Foreign Key Constraint based on specific values in other table

Question

在我的数据库中，我创建了以下 table:

CREATE TABLE [Category_Dim]
(
    [Id]                  INT NOT NULL PRIMARY KEY
    ,[__ParentCategoryId] INT 
    ,[Name]               VARCHAR(250)

    ,CONSTRAINT [FK1] FOREIGN KEY ([__ParentCategoryId]) REFERENCES [dbo].[Category_Dim] ([Id])
)

这允许我存储多个不同类型的分类（嵌套）列表，其根具有 __ParentCategoryId = NULL，然后按如下方式输入 children，例如：

INSERT INTO Category_Dim (Id, __ParentCategoryId, Name) VALUES
    (1, NULL, 'Dog Breeds'),
    (2, NULL, 'Bird Types'),

    (3, 1, 'Chihuahua'),
    (4, 1, 'Pug'),
    (5, 1, 'Pit Bull'),

    (6, 2, 'Macaw'),
    (7, 2, 'Finch'),
    ... etc

换句话说，在这种情况下，ID 3、4 和 5 是 children 的 1（不同的狗品种），6 和 7 是 children 的 2（鸟类的类型）。

现在，假设我正在尝试创建第二个 table，我希望 只允许犬种（Id = 1 中的 children）作为值 在列中，否则为错误。

到目前为止，我有以下定义：

CREATE TABLE [Trainers]
(
    [TrainerId] INT NOT NULL PRIMARY KEY IDENTITY (1, 1)
    ,[__DogBreedId] INT NOT NULL
    
    , ...

    ,CONSTRAINT [FK_DogBreeds] FOREIGN KEY ([__DogBreedId]) REFERENCES [dbo].[Category_Dim] ([Id])
)

这有外键约束，但它允许 Category_Dim 中的任何 Id 值作为我的 __DogBreedId，因此一个人可以输入数字，在这种情况下， 3-5 如我所愿。

有没有办法通过外键语句来实现？而且，如果不是，最好的方法是什么？或者这总体上是个坏主意吗？

谢谢！！

Answer 1

我最终为实现这一目标所做的是，我创建了一个返回 BIT 的函数，该函数基于 CategoryId 提供的是父 CategoryId 的子项：

CREATE FUNCTION [dbo].[IsChildOfCategory]
(
    @__CategoryId        INT
    ,@__ParentCategoryId INT
)
RETURNS BIT
AS
    BEGIN
    RETURN CASE
        WHEN EXISTS 
            (
                SELECT Id FROM Category_Dim 
                WHERE __ParentCategoryId = @__ParentCategoryId 
                AND Id = @__CategoryId
            )
            THEN 1
        ELSE 0
    END
    END;
GO

然后，我将以下检查约束添加到我的 table 定义中：

,CONSTRAINT [CHK_IsDogBreed] CHECK ([dbo].[IsChildOfCategory]([__DogBreedId], 1) = 1)

这与外键约束一起，似乎完全符合我的要求。

但是我真的想知道这是否是一个糟糕的使用模式（所有内容都作为数据存储在一个类别中 table 而不是单独的数据库 tables for each type of data），因为这让我不得不硬编码 Category Ids 就像在这个检查约束中一样，它作为数据存在于数据库中而不是特定的数据库对象中（换句话说，它留下我需要用非常具体的值来播种我的数据库 - 在这种情况下确保 Category Id 1 = 'Dog Breeds').

所以，它确实有效，但它确实让我质疑这是否是个坏主意。

Answer 2

对于单级层次结构（节点深度 = 1），您最好使用 supertype-subtype。

如果您确实需要可变节点深度的树，那么请考虑使用 闭包 table，而不是将 parent_id 放在同一个 table.
闭包 table 将所有路径存储在树中，因此每个祖先-后代 link 都是单独的一行。这样，给定节点的所有 ancestor/descendants 都会被公开。闭包 table 很容易查询，但维护起来有点困难，所以这是一个权衡。

-- Category CAT exists.
--
category {CAT}
      PK {CAT}


-- Data Sample
  (CAT)
------------------------
  ('Dogs')
, ('Big Dogs')
, ('Small Dogs')  
, ('Chihuahua')
, ('Pug')
, ('Pit Bull')
, ('Birds')
, ('Macaw')
, ('Finch')

-- Ancestor ANC has descendant DCS
--
category_tree {ANC, DCS}
           PK {ANC, DCS}

FK1 {ANC} REFERENCES category {CAT}
FK2 {DCS} REFERENCES category {CAT}


-- Data Sample, includes ANC=DCS
   (ANC, DCS)
------------------------
  ('Dogs'       , 'Dogs')
, ('Birds'      , 'Birds')
, ('Dogs'       , 'Big Dogs')
, ('Dogs'       , 'Small Dogs')
, ('Big Dogs'   , 'Big Dogs')
, ('Small Dogs' , 'Small Dogs')
, ('Dogs'       , 'Chihuahua')
, ('Small Dogs' , 'Chihuahua')
, ('Chihuahua'  , 'Chihuahua')
, ('Dogs'       , 'Pug')
, ('Small Dogs' , 'Pug')
, ('Pug'        , 'Pug')
, ('Dogs'       , 'Pit Bull')
, ('Big Dogs'   , 'Pit Bull')
, ('Pit Bull'   , 'Pit Bull')
, ('Birds'      , 'Macaw')
, ('Macaw'      , 'Macaw')
, ('Birds'      , 'Finch')
, ('Finch'      , 'Finch')

-- Trainer TRA trains all descendants of ancestor ANC.
--
trainer {TRA, ANC}
     PK {TRA, ANC}

FK {ANC, ANC} REFERENCES category_tree {ANC, DCS}


-- Data Sample
   (TRA, ANC)
------------------------
  ('Joe'    , 'Dogs')
, ('Jane'   , 'Small Dogs')
, ('Jane'   , 'Finch')
, ('Jill'   , 'Big Dogs')
, ('Jack'   , 'Birds')
, ('John'   , 'Pug')

-- Trainer TRA trains DCS, descendant of ANC.
-- (Resolved to leaf nodes.)
WITH
q_00 AS ( -- leaves only
select ANC, count(1) as cnt 
from category_tree
group by ANC
having count(1) = 1
)
SELECT t.TRA, x.DCS, t.ANC
FROM trainer       AS t
JOIN category_tree AS x ON x.ANC = t.ANC
JOIN q_00 as q ON q.ANC = x.DCS
ORDER BY TRA, t.ANC;
;

Returns:

TRA     DCS          ANC
----------------------------------
Jack'  'Finch'      'Birds'
Jack'  'Macaw'      'Birds'
Jane'  'Finch'      'Finch'
Jane'  'Pug'        'Small Dogs'
Jane'  'Chihuahua'  'Small Dogs'
Jill'  'Pit Bull'   'Big Dogs'
Joe'   'Pit Bull'   'Dogs'
Joe'   'Pug'        'Dogs'
Joe'   'Chihuahua'  'Dogs'
John'  'Pug'        'Pug'

注：

All attributes (columns) NOT NULL

PK = Primary Key
FK = Foreign Key

SQL 测试

CREATE TABLE category (
  CAT VARCHAR(32) NOT NULL
 
, CONSTRAINT pk_cat PRIMARY KEY (CAT)
);


CREATE TABLE category_tree (
  ANC VARCHAR(32) NOT NULL
, DCS VARCHAR(32) NOT NULL

, CONSTRAINT pk_ctre  PRIMARY KEY (ANC, DCS)

, CONSTRAINT fk1_ctre FOREIGN KEY (ANC)
              REFERENCES category (CAT)

, CONSTRAINT fk2_ctre FOREIGN KEY (DCS)
              REFERENCES category (CAT)
);


CREATE TABLE trainer (
  TRA VARCHAR(32) NOT NULL
, ANC VARCHAR(32) NOT NULL

, CONSTRAINT pk_tra PRIMARY KEY (TRA, ANC)

, CONSTRAINT fk1_tra FOREIGN KEY (ANC, ANC)
        REFERENCES category_tree (ANC, DCS)
);

INSERT INTO category (CAT)
VALUES
  ('Dogs')
, ('Big Dogs')
, ('Small Dogs')  
, ('Chihuahua')
, ('Pug')
, ('Pit Bull')
, ('Birds')
, ('Macaw')
, ('Finch')
;

INSERT INTO category_tree (ANC, DCS)
VALUES
  ('Dogs'       , 'Dogs')
, ('Birds'      , 'Birds')
, ('Dogs'       , 'Big Dogs')
, ('Dogs'       , 'Small Dogs')
, ('Big Dogs'   , 'Big Dogs')
, ('Small Dogs' , 'Small Dogs')
, ('Dogs'       , 'Chihuahua')
, ('Small Dogs' , 'Chihuahua')
, ('Chihuahua'  , 'Chihuahua')
, ('Dogs'       , 'Pug')
, ('Small Dogs' , 'Pug')
, ('Pug'        , 'Pug')
, ('Dogs'       , 'Pit Bull')
, ('Big Dogs'   , 'Pit Bull')
, ('Pit Bull'   , 'Pit Bull')
, ('Birds'      , 'Macaw')
, ('Macaw'      , 'Macaw')
, ('Birds'      , 'Finch')
, ('Finch'      , 'Finch')
;

INSERT INTO trainer (TRA, ANC)
VALUES
  ('Joe'    , 'Dogs')
, ('Jane'   , 'Small Dogs')
, ('Jane'   , 'Finch')
, ('Jill'   , 'Big Dogs')
, ('Jack'   , 'Birds')
, ('John'   , 'Pug')
;

编辑

如果整个 table 应该仅限于一个祖先，那么您可以：

-- Trainer TRA trains dog DCS; (ANC = 'Dogs').
--
dog_trainer {TRA, DSC, ANC}
         PK {TRA, DSC}

FK {ANC, DSC} REFERENCES category_tree {ANC, DCS}

CHECK (ANC = 'Dogs')

Foreign Key Constraint based on specific values in other table

Foreign Key Constraint based upon specific values in other table

sql

sql-server

database-design

foreign-keys

SQL 测试

编辑