6、 Data definition language of MySQL (1)

Preface

This section introduces MySQL Medium DDL（ Data definition language ）, The content mainly includes the management of data definition language library 、 The management of the table 、 Data types and common constraints （ The following section describes it separately ） The content such as .

Library management

establish （create)、 modify （alter)、 Delete （drop）

1. Library creation （create)

grammar
create database 【if not exists】 Library name 【character set Character set name 】;

2. Modification of the library （alter)

grammar
alter database Library name character set Character set name ;

You can change the character set of the library ：
ALTER DATEBASE books CHARACTER SET gbk;

3. Deletion of Library

grammar
drop database 【if exists】 Library name ;

The management of the table

establish （create)、 modify （alter)、 Delete （drop）

1. The creation of a table （create)

grammar
create table 【if not exists】 Table name （
Field name Field type 【 constraint 】,
Field name Field type 【 constraint 】,
…
Field name Field type 【 constraint 】
)

2. The modification of table

① Add columns
alter table Table name add column Name type 【first | after Field name 】;
② Modify the type or constraint of the column
alter table Table name modify column Name new type 【 New constraint 】;
③ Change column names
alter table Table name change column Old column names New column names type ;
④ Delete column
alter table Table name drop column Name ;
⑤ Modify the name of the table
alter table Table name rename 【to】 The new name of the table ;

3. The deletion of the table

grammar
drop table 【if exists】 Table name ;

4. Replication of tables

① Copy the structure of the table
create table Table name like Old table ;
② Copy the structure of the table + data
create table Table name
select Query list from Old table 【where Screening 】;

🧒 data type

1. Numerical type
   ① integer

type	byte
tinyint	1
smallint	2
mediumint	3
int/integer	4
bigint	8

characteristic
① Both unsigned and signed can be set , Default signed , adopt unsigned Set up unsigned
② If it's out of range , Will be submitted to the out or range abnormal , Insert threshold
③ The length may not be specified , There will be a length by default
The length represents the maximum width of the display , If not enough, use... On the left 0 fill , But it needs to match zerofill , And default to unsigned integer

② floating-point

Fixed-point number	decimal(M,D)
Floating point numbers	float（M,D） double(M,D)

characteristic
①M Represents the integral part + The number of decimal parts ,D Represents the decimal part
② If you go out of range , Then newspaper out or range abnormal , And insert the critical value
③M and D All can be omitted , But for fixed-point numbers ,M The default is 10,D The default is 0
④ If the accuracy is high , The fixed-point number is preferred

2. Character
   char 、varchar、binary、varbinary、enum、set、text、blob
   == Two... Are often used ==：
   char： Fixed length characters , It's written as char(M), The maximum length cannot exceed M, among M It can be omitted , The default is 1
   varchar： Variable length characters , It's written as varchar(M), The maximum length cannot exceed M, among M Don't omit

3. Date type
   year year
   date date
   time Time
   datetime date + Time ： byte 8
   timestamp date + Time ： byte 4 More vulnerable to time zone 、 Grammatical pattern 、 The impact of the version , Better reflect the real time in the current time zone

summary

This section mainly introduces MySQL Data definition language in , There are many common constraints , It will be introduced separately in the next section , Welcome friends to pay attention and learn ！