MySQL foundation 06 DDL

DDL operation

DDL(Data Definition Language), Data definition language , Used to define database objects ： database , surface , Column, etc. , For example, create 、 Delete 、 modify ： database 、 Table structure, etc . This article mainly talks about mysql In the database DDL The use of language , Of course, this is different from sql The standard DDL It's in the same vein , Only the data types of specific databases are different .

1. Operating the database DDL, Include database view 、 Switch 、 Create and delete

First, the user login of the client , such as windows Of dos State, ,mysql -uxxx -pyyy,xxx and yyy They are user name and password . Of course , Before logging in, make sure mysql The service for has started .

View all databases of the user ,show databases;
Pay attention to the implementation of sql Your order needs a semicolon , Otherwise, some commands will be considered as line feed .

install mysql after , The system will automatically create some databases , Yes 4 individual , Respectively information_schema、mysql、performance_schema、test, It has 5 individual , among mydb1 It's my new .

It should be noted that , There is no need to modify or operate the databases created by these systems , Otherwise, it may cause mysql Use exception , Serious ash makes the database unusable .
The command to create the database is ： create database Database name ;

The command to delete the database is ：drop database Database name ;

Finally, the switching of database , The order is ：use Database name ;

The above is the database DDL Common operations for

2.DDL Operation table , It mainly includes the creation of tables 、 Modification and deletion, etc . The modification of the table includes the addition and deletion of columns 、 Modify the column 、 Modify table name, etc .

First of all, let's talk about the operation of using morality to query all tables in the database , The order is ：show tables;

Common statements for creating database
Query all databases

show databases;

 Create database 

create database dbname;

 Using a database 

use dbname;

 Query all data tables 

show tables;

 Delete database 

drop database dbname;

Table common statements
1、 Create an article

For example, a common name is emp Table of , The fields in the table contain 3 A field ,ename( full name ),hiredate( Date of dismissal )、sal( salary ), The field types are varchar(10)、data、int(2) Field types will be introduced later .

create table emp(ename varchar(10),hiredate date,sal decimal(10,2),deptno int(2)); 

 see emp surface , The following information will be output .

desc emp;

 Always say desc Command to view table definitions , But the input information is still not comprehensive .

show create table emp;

 Processing can see SQL In the sentence , Processing can see that the table definition is unexpected , You can also see the watch engine( Storage engine )

2、 Deleted 

 Delete statement of table 

drop table tablename;

3、 Revision article 

3.1、 Modify the table type syntax as follows ：

alter table emp modify ename varchar(20); 

3.2、 Add table field 

alter table emp add column age int(3);
3.3、 Delete table fields 

alter table emp drop column age;

3.4、 Field modification name 

alter table emp change ename username varchar(20);

3.5、 Modify the field arrangement Book sequence 

 For example, the new field is in birth  Add to username after ;

 alter table emp add birth date after username; 

 Modify fields deptno, Put it in the front .

alter table emp modify deptno int(3) first;

3.6、  Clear the table 
truncate table emp;