accessing the database , Restrict the user's permission , Or limit the storage of data .

2, The concept of databases

         Keep it in a computer for a long time , Organized 、 A collection of large amounts of data that can be shared , It's a number “ Warehouse ”.

        effect ： Deposit 、 Management data .

        classification ： Relational database 、NoSQL database

3,MySQL The characteristics of database

        （1） Easy to operate

        （2） small , The function is all ready

        （3） free , Open source databases

        （4） Operable and windows or linux System

4, database structure         

database (Database)	Stored on disk as a file , That is, it corresponds to one or more physical files .
Field (Field)	Also called domain . Each column in the table is called a field . Each field has a corresponding description .
Indexes (Index)	An index is actually a special type of table , It contains the value of the keyword and the pointer to the actual record position , It can improve the efficiency of accessing the database .
Data sheet (Table)	Abbreviation table , It consists of a set of data records , The data in the database is organized in tables . A table is a set of related data arranged in rows ; Each table contains the same type of information .
Record (Record)	Each row in the table is called a record , It consists of several fields .
SQL sentence	Structured query statement commands , Used to get a set of specified records from one or more tables , Or perform the specified operation on a table .

5,SQL Sentence classification

name	explain	command
DDL ( Data definition language )	Define and manage data objects , Such as a database , Data sheets, etc .	CREATE	establish
		DROP	Delete
		ALTER	modify
DML ( Data operation language )	Operate the data contained in the database object .	INSERT	insert data
		UPDATE	Update data
		DELETE	Delete data
DQL ( Data query language )	Used to query database data	SELECT	Inquire about
DCL ( Data control language )	The language used to manage the database , Including management authority and data change	GRANT	to grant authorization
		COMMIT	Submit
		ROLLBACK	Roll back

6,DDL Statement operation database

        （1） Create database         CREATE DATABASE Database name ;

        （2） Delete database         DROP DATABASE Database name ;

        （3） view the database         SHOW DATABASES;

        （4） Select database         USE Database name ;

--  Create database 
CREATE DATABASE day001;
--  Delete database 
DROP DATABASE day001;
--  view the database 
SHOW DATABASES;
--  Select database 
USE day001;

7,DDL Create database tables

grammar ：

        CREATE TABLE [ IF NOT EXISTS ] ` Table name ` (

                ` Field name 1` Field type [ attribute ] [ Indexes ] [ notes ] ,

                ` Field name 2` Field type [ attribute ] [ Indexes ] [ notes ] ,

                … …

                ` Field name n` Field type [ attribute ] [ Indexes ] [ notes ]

        ) [ Table type ] [ Table character set ] ;

among ："[]" The contents contained can be omitted

           "`" The current symbol is a backquote , Used to distinguish MySQL Reserved words and ordinary characters are introduced .

8, Field type of database table

        Type at character

type	explain	Value range	Storage requirements
char[(M)]	Fixed long string , Retrieval is fast, but it takes space , 0 <= M <= 255	M character	char[(M)]
varchar[(M)]	Variable string ,0 <= M <= 65535	Variable length	varchar[(M)]
tinytext	Micro text string	16777215 length +3 Bytes	tinytext
text	Text string （4 individual G Size left and right ）	4294967295 length +4 Bytes	text

        char and varchar Compare

type	characteristic	In space	On the time	Use scenarios
char(M)	Fixed length	Waste storage space	Efficient	Not much storage , High speed requirement
varchar(M)	Variable length	Save storage space	Low efficiency	Not CHAR The situation of

        value type

tinyint	Very small data	There is a signed value ： -2 ^7 ~ 2^7-1, Unsigned value ：0 ~ 28-1	1 byte
smallint	Smaller data	There is a signed value ： -2 15 ~ 215 -1 , Unsigned value ： 0 ~ 216 -1	2 byte
mediumin	Medium data	There is a signed value ： -2 23 ~ 223 -1 , Unsigned value ： 0 ~ 224 -1
int	Integers	There is a signed value ： -2^31 ~ 2^31-1, Unsigned value ：0 ~ 2^32-1	4 byte
bigint	A large integer	There is a signed value ： -2^63 ~2^63-1, Unsigned value ：0 ~2^64-1	8 byte
float	Single-precision floating-point	±1.1754351e -38	4 byte
double	Double precision floating point	±2.2250738585072014e -308	8 byte
Decimal	Floating point numbers in string form	decimal(m, d)	m Bytes

        Date and time type

type	explain	Value range	Storage requirements
DATE	YYYY-MM-DD, Date format	1000-01-01~ 9999-12-31	DATE
TIME	Hh:mm:ss , Time format	-838:59:59~838:59:59	TIME
DATETIME	YY-MM-DD hh:mm:ss	1000-01-01 00:00:00 to 9999-12-31 23:59:59	DATETIM
TIMESTAMP	YYYYMMDDhhmmss Time in format	197010101000000 ~2037 Some time in the year	TIMESTAMP
YEAR	YYYY The year value of the format	1901~2155	YEAR

9, Database field comments

        CREATE TABLE [ IF NOT EXISTS ] ` Table name ` (

                ` Field name 1` Field type [ attribute ] [ Indexes ] [ notes ] ,

                ` Field name 2` Field type [ attribute ] [ Indexes ] [ notes ] ,

                … …

                ` Field name n` Field type [ attribute ] [ Indexes ] [ notes ]

        ) [ Table type ] [ Table character set ] ;

10, Query the database table structure created

Mode one ：describe Table name     or      desc        Table name         

DESCRIBE teacher;
DESC teacher;

Mode two ：show create table Table name

SHOW CREATE TABLE teacher;

11, Modify and delete database tables

（1） Modify the name of the table ALTER TABLE The old name of the table RENAME AS The new name of the table ;

（2） Add fields  ALTER TABLE Table name ADD Field name Column type [ attribute ];

（3） Delete field  ALTER TABLE Table name DROP Field name ;

（4） Modify fields  

                Mode one ：ALTER TABLE Table name MODIFY Field name Column type [ attribute ];

                Mode two ：ALTER TABLE Table name CHANGE Old field name new field name data type attribute ;        

（5） Delete table  DROP TABLE Table name ;

--  Modify the name of the table 
ALTER TABLE teacher RENAME AS laoshi;
--  Add fields 
ALTER TABLE teacher ADD t_phone CHAR(11) COMMENT ' Teacher's phone number ';
--  Delete field （ Dangerous operation ）
ALTER TABLE teacher DROP tea_name;

--  Modify fields   Substitution , If you don't write, the default value will be used 
--  Data can be converted to modified data types , The length meets the modified length .

--  The way 1：modify
ALTER TABLE teacher MODIFY t_phone CHAR(20);

--  The way 2：change
-- alter table teacher change  Old field name   new field name   data type   attribute ;
ALTER TABLE teacher CHANGE t_phone dianhua CHAR(11) COMMENT ' Telephone ';

--  Delete table （ Dangerous operation ）
DROP TABLE teacher;

12, constraint

Concept ： Constraints are actually constraints on the data in the table

effect ： The purpose of adding constraints in the table design is to ensure that the records in the table are complete and effective

species ：

         Non empty constraint (not null)

         Uniqueness constraint (unique)

         Primary key constraint (primary key) PK

         Foreign key constraints (foreign key) FK

         Check constraint ( at present MySQL I won't support it 、Oracle Support )

13, Non empty constraint

        Concept ： use not null Constraint field cannot be null value , Specific data must be given

--  Non empty constraint 
CREATE TABLE teacher2(
	t_name VARCHAR(10) NOT NULL DEFAULT ' A passer-by ',
	t_age TINYINT
);

  If the field is empty, an error will be reported

14, Unique constraint

        Concept ：unique Constrained fields , Have uniqueness , Do not repeat , But it can be null        

--  Unique constraint 
--  Mode one ： Row level unique constraint 
CREATE TABLE teacher3(
	t_name VARCHAR(10) UNIQUE,
	t_age TINYINT
);

--  Mode two ： Table level unique constraint 
CREATE TABLE teacher4(
	t_name VARCHAR(10),
	t_age TINYINT,
--  Table level constraints name constraints 
	CONSTRAINT uniqu_tname_tage UNIQUE(t_name,t_age) --  Union sole constraint （ As like as two peas , It is a violation of the only constraint ）
);

If the fields are the same, an error will be reported

 15, Primary key constraint

Concept ： Primary key (primary key) Is one or more fields in a table , Its value is used to uniquely identify a record in the table

Primary key constraints and “not null unique” difference

1, As Primary Key The domain of / Domain group cannot be null, and Unique Key Sure .

2, There can only be one... In a table Primary Key, The multiple Unique Key Can exist at the same time .unique not null You can define one or more columns of a table as unique attributes , and primary key When set to multiple columns , Only the sum of multiple columns can be guaranteed to be unique , Specific to a column may be repeated .

3, The bigger difference is in logical design .Primary Key It is generally used as record identification in logic design , This is also the setting

Primary Key My original intention , and Unique Key Just to ensure that the domain / Uniqueness of domain groups .

Be careful ： A table should have primary key fields , without , Indicates that the table is invalid

Primary key value ： It is the unique identification of the current row data 、 Is the ID number of the current line data , Even if the relevant data of the two rows of records in the table are the same , However, due to different primary key values , So I also think it's two different lines of records .

--  Primary key constraint 
-- 1, All tables must have a primary key 
-- 2, Only one primary key is allowed for each table 
-- 3, The columns of the set primary key cannot be duplicated （ Uniqueness ）
-- 4, Not allowed to be empty （ Not for null）
-- 5, Generally, it is the type of primary key  INT,BIGINT, In turn, increasing （auto_increment）
CREATE TABLE teacher5(
	t_name VARCHAR(10) PRIMARY KEY,
	t_age TINYINT
);

CREATE TABLE teacher6(
	t_no INT PRIMARY KEY auto_increment,
	t_name VARCHAR(10),
	t_age TINYINT
);

--  Table level primary key 
CREATE TABLE teacher7(
	t_email VARCHAR(20),
	t_passwd VARCHAR(10),
	t_name VARCHAR(10),
	PRIMARY KEY(t_email,t_name), --  There can only be one 
	UNIQUE(t_email),
	UNIQUE(t_passwd),
	UNIQUE(t_email,t_name)
);

16, Foreign key constraints

        Foreign keys ： Foreign keys are used to associate with another table . It's a field that can determine another table record , Used to keep data consistent . If there are two tables A、B,id yes A Primary key of , and B There are also id Field , be id It's a watch B The foreign key .

        A Is the basic table or parent table , Main table ,B For information sheet , Sub table , Side table

        Definition ：foreign key( The field name of the table ) references Parent table name ( Field name of parent table )

After adding a foreign key constraint to a field , This field is called outer Key fields , Every data in the foreign key field is a foreign key value

--  Foreign key constraints 
--  First create the parent table , Create a sub table 
--  Foreign key data can be empty 
--  When adding data , First, add the data of the parent table , Add sub table 
--  When deleting data , First, delete the data of the sub table , Then delete the data of the parent table 
--  Like what? （ Parent table ） It can be used as （ Sub table ） The foreign key （ Or primary key , Or the only ）

CREATE TABLE A(
	id INT PRIMARY KEY auto_increment,
	a_name VARCHAR(10)	
);
CREATE TABLE B(
	bid INT PRIMARY KEY auto_increment,
	b_name VARCHAR(10),
	id INT,
	FOREIGN KEY(id) REFERENCES A(id)
);

If the value primary key of the added foreign key data does not exist

17, Add constraints

Add non empty constraints	alter table Table name modify Field name not null
Add unique constraints	alter table Table name add unique( Field name )
Add primary key constraint	alter table  Table name add primary key( Field name )
Add a foreign key constraint	alter table  Table name add constraint N1 foreign key( Field name ) references Parent table （ Parent table field name ）

SHOW CREATE TABLE teacher;
CREATE TABLE `teacher` (
  `tea_age` int(11) DEFAULT NULL,
  `dianhua` char(11) DEFAULT NULL COMMENT ' Telephone '
) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_0900_ai_ci

--  to tea_age Add non empty constraints 
ALTER TABLE teacher MODIFY tea_age INT(11) NOT NULL;

CREATE TABLE `teacher` (
  `tea_name` varchar(10) DEFAULT NULL,
  `tea_age` int(11) NOT NULL
) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_0900_ai_ci

--  to tea_name Add unique constraints 
ALTER TABLE teacher MODIFY tea_name VARCHAR(20) UNIQUE;

CREATE TABLE `teacher` (
  `tea_name` varchar(20) NOT NULL,
  `tea_age` int(11) NOT NULL,
  PRIMARY KEY (`tea_name`),
  UNIQUE KEY `tea_name` (`tea_name`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_0900_ai_ci

--  to tea_name Add primary key constraint 
ALTER TABLE teacher MODIFY tea_name VARCHAR(10) PRIMARY KEY;

CREATE TABLE `teacher` (
  `tea_name` varchar(10) NOT NULL,
  `tea_age` int(11) NOT NULL,
  PRIMARY KEY (`tea_name`),
  UNIQUE KEY `tea_age` (`tea_age`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_0900_ai_ci

18, Deletion of constraints

Delete not null constraint	alter table Table name modify Name type ;
Delete unique constraint	alter table Table name drop index Unique constraint name ;
Delete primary key constraint	alter table Table name drop primary key;
Delete foreign key constraint	alter table Table name drop foreign key Foreign key name ;

--  Delete tea_age Non empty constraint of 
ALTER TABLE teacher MODIFY tea_age INT(11);

CREATE TABLE `teacher` (
  `tea_name` varchar(20) NOT NULL,
  `tea_age` int(11) DEFAULT NULL,
  PRIMARY KEY (`tea_name`),
  UNIQUE KEY `tea_name` (`tea_name`),
  UNIQUE KEY `tea_name_2` (`tea_name`),
  UNIQUE KEY `tea_age` (`tea_age`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_0900_ai_ci

--  Delete tea_age The only constraint 
ALTER TABLE teacher DROP INDEX tea_age;

CREATE TABLE `teacher` (
  `tea_name` varchar(20) NOT NULL,
  `tea_age` int(11) DEFAULT NULL,
  PRIMARY KEY (`tea_name`),
  UNIQUE KEY `tea_name` (`tea_name`),
  UNIQUE KEY `tea_name_2` (`tea_name`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_0900_ai_ci

--  Delete the primary key of the table 
ALTER TABLE teacher DROP PRIMARY KEY;

CREATE TABLE `teacher` (
  `tea_name` varchar(20) NOT NULL,
  `tea_age` int(11) DEFAULT NULL,
  UNIQUE KEY `tea_name` (`tea_name`),
  UNIQUE KEY `tea_name_2` (`tea_name`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_0900_ai_ci