设计多对多实体关系的最佳方式

Question

我有两个table权限和多对多关系组

CREATE TABLE `permissions` (
 `Permission_Id` int(11) NOT NULL AUTO_INCREMENT,
 `Permission_Name` varchar(50) DEFAULT NULL,
 PRIMARY KEY (`Permission_Id`)
)

群组table

CREATE TABLE `groups` (
 `Group_Id` int(11) NOT NULL AUTO_INCREMENT,
 `Group_Desc` varchar(100) DEFAULT NULL,
 PRIMARY KEY (`Group_Id`)
)

我很困惑如何实现多对多关系

在新的table

中创建Group_id和Permission_id的复合主键哪个更好

或者使用 join 关键字从两个 table 中创建一个新的 table & select 列。

Answer 1

我知道解决方法。

在这种情况下，我需要创建 "junction" table 来保持 many-to-many 关系。

CREATE TABLE Groups_Permissions
(
Group_Id INT,
Permission_Id INT,
)

Group_Id 和 Permmission_Id 的组合应该是唯一的，并且对组具有 FK 和权限 tables。

Answer 2

来自 my blog:

这样做。

CREATE TABLE XtoY (
    # No surrogate id for this table
    x_id MEDIUMINT UNSIGNED NOT NULL,   -- For JOINing to one table
    y_id MEDIUMINT UNSIGNED NOT NULL,   -- For JOINing to the other table
    # Include other fields specific to the 'relation'
    PRIMARY KEY(x_id, y_id),            -- When starting with X
    INDEX      (y_id, x_id)             -- When starting with Y
) ENGINE=InnoDB;

备注：

⚈  Lack of an AUTO_INCREMENT id for this table -- The PK given is the 'natural' PK; there is no good reason for a surrogate.
⚈  "MEDIUMINT" -- This is a reminder that all INTs should be made as small as is safe (smaller ⇒ faster). Of course the declaration here must match the definition in the table being linked to.
⚈  "UNSIGNED" -- Nearly all INTs may as well be declared non-negative
⚈  "NOT NULL" -- Well, that's true, isn't it?
⚈  "InnoDB" -- More effecient than MyISAM because of the way the PRIMARY KEY is clustered with the data in InnoDB.
⚈  "INDEX(y_id, x_id)" -- The PRIMARY KEY makes it efficient to go one direction; this index makes the other direction efficient. No need to say UNIQUE; that would be extra effort on INSERTs.
⚈  In the secondary index, saying just INDEX(y_id) would work because it would implicit include x_id. But I would rather make it more obvious that I am hoping for a 'covering' index.

要有条件地插入新链接，请使用 IODKU

请注意，如果您在此 table 中有一个 AUTO_INCREMENT，IODKU 会很快 "burn" ID。

更多

A FOREIGN KEY 在涉及的列上隐式创建索引。

PRIMARY KEY(a,b) (1) 表示组合 (a,b) 是 UNIQUE，并且 (2) 按 (a,b).

对数据进行排序

INDEX(a), INDEX(b)（无论是由FOREIGN KEY生成还是手动生成）与INDEX(a,b).

不相同

InnoDB 确实需要一个 PRIMARY KEY，所以你不妨说 PRIMARY KEY (a,b) 而不是 UNIQUE(a,b).

设计多对多实体关系的最佳方式

The best way for designing many to many entities relationship

mysql

database

database-design

relational-database