MySQL basics 03 introduction to MySQL types

Connect to database

mysql It's already installed , How to connect to the database ？

• stay DOS Command line window to enter The installation directory \mysql\bin
• You can set environment variables , Set the environment variable , You can open it in any directory ！

mysql -uroot -p123456

Operating the database

Classification of structured query statements :

name explain command
DDL （ Data definition language ） Define and manage data objects , Such as a database , Data sheets, etc CREATE、DROP、ALTER
DML （ Data operation language ） Used to manipulate data contained in database objects INSERT、UPDATE、DELETE
DQL （ Data query language ） Used to query database data SELECT
DCL （ Data control language ） Language for managing databases , Including management authority and data change GRANT、commit、rollback

Database operation

Create database : create database ceshi;
Delete database : drop database ceshi;
view the database : show databases;
Using a database : use ceshi;

Common database words :

create 、drop、use、show、table、column、user、select、insert、delete、update、join、on、
where、left、right、ddl,dml, alter,index,sequence

Column type : Specify the data type of the column in the database

mysql Common data types : MySQL Support for multiple types :
It can be roughly divided into four categories ： Numerical type 、 floating-point 、 date / Time and string ( character ) type

decimal(M,N) M Represents the total number of digits ,N It means the number of decimal places , If M More than an error , Decimal exceeded , rounding :

Example :
create table test(a int,b decimal(3,2));
Create a table test, As a a,b among a The type is int type ,b The type is decimal, The total number of digits is 3 position , among 2 Decimal place

Decimal point can be truncated , But the integer part directly reports an error

The most commonly used is varchar(20) type , Indicates that the maximum number of entries can be 20 Characters :

create table test1(a int,b varchar(20));

among char(5) Means if you define 5 Characters , Even if you insert one , Also inserted 5 Characters :

The date type

insert into test3 values(1,‘2022-01-01 12:00:00’);

NULL value

• Understood as a “ No value ” or “ Unknown value ” Do not use NULL Perform arithmetic operations , The result is still the NULL

Auto_InCrement

Automatic growth , Every time you add a piece of data , Automatically add... To the number of previous records 1( Default )
Usually used to set the primary key , And it is an integer type. You can define the starting value and step size
Current table setting step (AUTO_INCREMENT=100) : Only the current table
SET @@auto_increment_increment=5 ; Affect all tables using auto increment ( overall situation )

create table test4(a int,b int AUTO_INCREMENT primary key);

Automatic growth is generally associated with primary keys :

NULL and NOT NULL

The default is NULL , That is, the value of the column is not inserted
If set to NOT NULL , Then the column must have a value of

DEFAULT

default
Used to set the default value
for example , Gender field , The default is " male " , Otherwise “ Woman ” ; If the value of this column is not specified , The default value is " male " Value

create table test6(a int ,b int default 5);

– The goal is : Create a school database
– Create student table ( Column , Field )
– Student number int The login password varchar(20) full name , Gender varchar(2), Date of birth (datatime), Family residence
site ,email
– Before creating the table , Be sure to select the database first

CREATE TABLE IF NOT EXISTS student (
id int(4) NOT NULL AUTO_INCREMENT COMMENT ‘ Student number ’,
name varchar(30) NOT NULL DEFAULT ‘ anonymous ’ COMMENT ‘ full name ’,
pwd varchar(20) NOT NULL DEFAULT ‘123456’ COMMENT ‘ password ’,
sex varchar(2) NOT NULL DEFAULT ‘ male ’ COMMENT ‘ Gender ’,
birthday datetime DEFAULT NULL COMMENT ‘ Birthday ’,
address varchar(100) DEFAULT NULL COMMENT ‘ Address ’,
email varchar(50) DEFAULT NULL COMMENT ‘ mailbox ’,
PRIMARY KEY (id)
) ENGINE=InnoDB DEFAULT CHARSET=utf8

– View the definition of the database
SHOW CREATE DATABASE school;
– Look at the definition of the data table
SHOW CREATE TABLE student;
– Display table structure
DESC student;

CREATE TABLE Table name (
– Omit some code
– Mysql notes
– 1. # Single-line comments
– 2. /…/ Multiline comment
)ENGINE = MyISAM (or InnoDB) # Now the default is innodb

MySQL Data tables are stored on disk as files
Include table files , Data files , And the database options file
Location : Mysql The installation directory \data\ The data table is stored in the table below . The directory name corresponds to the database name , The file name in this directory corresponds to the data table

Be careful :
InnoDB There is only one type table *.frm file , And the next level directory ibdata1 file

View the data file directory :