DDL language (data definition language)

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Preface

The data definition language mainly includes the management of libraries and tables.
1. Library management
Create, modify, delete
2. Table management
Create, modify, delete
Keywords
Create: create
modify: alter
delete: drop
here is to perform the definition operation with the previous data, distinguish between delete and truncate

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1. Library management

1. The creation of the library

grammar:

create database 库名;

Case: Creating Books

CREATE DATABASE books;
#CREATE DATABASE if not exists books;如果不存在，就创建

2. Modification of the library

RENAME DATABASE books TO 新库名;

You can change the character set of the library:

ALTER DATABASE books CHARACTER SET gbk;

3. Deletion of the library

DROP DATABASE books;
#DROP DATABASE IF EXISTS books;如果存在就删除

Second, the management of the table

1. Table creation

grammar:

create table 表名(
	列名 列的类型【(长度) 约束】,
	列名 列的类型【(长度) 约束】,
	列名 列的类型【(长度) 约束】,
	...
	列名 列的类型【(长度) 约束】
)

Case 1: Create a table Books

USE books ;

CREATE TABLE book (
  id INT,#编号 
  bName VARCHAR (20),#图书名 
  price DOUBLE,#价格 
  authorId INT,#作者编号 
  publishDate DATETIME#出版日期 
) ;

DESC book ;

Result:

Case 2: Create table author

CREATE TABLE author (
  id INT,
  au_name VARCHAR (20),
  nathion VARCHAR (10)
) ;

DESC author;

result:

2. Modification of the table

Core syntax:

alter table 表名 add(添加)|drop(删除)|modify(修改)|change(改变)  column  列名 【列类型  约束】；

①Modify column name

ALTER TABLE book 
  CHANGE COLUMN publishdate pubDate DATETIME ;

②Modify the type or constraint of the column

ALTER TABLE book 
  MODIFY COLUMN pubDate TIMESTAMP ;

③Add a new column

ALTER TABLE author 
  ADD COLUMN annual DOUBLE ;

④Delete column

ALTER TABLE author 
  DROP COLUMN annual;

⑤ Modify the table name

ALTER TABLE author 
  RENAME TO book_author ;

3. The deletion of the table

DROP TABLE book_author;
DROP TABLE IF EXISTS book_author;
SHOW TABLES;

General writing:

DROP DATABASE IF EXISTS 旧库名;
CREATE DATABASE 新库名;

DROP TABLE IF EXISTS 旧表名;
CREATE TABLE 表名();

4. Copy of the table

(1) Only copy the structure of the table

CREATE TABLE copy LIKE author ;

(2) Copy table structure + data (all)

CREATE TABLE copy2 
SELECT 
  * 
FROM
  author ;

(3) Copy only part of the data

CREATE TABLE copy3 
SELECT 
  id,
  au_name 
FROM
  author 
WHERE nation='中国' ;

(4) Only copy certain fields (as long as the column structure, no data)

CREATE TABLE copy4 
SELECT 
  id,
  au_name 
FROM
  author 
WHERE 1 = 2 ;

test

1. Create table deptl

name	Null	type
id		int(7)
name		varchar(25)

USE test ;#创建表首先要找一个库
CREATE TABLE dept1 (id INT (7), NAME VARCHAR (25)) ;

2. Insert the data in the departments table into the new table dept2

CREATE TABLE dept2 
SELECT 
  * 
FROM
  myemployees.departments ;

3. Create table emp5

name	Null	type
id	not null	int(7)
first_name		varchar(25)
last_name		varchar(25)
dept_id	not null	int(7)

CREATE TABLE emp5(
	id INT(7) NOT NULL,
	first_name VARCHAR(25),
	last_name VARCHAR(25),
	dept_id INT(7) NOT NULL
);

4. Increase the length of the column Last_name to 50

ALTER TABLE emp5 
  MODIFY COLUMN last_name VARCHAR (50) ;

5. Create employees2 based on the table employees

CREATE TABLE employees2 LIKE myemployees.employees ;

6. Delete table emp5

DROP TABLE IF EXISTS emp5;

7. Rename the table employees2 to emp5

ALTER TABLE employees2 
  RENAME TO emp5 ;

8. Add a new column test_column in the table emp5, and check the operation

ALTER TABLE emp5 
  ADD COLUMN test_column VARCHAR (10) ;

DESC emp5 ;

9. Directly delete the column dept_id in the table emp5

ALTER TABLE emp5 
  DROP COLUMN dept_id ;