SQL learning notes (02) - database table operation

One 、Mysql Database operation

1. Create database Format ：create database Database name ;

create database student;

2. Delete database Format ：drop database Database name ;

drop database student;

3. Look directly at the existing database ：show databases;

Two 、Mysql Data type in

   1. Integer and decimal

  Integer types ：int
  Decimal type ：decimal Format ：decimal( Total length , Decimal places )
  decimal(5,2) The total length of data required to be transmitted is 5 position , The decimal places are 2 position

   2. Date and time type

  datetime       YYYY-MM-DD HH:MM:SS
  date         YYYY-MM-DD

   3. String type

  char( Length of string )         Fixed length
  varchar( Length of string )       Lengthening
  for example ：
  full name char(10)     tom–> Occupy 3 Characters And then there were 7 The space of characters will not be released , The system will fill with blanks
  full name varchar(10)   tom–> Occupy 3 Characters And then there were 7 Characters were released

3、 ... and 、 Create and delete tables

Create table syntax format

create table  Table name (
 Name 1  data type ,
 Name 2  data type ,
 Name 3  data type 
);

Comments in the database

Single-line comments ：-- Content of notes
Multiline comment ：/* Content of notes */

Switch database

Format ：use Database name

Open the table ： Check whether there is data stored in the table

Design table ： View table structure

Delete table

Delete single table ：drop table Table name ;
Delete multiple tables ：drop table Table name 1, Table name 2;

Four 、 Modify table structure

Add columns

alter table  Table name  add  Column name data type ;

Delete column

alter table  Table name  drop  Name ;

Change the data type of the column

alter table  Table name  modify  Name   New data types ;

Change column names

alter table  Table name  change  Old column names   New column names   data type ;

Display table structure

desc  Table name ;

Check constraints check（ Limit the range of values in the column ）

create table  Table name (
 Name 1  data type  check( constraint condition )
);

give an example ： In the table id The value range is 5-20

create table students(
id int check(id>5 and id<20)
);