First knowledge of database

1 database （DataBase）

database ： A warehouse for storing data

characteristic ： You can think of it as an improvement on the file system . It solves the problem between different operating systems , Compatibility of data formats . in other words , As long as it is a data file of the same database , Even from Windows Moved to Linux On , It can also be handled normally .

2 Database classification

It can be divided into two categories: relational and non relational

Relational database ： about java for , A class can correspond to a table , A class object can correspond to a row of data in the table , A member variable can correspond to a column in the table , Be able to map one by one , In the database, the .. besides It is also mainly reflected in its own relationship model , It reflects the relationship between tables

Non relational database ： Is to store key value pairs , There is no association between each table , They're all independent . But it has the convenience of processing data in relational database , It can also process a large amount of data , What's hot is Redis、mongoDB.

 3MySQL brief introduction

MySQL What is used SQL Language is the most commonly used standardized language for accessing database .MySQL The software adopts a dual licensing policy , It is divided into community version and commercial version , Because of its small size 、 Fast 、 Low total cost of ownership , Especially open source , In general, small and medium-sized sites are chosen for development MySQL As website database .

Database objects ： surface , View , function , stored procedure , Index, etc. .

surface

It's where data is stored , There are records in it , Records are made up of fields . Each field has its own field name 、 field value 、 Field type （ data type ）. Fields are the basic unit for storing data . similar Java Objects in the , For storing data .

View

We can understand it as a virtual table （ It doesn't really exist on physical disks ）. The view is based on select The result set generated by the statement query . When one select When the statement is complex and the call frequency is high , We don't want to write the sentence again , At this time, you can put this select Statement is created as a view . Every time you use a view , You can use it as a watch .

function

It is usually the implementation of a function , Most of us use system functions （MYSQL Provided ）, We can also customize functions . Its calling routine is through the use of select Function name ( Parameters 1, Parameters 2..); To call . It has only one return value .

stored procedure

It is also the realization of a function . What we call database programming ,SQL Statement programming , It mainly refers to customizing a stored procedure . Calling a stored procedure uses exec Stored procedure name Parameters 1 Wait to call . It can return multiple values , There can be no return value .

Indexes

We've been in contact with , It's like a book catalog , It is established to speed up our query .

5.2 ordinary sql The command operation

// Database view version number

select version();

// View all local databases

show databases;

// Write what you check

select 'xxx'

// Switch database

use test ;

// Look at all the tables

show tables;

// sign out    Exit

5.3 User creation and Authorization

1） Create user ： CREATE USER 'USERNAME' @ 'host' IDENTIFIED BY  'password' ; 

Need to switch to mysql  Under database       operation user surface

username： user name ,  host： Specify which host can log in , This machine is available localhost,% General configuration of all remote hosts ;password： User login password ;

2） to grant authorization ： GRANT ALL ON ”." TO username'@'%'

Format ： grant jurisdiction on Database name . indicate .to user @ Log on to the host   identified by " User password ";

jurisdiction ：

Select  Inquire about

Insert insert data

Update Update data

Delete Delete data

Drop Delete table

Create Create table

3） Cancel the authorization

revoke Follow grant The syntax is similar , Just put the keywords “to” Switch to “from” that will do

grant all on . to 

 Delete user , Will synchronize user and db Delete the corresponding data in all tables 

drop user  user name @'%’;

6 data type

When creating a table , Data type used , The purpose is to limit the type of data , Commonly used ： floating-point , Character , Date time type, etc

Numerical type ：

  Time type

  String type

 

 7SQL The classification of

 1）DCL Data control language , Represent keyword ： grant,revoke

2)  DDL Data definition language   , Represent keyword ： create,drop

          a Database creation Delete ： Creat, drop

           b The creation and deletion of tables in the database     

                grammar ：   create table XXX(

                                Name     type ,

                                listed       type

);

3) DML  Data operation language ,    Represent keyword ： insert,delete, update

        a It is mainly used to insert... Into the database , modify , Keywords involved in deleting data include : insert delete update

        b                Insert

       inser into Table name ( Name 1, Name 2)       values( value 1. value 2);

        c                Update

        update    indicate     set  Name 1 = value , Name 2= value    where Name = value ;

        D              delete

        delete   from  Table name     where  Name    =  value ;

4) DQL Data query language ： Represent keyword ：select

        select    Column qualification      from Table limit        where    Row qualification

5） TCL Transaction control language ： Represent keyword ： commit,rollback