如何在SQL Server中创建外键？

我从未为SQL Server编写过“手工编码”的对象创建代码，外键解密似乎在SQL Server和Postgres之间是不同的。到目前为止，这是我的SQL：

drop table exams;
drop table question_bank;
drop table anwser_bank;

create table exams
(
    exam_id uniqueidentifier primary key,
    exam_name varchar(50),
);
create table question_bank
(
    question_id uniqueidentifier primary key,
    question_exam_id uniqueidentifier not null,
    question_text varchar(1024) not null,
    question_point_value decimal,
    constraint question_exam_id foreign key references exams(exam_id)
);
create table anwser_bank
(
    anwser_id           uniqueidentifier primary key,
    anwser_question_id  uniqueidentifier,
    anwser_text         varchar(1024),
    anwser_is_correct   bit
);

当我运行查询时，出现以下错误：

消息8139，级别16，状态0，第9行外键中引用列的数量与表'question_bank'的引用列的数量不同。

您能发现错误吗？

create table question_bank
(
    question_id uniqueidentifier primary key,
    question_exam_id uniqueidentifier not null,
    question_text varchar(1024) not null,
    question_point_value decimal,
    constraint fk_questionbank_exams foreign key (question_exam_id) references exams (exam_id)
);

而且，如果您只想自己创建约束，则可以使用ALTER TABLE

alter table MyTable
add constraint MyTable_MyColumn_FK FOREIGN KEY ( MyColumn ) references MyOtherTable(PKColumn)

我不推荐Sara Chipps提到的用于内联创建的语法，因为我宁愿命名自己的约束。

您还可以使用以下方法来命名外键约束：

CONSTRAINT your_name_here FOREIGN KEY (question_exam_id) REFERENCES EXAMS (exam_id)

我喜欢AlexCuse的答案，但是每当添加外键约束时，您都应该注意的是如何处理对引用表行中的引用列的更新，尤其是如何删除引用表中的行表要处理。

如果这样创建约束：

alter table MyTable
add constraint MyTable_MyColumn_FK FOREIGN KEY ( MyColumn ) 
references MyOtherTable(PKColumn)

..然后，如果引用表中有相应的行，则引用表中的更新或删除将因错误而崩溃。

那可能是您想要的行为，但是根据我的经验，它通常不是。

如果改为这样创建它：

alter table MyTable
add constraint MyTable_MyColumn_FK FOREIGN KEY ( MyColumn ) 
references MyOtherTable(PKColumn)
on update cascade 
on delete cascade

..then然后在父表中进行更新和删除将导致在引用表中相应行的更新和删除。

（我不建议更改默认值，这是谨慎的做法，这是很好的。我只是说这是创建内容的人应该经常注意的事情。）

顺便说一下，这可以在创建表时完成，如下所示：

create table ProductCategories (
  Id           int identity primary key,
  ProductId    int references Products(Id)
               on update cascade on delete cascade
  CategoryId   int references Categories(Id) 
               on update cascade on delete cascade
)

create table question_bank
(
    question_id uniqueidentifier primary key,
    question_exam_id uniqueidentifier not null constraint fk_exam_id foreign key references exams(exam_id),
    question_text varchar(1024) not null,
    question_point_value decimal
);

-那也行。也许更直观的构造？

在任何表上创建外键

ALTER TABLE [SCHEMA].[TABLENAME] ADD FOREIGN KEY (COLUMNNAME) REFERENCES [TABLENAME](COLUMNNAME)
EXAMPLE
ALTER TABLE [dbo].[UserMaster] ADD FOREIGN KEY (City_Id) REFERENCES [dbo].[CityMaster](City_Id)

如果要使用查询在一个关系中创建两个表的列，请尝试以下操作：

Alter table Foreign_Key_Table_name add constraint 
Foreign_Key_Table_name_Columnname_FK
Foreign Key (Column_name) references 
Another_Table_name(Another_Table_Column_name)

像您一样，我通常不手动创建外键，但是如果出于某些原因需要脚本，我通常使用ms sql服务器管理工​​作室创建它，然后在保存然后进行更改之前，选择“表设计器” |“表设计器”。生成变更脚本

该脚本是关于使用外键创建表的，并且添加了参照完整性约束sql-server。

create table exams
(  
    exam_id int primary key,
    exam_name varchar(50),
);

create table question_bank 
(
    question_id int primary key,
    question_exam_id int not null,
    question_text varchar(1024) not null,
    question_point_value decimal,
    constraint question_exam_id_fk
       foreign key references exams(exam_id)
               ON DELETE CASCADE
);

死灵法师。
实际上，正确执行此操作比较麻烦。

首先，您需要检查要设置外键引用的列是否存在主键。

在此示例中，在表T_ZO_SYS_Language_Forms上创建了一个外键，引用了dbo.T_SYS_Language_Forms.LANG_UID

-- First, chech if the table exists...
IF 0 < (
    SELECT COUNT(*) FROM INFORMATION_SCHEMA.TABLES 
    WHERE TABLE_TYPE = 'BASE TABLE'
    AND TABLE_SCHEMA = 'dbo'
    AND TABLE_NAME = 'T_SYS_Language_Forms'
)
BEGIN
    -- Check for NULL values in the primary-key column
    IF 0 = (SELECT COUNT(*) FROM T_SYS_Language_Forms WHERE LANG_UID IS NULL)
    BEGIN
        ALTER TABLE T_SYS_Language_Forms ALTER COLUMN LANG_UID uniqueidentifier NOT NULL 

        -- No, don't drop, FK references might already exist...
        -- Drop PK if exists 
        -- ALTER TABLE T_SYS_Language_Forms DROP CONSTRAINT pk_constraint_name 
        --DECLARE @pkDropCommand nvarchar(1000) 
        --SET @pkDropCommand = N'ALTER TABLE T_SYS_Language_Forms DROP CONSTRAINT ' + QUOTENAME((SELECT CONSTRAINT_NAME FROM INFORMATION_SCHEMA.TABLE_CONSTRAINTS 
        --WHERE CONSTRAINT_TYPE = 'PRIMARY KEY' 
        --AND TABLE_SCHEMA = 'dbo' 
        --AND TABLE_NAME = 'T_SYS_Language_Forms' 
        ----AND CONSTRAINT_NAME = 'PK_T_SYS_Language_Forms' 
        --))
        ---- PRINT @pkDropCommand 
        --EXECUTE(@pkDropCommand) 

        -- Instead do
        -- EXEC sp_rename 'dbo.T_SYS_Language_Forms.PK_T_SYS_Language_Forms1234565', 'PK_T_SYS_Language_Forms';


        -- Check if they keys are unique (it is very possible they might not be) 
        IF 1 >= (SELECT TOP 1 COUNT(*) AS cnt FROM T_SYS_Language_Forms GROUP BY LANG_UID ORDER BY cnt DESC)
        BEGIN

            -- If no Primary key for this table
            IF 0 =  
            (
                SELECT COUNT(*) FROM INFORMATION_SCHEMA.TABLE_CONSTRAINTS 
                WHERE CONSTRAINT_TYPE = 'PRIMARY KEY' 
                AND TABLE_SCHEMA = 'dbo' 
                AND TABLE_NAME = 'T_SYS_Language_Forms' 
                -- AND CONSTRAINT_NAME = 'PK_T_SYS_Language_Forms' 
            )
                ALTER TABLE T_SYS_Language_Forms ADD CONSTRAINT PK_T_SYS_Language_Forms PRIMARY KEY CLUSTERED (LANG_UID ASC)
            ;

            -- Adding foreign key
            IF 0 = (SELECT COUNT(*) FROM INFORMATION_SCHEMA.REFERENTIAL_CONSTRAINTS WHERE CONSTRAINT_NAME = 'FK_T_ZO_SYS_Language_Forms_T_SYS_Language_Forms') 
                ALTER TABLE T_ZO_SYS_Language_Forms WITH NOCHECK ADD CONSTRAINT FK_T_ZO_SYS_Language_Forms_T_SYS_Language_Forms FOREIGN KEY(ZOLANG_LANG_UID) REFERENCES T_SYS_Language_Forms(LANG_UID); 
        END -- End uniqueness check
        ELSE
            PRINT 'FSCK, this column has duplicate keys, and can thus not be changed to primary key...' 
    END -- End NULL check
    ELSE
        PRINT 'FSCK, need to figure out how to update NULL value(s)...' 
END 

我总是使用这种语法在2个表之间创建外键约束

Alter Table ForeignKeyTable
Add constraint `ForeignKeyTable_ForeignKeyColumn_FK`
`Foreign key (ForeignKeyColumn)` references `PrimaryKeyTable (PrimaryKeyColumn)`

即

Alter Table tblEmployee
Add constraint tblEmployee_DepartmentID_FK
foreign key (DepartmentID) references tblDepartment (ID)