What is the commonly heard sub table of MySQL? Why does MySQL need tables?

Why should we divide the tables

A single table is accessed and written frequently , At this time, tables can be divided according to certain business rules , This will reduce the pressure on the single gauge , The interaction between the various modules can also solve the problem frequently . It is generally recommended to control in 20 The left and right fields are appropriate

Another example is the user information table , We put the basic information of users （ Such as user id, user name ） Put it in a table , Then put other user extension information in other tables , This is because basic information is accessed very frequently , In addition, it is convenient for us to use mencache Wait for the cache to replace the query function of this table , If caching is enabled , Program changes will also be very small .

There are two ways to split a table ：

1 Horizontal segmentation ： Place data rows in two separate tables based on the values of one or more columns of data .

For example, the same fields are stored in two tables In some way such as 1-100 Data in user1 surface The first 101-200 Pieces of data are stored in user2 surface Or take the mold, such as the user ID%100, branch 100 A watch Bottom example ：user*

CREATE TABLE `user{
   {
   1-100}}` (
  `id` int(11) unsigned NOT NULL AUTO_INCREMENT,
  `uid` int(11) NOT NULL DEFAULT '0' COMMENT ' Owner uid',
  `source_id` int(11) NOT NULL DEFAULT '0' COMMENT ' resources id',
  `type_mark` int(10) NOT NULL DEFAULT '0' COMMENT ' The resource type ',
  ......
  `create_time` char(10) DEFAULT '0' COMMENT ' Input time ',
  `update_time` char(10) DEFAULT '0' COMMENT ' Update time ',
  PRIMARY KEY (`id`),
 UNIQUE KEY `st_index` (`source_id`,`type_mark`) USING BTREE
) ENGINE=InnoDB AUTO_INCREMENT=1 DEFAULT CHARSET=utf8 COMMENT=' User resource access records ';

Horizontal segmentation is usually used in the following situations . Or monthly access that the user operation log is not often viewed
　　• The watch is big , After segmentation, it can reduce the number of pages of data and index that need to be read during query , At the same time, it also reduces the number of layers of index , Improve query speed .
　　• The data in the table is inherently independent , For example, the data of each region or different periods are recorded in the table , In particular, some data are commonly used , Other data are not commonly used .
　　• You need to store data on multiple media .
Horizontal segmentation adds complexity to the application , It usually requires multiple table names when querying , To query all data, you need union operation . In many database applications , This complexity will outweigh the benefits it brings , Because it's just The keywords to be indexed are not large , When the index is used for queries , Add two to three times the amount of data in the table , Query also increases the number of times to read an index layer disk .

2 Vertical segmentation ： Put the main code and some columns in a table , Then put the main code and other columns in another table .
　　 If some columns in a table are commonly used , And other columns are not often used , Then vertical segmentation can be used , In addition, vertical segmentation can make data rows smaller , One data page can hold more data , It will be reduced when querying I/O frequency . The disadvantage is the need to manage redundant columns , To query all data, you need join operation .　
　　 adopt uid Associate two tables

CREATE TABLE `user` (
  `uid` int(11) unsigned NOT NULL AUTO_INCREMENT,
  `user_name` varchar(50) DEFAULT '' COMMENT ' User login ',
  `password` char(32) DEFAULT '' COMMENT ' User login password ',
  `is_del` tinyint(1) unsigned DEFAULT '1' COMMENT '1, normal  2, Delete ',
  `create_time` char(10) DEFAULT '0' COMMENT ' Input time ',
  `update_time` char(10) DEFAULT '0' COMMENT ' Update time ',
  PRIMARY KEY (`uid`)
) ENGINE=InnoDB AUTO_INCREMENT=1 DEFAULT CHARSET=utf8 COMMENT=' User basic table ';

CREATE TABLE `user_info` (
  `uid` int(11) unsigned NOT NULL,
  `head_img` varchar(50) DEFAULT '' COMMENT ' Head portrait ',
  `nick_name` varchar(50) DEFAULT '' COMMENT ' nickname ',
  `sign_content` varchar(255) DEFAULT '' COMMENT ' Individuality signature ',
  `update_time` char(10) DEFAULT '0' COMMENT ' Update time ',
  PRIMARY KEY (`uid`)
) ENGINE=InnoDB AUTO_INCREMENT=1 DEFAULT CHARSET=utf8 COMMENT=' User information sheet ';