如何添加约束在SQL

Question

删除我的书，bookcategories表和类别

它看起来像这样

Book 
    Id uniqueidentifier, 
    Title varchar(255), 
    Author varchar(255), 
    Deleted datetime 

BookCategory 
    BookId 
    CategoryId 

Category 
    Id uniqueidentifier 
    Name varchar(255) 
    Deleted datetime 

我想写一个约束，这将阻止删除一本书，如果该书的类别存在

意思是，如果一本书（可以说哈利波特）有一个小说类别，例如表BookCategory将包括这两个Id的当然。如果用户想要删除具有特定类别的图书，他将无法执行此操作。

Can some1能帮助我吗？

PS当我删除项目，我没有实际删除它们，但设置属性删除datetime。

Answer 1

既然要执行软删除，你可以完成你想要的东西通过添加一些额外的辅助列和外键：

create table Books (
    Id uniqueidentifier primary key, 
    Title varchar(255) not null, 
    Author varchar(255) not null, 
    Deleted datetime null, 
    _DelXRef as CASE WHEN Deleted is null then 0 else 1 END persisted, 
    constraint UQ_Books_DelXRef UNIQUE (Id,_DelXRef) 
) 
create table Categories (
    Id uniqueidentifier primary key, 
    Name varchar(255) not null, 
    Deleted datetime null, 
    _DelXRef as CASE WHEN Deleted is null then 0 else 1 END persisted, 
    constraint UQ_Categories_DelXRef UNIQUE (Id,_DelXRef) 
) 
create table BookCategories (
    BookId uniqueidentifier not null, 
    CategoryId uniqueidentifier not null, 
    _DelXRef as 0 persisted, 
    constraint FK_BookCategories_Books foreign key (BookID) references Books(Id), 
    constraint FK_BookCategories_Books_DelXRef foreign key (BookID,_DelXRef) references Books(Id,_DelXRef), 
    constraint FK_BookCategories_Categories foreign key (CategoryId) references Categories(Id), 
    constraint FK_BookCategories_Categories_DelXRef foreign key (CategoryId,_DelXRef) references Categories(Id,_DelXRef) 
) 

希望，你可以看到外键如何确保引用表中的_DelXRef列始终保持0，因此无法将Deleted设置为任何非NULL值，同时从BookCategories表中引用该行。 （现在，“原始”外键，FK_BookCategories_Books和FK_BookCategories_Categories似乎是多余的。我更喜欢让他们在模型中记录真正的FK关系。我也使用我自己的约定前缀对象与_其中，它并不意味着他们可以使用到数据库的用户 - 他们只是存在，使DRI予以强制执行）

Answer 2

这回答了编辑前的原始问题。

你正在寻找一个外键约束：

alter table BookCategory add constraint fk_BookCategory_Book 
    foreign key (BookId) references Book(Id); 

默认情况下，这样的限制不会允许你删除一本书，有一个类别。您可以了解cascading选项以提供更精确的行为。不提供级联操作时的默认值是ON DELETE NO ACTION，这意味着Book中的行不会被删除。

Answer 3

在表类别中为id创建一个主键，并用外键将它与表bookcategory（categoryid）相关联。这将确保您不会在不能删除同时不引用父表类别的情况下添加一个id类别表中的记录也是如此。

Answer 4

如果我明白，你要做的是限制对表格的更新以将记录标记为“已删除”。表上的约束不能做到这一点，但你可以通过更新触发器来完成。此外，你应该尝试在你的应用程序中处理这种逻辑。阅读这问那与你相似：Can an SQL constraint be used to prevent a particular value being changed when a condition holds?

Answer 5

CREATE TABLE Book 
(
    Id  UNIQUEIDENTIFIER 
    , Title VARCHAR(255) 
    , Author VARCHAR(255) 
    , Deleted DATETIME 
); 

CREATE TABLE BookCategory 
(
    BookId  UNIQUEIDENTIFIER 
    , CategoryId UNIQUEIDENTIFIER 
); 

CREATE TABLE Category 
(
    Id  UNIQUEIDENTIFIER 
    , Name VARCHAR(255) 
    , Deleted DATETIME 
); 

INSERT INTO dbo.Book ( Id 
         , Title 
         , Author 
         , Deleted 
        ) 
VALUES ( 'BCF8DE45-D26F-43EE-82CD-9975E35E51B1' -- Id - uniqueidentifier 
     , 'Harry Potter'       -- Title - varchar(255) 
     , 'RK'         -- Author - varchar(255) 
     , NULL         -- Deleted - datetime 
     ); 

INSERT INTO dbo.Category ( Id 
          , Name 
          , Deleted 
         ) 
VALUES ( 'EB6E823D-DFE8-448D-B648-EAE1EFE06358' -- Id - uniqueidentifier 
     , 'Fantasy'        -- Name - varchar(255) 
     , NULL         -- Deleted - datetime 
     ); 

INSERT INTO dbo.Category ( Id 
          , Name 
          , Deleted 
         ) 
VALUES ( '9B53C866-0DAA-4637-8169-5B8885A2E644' -- Id - uniqueidentifier 
     , 'Category without books'    -- Name - varchar(255) 
     , NULL         -- Deleted - datetime 
     ); 

INSERT INTO dbo.BookCategory ( BookId 
           , CategoryId 
          ) 
VALUES ( 'BCF8DE45-D26F-43EE-82CD-9975E35E51B1' -- BookId - int 
     , 'EB6E823D-DFE8-448D-B648-EAE1EFE06358' -- CategoryId - int 
     ); 
GO 

CREATE TRIGGER trg_Category_Check_Category_Usage 
ON Category 
INSTEAD OF UPDATE 
AS 
    BEGIN 
     IF UPDATE(Deleted) 
      BEGIN 
       IF EXISTS ( SELECT * 
           FROM INSERTED   i 
           JOIN dbo.BookCategory AS b ON b.CategoryId = i.Id 
           WHERE i.Deleted IS NOT NULL 
         ) 
        THROW 60000, 'record exists', 1; 
      END; 

     UPDATE b 
      SET b.deleted = i.deleted 
      , b.Name = i.Name 
      FROM Category b 
      JOIN INSERTED i ON i.Id = b.Id; 
    END; 
GO 
UPDATE dbo.Category SET Deleted = GETDATE()WHERE Name = 'Fantasy'; --error 

UPDATE dbo.Category 
    SET Deleted = GETDATE() 
WHERE Name = 'Category without books'; --no error