SQL_DDL_ operation database/table

Operating database: CRUD

1. C(Create): Create

• Create a database:
  * create database database name;
• Create a database, determine that it does not exist, and then create:
  * create database if not exists database name;
• Create a database, and specify the character set
  * create database database name character set character set name;

Exercise: Create a db4 database, determine whether it exists, and set the character set to gbk
* create database if not exists db4 character set gbk;

2. R(Retrieve): query

• Query the names of all databases:
  * show databases;
• Query the character set of a database: query the creation statement of a database
  * show create database database name;
  example:

Quietly Mimi spit out, this capitalization is enough!

3. U(Update): modify

• Modify the character set of the
  database * alter database database name character set character set name;

**4. D(Delete):删除**

• Delete database
  * drop database database name;
• Determine that the database exists, and then delete it
  if it exists* drop database if exists database name;
  

5. Use the database

• Query the name of the database currently in use
  * select database();

• Use database
  * use database name;

  

Operation table

1. C(Create): Create

1. grammar:

create table 表名(
						列名1 数据类型1,
						列名2 数据类型2,
						....
						列名n 数据类型n
				);

• Note: There is no need to add a comma (,) in the last column

• Database type:
  1. int: integer type
  * age int,
  2. double: decimal type
  * score double(5,2)
  3. date: date, only contains year, month and day, yyyy-MM-dd
  4. datetime: date, Contains year, month, day, hour, minute, and second yyyy-MM-dd HH:mm:ss
  5. timestamp: Time error type includes year, month, day, hour, minute, and second yyyy-MM-ddHH:mm:ss
  * If you don’t assign a value to this field in the future, or assign a value null, the current system time is used by default to automatically assign the value
  6. varchar: string
  * name varchar(20): name maximum 20 characters
  * zhangsan 8 characters Zhang San 2 characters

  • Create table

	create table student(
					id int,
					name varchar(32),
					age int ,
					score double(4,1),
					birthday date,
					insert_time timestamp
				);

• Copy table:
  * create table table name like the name of the table being copied;
  

2. R(Retrieve): query

• Query all the table names in a database
  * show tables;
  * Query table structure
  * desc table name;

3. U(Update): modify

1. Modify the table name
   alter table table name rename to the new table name;
   

2. Modify the character set of the
   table alter table table name character set character set name;
   
   

3. Add a column of
   alter table table name add column name data type;
   

4. Modify column name type
   alter table table name change column name new column name new data type;
   alter table table name modify column name new data type;
   
   

5. Delete column
   alter table table name drop column name;

	**4. D(Delete):删除**

drop table 表名;
drop table if exists 表名 ;