sql-server - 如何创建外键，因此当我们删除子项中的父值时，将更新为 null

Question

我是 T-SQL 新手，请帮忙：我有两个表

User
  Id
  Name

History
  Id
  Result
  UserId

如何创建关系，因此一旦我从用户表中删除用户，历史表将针对列 UserId 更新为 NULL

Answer 1

你可以on delete set null这样使用

ALTER TABLE USERS 
ADD CONSTRAINT Users_ThemeID_FK 
FOREIGN KEY (ThemeID) REFERENCES Themes(ThemeID) 
ON DELETE SET NULL

Answer 2

ALTER TABLE dbo.History ADD CONSTRAINT
    FK_History_User FOREIGN KEY
    (
    UserID
    ) REFERENCES dbo.[User]
    (
    ID
    ) ON UPDATE  NO ACTION 
     ON DELETE  SET NULL 

Answer 3

确保打开Cascading deletes，然后设置 pK 关系，然后您只需使用常规 Delete 语法，如Delete From Table Where PK =

Answer 4

您可以将 FK 关系上的操作更改为设置为 null：

-- Create FK relationship
IF EXISTS (SELECT * FROM sys.objects
WHERE name = N'FK_EmpEducation_Employees' AND [type] = 'F')
ALTER TABLE EmpEducation
DROP Constraint FK_EmpEducation_Employees
GO

ALTER TABLE EmpEducation
ADD CONSTRAINT [FK_EmpEducation_Employees]
FOREIGN KEY (empno)REFERENCES employees(empno)
ON DELETE SET NULL ON UPDATE SET NULL
GO

见这里：http ://www.mssqltips.com/sqlservertip/2365/sql-server-foreign-key-update-and-delete-rules/