Question

I’ve got a database with three tables: Books (with book details, PK is CopyID), Keywords (list of keywords, PK is ID) and KeywordsLink which is the many-many link table between Books and Keywords with the fields ID, BookID and KeywordID.

I’m trying to make an advanced search form in my app where you can search on various criteria. At the moment I have it working with Title, Author and Publisher (all from the Book table). It produces SQL like:

SELECT * FROM Books WHERE Title Like '%Software%' OR Author LIKE '%Spolsky%'; 

I want to extend this search to also search using tags – basically to add another OR clause to search the tags. I’ve tried to do this by doing the following

SELECT *     FROM Books, Keywords, Keywordslink     WHERE Title LIKE '%Joel%'        OR (Name LIKE '%good%' AND BookID=Books.CopyID AND KeywordID=Keywords.ID) 

I thought using the brackets might separate the 2nd part into its own kinda clause, so the join was only evaluated in that part – but it doesn’t seem to be so. All it gives me is a long list of multiple copies of the one book that satisfies the Title LIKE '%Joel%' bit.

Is there a way of doing this using pure SQL, or would I have to use two SQL statements and combine them in my app (removing duplicates in the process).

I’m using MySQL at the moment if that matters, but the app uses ODBC and I’m hoping to make it DB agnostic (might even use SQLite eventually or have it so the user can choose what DB to use).

Answer 1

What you’ve done here is made a cartesian result set by having the tables joined with the commas but not having any join criteria. Switch your statements to use outer join statements and that should allow you to reference the keywords. I don’t know your schema, but maybe something like this would work:

SELECT    *  FROM    Books    LEFT OUTER JOIN KeywordsLink ON KeywordsLink.BookID = Books.CopyID    LEFT OUTER JOIN Keywords ON Keywords.ID = KeywordsLink.KeywordID  WHERE Books.Title LIKE '%JOEL%'        OR Keywords.Name LIKE '%GOOD%'