Mysql database foundation

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  Catalog

🥬 Common data types

🥬 Basic operation of database

 🥬 Constraint type

🥬 Summary

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🥬 Common data types

1、int： plastic

2、double(m,d)

     decimal（m,d）： Floating point type （m Specify the length ,d It means the number of decimal places ）

3、varchar（size）： String type

4、timestamp： The date type

🥬 Basic operation of database

No matter what statement is executed , Add... At the end of the sentence ;（ A semicolon ）.

1、 Create database

create database  Database name ;

2、 Show current database

show databases;

3、 Delete database

drop database  Database name ;

4、 Using a database

use  Database name ;

5、 Create table

create table  Table name ;

6、 View table structure

desc  Table name ;

7、 insert data

insert into  Table name  values（ Write... Against the type of the column ）;

8、 Look for statements

Full column lookup

select * from  Table name ;

Specified column query

select  Name , Name …… from  Table name ;

Specifies that the query field is an expression （ In the query , Perform some operations at the same time ）

select  Name +（-*/） Name  from  Table name ;

Specify alias for query field

select  Name …… as  Alias  from  Table name ;

De duplication of query results

select distinct  Name  from  Table name ;

Sort the query results

select * from  Table name  order by  Name  asc/desc;// Ascending / Descending 

Paging query

select * from  Table name  limit n;//  from 0 Start , Screening n Bar result 
select * from  Table name  limit n offset s;// from s Start , Screening n Bar result 

Conditions of the query

select  Name  from  Table name  where  Conditions ;

Common operators ：

and,or,not And or not （and Priority ratio or high ）

>,>=,<,<= Greater than , Greater than or equal to ……

= More equal , No comparison NULL

<=> More equal , Compare NULL

！=,<> It's not equal to

between a and b Range match , In the closed zone [a,b] Within the scope of

in (option, ...) If it is option Any one of , return TRUE(1)

is NULL    yes NULL

is not NULL   No NULL

like Fuzzy matching ,% Denotes any number of （ Include 0 individual ） Any character ;_ Represents any character  

    The results of the above search are all one " A temporary table ", The tables of the database are all on the hard disk , The query result temporary table here , Not on the hard drive , It's in memory , With the output , The data is released . So the above operations are all temporary table operations , No impact on raw data .

    When user input sql after , The client will put this sql Packaged as a network request , To the server , After the server receives the request , Will operate the hard disk , Read data from the hard disk and package the data into a response . After the client receives the response data , The response result will be temporarily saved in memory and output on the display ( Print on standard output ) As the printing is completed , The result data saved in the client memory is also released .

9、 Modify the statement

update  Table name  set  Name = value …… where  Conditions ;

10、 Delete statements

delete from  Table name  where  Conditions ;

What is modified here is the original data .

Illustrate with examples ：

Create database ：

Show current database ：

  Create a student Table of （ When you need to manipulate a table in a database , You need to use the database first ）：

View table structure ：

Insert and view ：

insert When inserting , You can insert only one column or several columns . At this time, the other columns will adopt the default value

  Multiple columns can be inserted at one time , use （）, separate

  If a certain type is datetime, There are three ways to insert

 

  lookup ：

Specify column lookup

Specifies that the query field is an expression

  Specify alias for query field

 

  De duplication of query results

  Sort the query results

Ascending

Descending

  If there is NULL value , Default NULL The value is the minimum

  Conditions of the query

 % Match any number of （ Include 0 individual ） character   , You can match the name starting with any piece

  _ Match a strict arbitrary character , One _ Can match a character .

 

Paging query

 

 

  Check the results of the top three

modify ：

Change Zhang San's Chinese score to 80

  Revise everyone's Chinese grades

  Add the Chinese scores of everyone in the top three of the total scores 10

  Delete ：

 🥬 Constraint type

1、not null--- Indicates that a column cannot store NULL value .

 

 2、unique--- Ensure that each row of a column must have a unique value .

 3、default--- Specifies the default value when the column is not assigned a value .

When a default value is specified , When the column is not assigned a value , The default value of this column is the specified default value .

 

 4、primary key（ The most important constraint ）not null and unique The combination of . Make sure that a column （ Or a combination of two or more columns ） There is a unique identification , Helps to find a specific record in a table more easily and quickly . Primary key constraint , Equivalent to the unique identification of data . For a table , Only one column can be specified as the primary key .

Since the primary key primary key auto_increment, When the auto increment primary key is set , The record inserted at this time , You can not specify the value of self incrementing primary key ( Use it directly null To express ) , hand mysql Just distribute it yourself .

 5、foreign key --- Ensure referential integrity of data in one table matching values in another table .

Also known as foreign key constraints , Describes the relationship between two columns of two tables " Dependency relationship ” The child table depends on the parent table ( The child table is referenced from the parent table ) The corresponding records in the child table must exist in the parent table .

for instance ： Create a class table and student table as follows , Every student should belong to a specific class , This class must exist first , Here the student table depends on the class table , Just call the student list ” Sub table “, The class schedule is called ” Parent table “.

This situation can be described by foreign key constraints .

Foreign key constraints , Describes the relationship between two columns of two tables " Dependency relationship ", The child table depends on the parent table ( The child table is referenced from the parent table ) The corresponding records in the child table must exist in the parent table , The classes in the class table are only 1,2,3, At this time, I insert a class as 10, There will be errors , Insert the failure .

 

  Insert some classes in the class table and they will be inserted successfully .

 

  Foreign key constraints also constrain the parent table , When a record in the parent table is dependent on the child table , At this point, any attempt to delete the modification will fail .

🥬 Summary

That's what we have today , If you have any questions, you can leave a message in the comment area