Research on the scheme of MySQL advanced (VIII) sorting problem

One 、 Preface

stay

mysql

Default

order by

  Only numbers and date types can be sorted , But for the

varchar

Character type sorting seems useless , Now let me introduce to you varchar How to solve the type sorting problem .

Today, I found an interesting problem when sorting the national telephone number list , I want to make isdcode The fields are sorted from small to large , So I wrote it like this

SELECT * FROM gb_country_isdcode ORDER BY isdcode asc;

give the result as follows , I found that it was not the result I wanted ,

asc

The sorting is right , So I searched and searched , Look and look , Finally, I found the reason ;

isdcode yes varcher Type of , If sorting, use

asc

Obviously not , He must be converted into

int

Types can then be sorted normally , as long as

isdcode + 0

That's all right.

So it was written like this

SELECT * FROM gb_country_isdcode ORDER BY (isdcode+0) asc;

It seems that the data you want is larger .. But why +0 That's it ？ original ,+0 And then it's converted

INT

The types are sorted . So you can sort by size .

Two 、 Chinese sorting

What if it's not the phone but Chinese characters , We can sort Chinese characters by simply converting .

stay

mysql

Use in

order by

For fields that store Chinese information , The default results are not sorted by the order of Chinese Pinyin , If you want to sort by the Pinyin of Chinese characters , You need to set the character set of the database to

UTF8

, And then in

order by

When forced to convert the field information into

GBK

, The result is sorted in pinyin order . for example ：

SELECT * FROM table_name ORDER BY CONVERT(column_name USING gbk);

stay mysql It's a pilot , The results are very satisfactory .

The conclusion is that ： When querying , adopt

convert

function , The character set used to query the data

gb2312

Just code it , And then use

convert

After the Chinese sort . But if you really change the character set of the fields in the table to

gb2312

, It will also involve many coding problems , Page value transfer , Access from the database , so much trouble . Just at the time of inquiry , Specify the following character set , It's not really changing the physical field to

gb2312

, Relatively simple .

3、 ... and 、 Extended reading  MySQL The role of foreign keys in databases

MySQL

The purpose of foreign keys is to control the data stored in the foreign key table , Associate two tables , yes

MySQL

A very important part of the database , It is worth us to have a deep understanding . that ,

MySQL

What role do foreign keys play ？ The following will take you to explore the secret .

3.1 MySQL The role of foreign keys

Keep data consistent , integrity , The main purpose is to control the data stored in the foreign key table . Associate two tables , Foreign keys can only refer to the values of columns in the surface ！

3.2  The premise of establishing foreign key

The columns in this table must be of the same type as the foreign key ( Foreign keys must be external primary keys ).

Specify the primary key keyword ：

 foreign key( Name )

Keyword reference foreign key ：

&nbsp;references&nbsp;< Foreign key table name >( Foreign key column name )

3.3  Event trigger limit

on delete

and

on update

,  Parameters can be set

cascade

( Follow foreign key changes ), 

restrict

( Limit foreign key changes in appearance ),

set Null

( Set null ）,

set Default

（ Set the default value ）,[ Default ]

no action

.

      

outTable surface   Primary key  id  type  int

  Create a table with foreign keys ：

create table temp(

   id int,

   name char(20),

   foreign key(id) references outTable(id) on delete cascade on update cascade);

explain ： hold id Set column to MySQL Foreign keys , Refer to the appearance outTable Of id Column . When the value of the foreign key is deleted , Delete the corresponding column in this table ; When the value of the foreign key changes   The corresponding column value in this table changes .

MySQL

A table in can only have one primary key , A primary key can consist of multiple fields .