How to do a Join Using Foreign Keys in Stored in a Database Column?

[Chris]

I have a database column that stores a comma delimited list of foreign
keys. Would someone show me how to do a join using the values from a
list stored within a record?

For example, a record in tbl_cds.genre_id might have a value of "2,
5, 6" corresponding to genre_ids 2 , 5 and 6. I want to join
tbl_cds.genre_id to tbl_genre.genre_id using the values in that data
field.

It seems I need a loop like this:
SELECT * FROM tbl_cds
WHERE
Begin Loop
tbl_cds.genre_id[i] = tbl_genre.genre_id
End Loop.

Would someone give me the correct syntax?
Is there an alternative method that would create less overhead?

Sorry for such a novice post.

[Hugo Kornelis]

On 17 Apr 2007 14:55:36 -0700, Chris wrote:

>I have a database column that stores a comma delimited list of foreign
keys. Would someone show me how to do a join using the values from a
list stored within a record?

For example, a record in tbl_cds.genre_id might have a value of "2,
5, 6" corresponding to genre_ids 2 , 5 and 6. I want to join
tbl_cds.genre_id to tbl_genre.genre_id using the values in that data
field.

It seems I need a loop like this:
SELECT * FROM tbl_cds
WHERE
Begin Loop
tbl_cds.genre_id[i] = tbl_genre.genre_id
End Loop.

Would someone give me the correct syntax?
Is there an alternative method that would create less overhead?

Sorry for such a novice post.

Hi Chris,

You can't. And that's because the database design volates one of the
basic principles of databases - you store a single value in a column.

If a CD can belong to three genres, you'll have to add a table like
this:

CREATE TABLE CDGenres
(CD_ID int NOT NULL,
Genre_ID int NOT NULL,
PRIMARY KEY (CD_ID, Genre_ID),
FOREIGN KEY (CD_ID) REFERENCES tbl_cds(CD_ID),
FOREIGN KEY (Genre_ID) REFERENCES Genres(Genre_ID)
);

If you don't have a master table of all genres, then you can leave out
the last foreign key constraint.

--
Hugo Kornelis, SQL Server MVP
My SQL Server blog: http://sqlblog.com/blogs/hugo_kornelis

[Erland Sommarskog]

Chris writes:

I have a database column that stores a comma delimited list of foreign
keys. Would someone show me how to do a join using the values from a
list stored within a record?

For example, a record in tbl_cds.genre_id might have a value of "2,
5, 6" corresponding to genre_ids 2 , 5 and 6. I want to join
tbl_cds.genre_id to tbl_genre.genre_id using the values in that data
field.

It seems I need a loop like this:
SELECT * FROM tbl_cds
WHERE
Begin Loop
tbl_cds.genre_id[i] = tbl_genre.genre_id
End Loop.

Would someone give me the correct syntax?
Is there an alternative method that would create less overhead?

As Hugo said, you are in desperate need of a redesign.

On SQL 2005 you can nevertheless do:

SELECT *
FROM tbl_cds c
OUTER APPLY list_to_table(c.genre_id) l
JOIN tbl_genre g ON l.num = g.genre_id

where list_to_table is a table-valued function that unpacks the
comma-separated list into a table. See
http://www.sommarskog.se/arrays-in-sql.html for examples of such functions.
--
Erland Sommarskog, SQL Server MVP, es****@sommarskog.se

Books Online for SQL Server 2005 at
http://www.microsoft.com/technet/pro...ads/books.mspx
Books Online for SQL Server 2000 at
http://www.microsoft.com/sql/prodinf...ons/books.mspx