如何解决多个级联路径？答案

【问题标题】：How to solve multiple cascade paths?如何解决多个级联路径？
【发布时间】：2015-10-14 13:23:41
【问题描述】：

在我的数据库中，我有一个 Person 和 Member 表。 Member 表有 2 个外键，它们都引用 People 表中的同一列。约束如下所示：

CONSTRAINT [FK_Members_People_1] 
  FOREIGN KEY ([PersonID])      
  REFERENCES [People].[People]([ID])
  ON DELETE CASCADE
  ON UPDATE NO ACTION,

CONSTRAINT [FK_Members_People_2] 
  FOREIGN KEY ([EnquiryTakenBy]) 
  REFERENCES [People].[People]([ID])
  ON DELETE SET NULL
  ON UPDATE NO ACTION

我得到的错误如下：

Introducing FOREIGN KEY constraint 'FK_Members_People_2' on table 'Members' may cause cycles or multiple cascade paths. Specify ON DELETE NO ACTION or ON UPDATE NO ACTION, or modify other FOREIGN KEY constraints.

我将如何解决这个问题？

基本上这是我想要得到的行为：

当 PersonID 引用的记录被删除时。我需要它来删除其他表中的所有子记录。（有 8 个子表）
当 EnquiryTakenBy 引用的记录被删除时。我需要将 EnquiryTakenBy 设置为 null。

编辑：表结构如下：

CREATE TABLE [People].[People]
(
    [ID]                    INT          NOT NULL IDENTITY,
    [PersonType]            INT          NOT NULL,         
    [Forename]              VARCHAR(16)  NOT NULL,
    [Surname]               VARCHAR(32)  NOT NULL,
    [Gender]                CHAR(1)      NOT NULL,
    [DateOfBirth]           DATE         NULL,
    [HobbiesAndInterests]   VARCHAR(256) NULL,
    [AdditionalInformation] VARCHAR(512) NULL,
    [LocalCentre]           INT          NOT NULL  DEFAULT 0,

    CONSTRAINT [PK_People] PRIMARY KEY ([ID]),

    CONSTRAINT [FK_People_PersonType] 
      FOREIGN KEY ([PersonType])  
      REFERENCES [Lookups].[PersonTypes]([ID])
      ON DELETE CASCADE
      ON UPDATE CASCADE,

    CONSTRAINT [FK_People_Centres]    
      FOREIGN KEY ([LocalCentre]) 
      REFERENCES [Lookups].[Centres]([ID])
      ON DELETE CASCADE
      ON UPDATE CASCADE
)

CREATE TABLE [People].[Members]
(
    [PersonID]         INT          NOT NULL,
    [IsActive]         BIT          NOT NULL  DEFAULT 0,
    [Issues]           VARCHAR(500) NULL,
    [InTreatment]      BIT          NOT NULL  DEFAULT 0,
    [ProblemSubstance] VARCHAR(64)  NOT NULL,
    [WantsHelpWith]    VARCHAR(128) NULL,
    [EnquiryTakenBy]   INT          NOT NULL,
    [IsVolunteer]      BIT          NOT NULL  DEFAULT 0,

    CONSTRAINT [PK_Members] PRIMARY KEY ([PersonID]),

    CONSTRAINT [FK_Members_People_1] 
      FOREIGN KEY ([PersonID])      
      REFERENCES [People].[People]([ID])
      ON DELETE CASCADE
      ON UPDATE NO ACTION,

    CONSTRAINT [FK_Members_People_2] 
      FOREIGN KEY ([EnquiryTakenBy]) 
      REFERENCES [People].[People]([ID])
      ON DELETE SET NULL
      ON UPDATE NO ACTION

)

【问题讨论】：

标签： sql-server class-table-inheritance

【解决方案1】：

好的，找到了解决方案。显然我的问题与 SQL Server 不知道哪个约束优先，所以我用这个触发器替换了第二个外键约束：

CREATE TRIGGER Member_UpdateEnquiryID ON People.People
AFTER DELETE
AS
BEGIN
   DECLARE @id int = (select id from deleted);
   UPDATE People.Members
   SET EnquiryTakenBy = NULL
   where EnquiryTakenBy = @id
END

【讨论】：