Question

Say I have two tables, author and book. Each author can have multiple books, identified by the authorId column of the book table which is linked to the id field of author table.

I want to fetch all the books with a certain category Id, and for each book I want to get the author’s name and photo url to be displayed next to the book title.

So I do a query like this:

SELECT author.name, author.photoUrl, book.title
    FROM author, book
    WHERE book.categoryId = '3' AND author.id = book.authorId

The problem is, what if an author has multiple books in the category? (E.g a fiction writer writes multiple fiction books). In that case, will the author’s info be fetched separately for each row, hence fetching duplicate info, or is there any way such as using DISTINCT so an author’s info is only fetched once?

Answer 1

Yes, it would return duplicates on the author but I dont think that is a problem.
An alternative could be to have two queries instead, one for the books and one for the authors, and than in code set up a relationship (in a DataSet or using a Dictionary<int, Author>) between the results on AuthorId.

Books

SELECT 
  book.title,
  book.authorId
FROM 
  book
WHERE 
  book.categoryId = '3'

Authors

SELECT 
  author.Id,
  author.name, 
  author.photoUrl
FROM
  author
WHERE
  EXISTS(
     SELECT NULL
     FROM book
     WHERE 
       book.categoryId = '3' AND
       author.id = book.authorId
  )