问：
In SQL Server , I got this error ->
"There are no primary or candidate keys in the referenced table 'BookTitle' that match the referencing column list in the foreign key 'FK_BookCopy_Title__2F10007B'."
I first created a relation called the BookTitle relation.
CREATE TABLE BookTitle ( ISBN CHAR(17) NOT NULL, Title VARCHAR(100) NOT NULL, Author_Name VARCHAR(30) NOT NULL, Publisher VARCHAR(30) NOT NULL, Genre VARCHAR(20) NOT NULL, Language CHAR(3) NOT NULL, PRIMARY KEY (ISBN, Title))
Then I created a relation called the BookCopy relation. This relation needs to reference to the BookTitle relation's primary key, Title.
CREATE TABLE BookCopy ( CopyNumber CHAR(10) NOT NULL, Title VARCHAR(100) NOT NULL, Date_Purchased DATE NOT NULL, Amount DECIMAL(5, 2) NOT NULL, PRIMARY KEY (CopyNumber), FOREIGN KEY (Title) REFERENCES BookTitle(Title))
But I can't create the BookCopy relation because the error stated above appeared.
I really appreciate some useful help.
答：
Foreign keys work by joining a column to a unique key in another table, and that unique key must be defined as some form of unique index, be it the primary key, or some other unique index.
At the moment, the only unique index you have is a compound one on ISBN, Title which is your primary key.
There are a number of options open to you, depending on exactly what BookTitle holds and the relationship of the data within it.
I would hazard a guess that the ISBN is unique for each row in BookTitle. ON the assumption this is the case, then change your primary key to be only on ISBN, and change BookCopy so that instead of Title you have ISBN and join on that.
If you need to keep your primary key as ISBN, Title then you either need to store the ISBN in BookCopy as well as the Title, and foreign key on both columns, OR you need to create a unique index on BookTitle(Title) as a distinct index.
More generally, you need to make sure that the column or columns you have in your REFERENCES clause match exactly a unique index in the parent table: in your case it fails because you do not have a single unique index on Title alone.
所以我们可以看到，在SQL Server中，一个表的外键必须引用的是另一个表的主键或唯一键，否者外键关系是无法被成功建立的，会报错。