As I'm working my way through Celkos "SQL for Smarties", he points out an interesting way to map such boolean queries by using GROUP BY and HAVING:

-- find all book ids coauthored by Smith and Jones:
-- (let's assume that the author_name is unique)
select book_id, count(*) from authors
where 
  author_name = 'Smith'
  or author_name = 'Jones'
group by book_id
having count(*) = 2

[download]

Whether the performance of this is better or worse than selecting all books by one and all books by the other and scanning the lists in Perl, or worse than dynamically creating the self joins needs to be benchmarked. Another way to make this slightly less dynamic could be to use the IN predicate:

-- find all book ids coauthored by Smith and Jones:
-- (let's assume that the author_name is unique)
select book_id, count(*) from authors
where 
  author_name in ('Smith', 'Jones')
group by book_id
having count(*) = 2

[download]

To make it completely static, you could insert the authors into a (session-local) temporary table and cross-join that table.

Update: Oops - this approach will also return books co-authored by 'Smith', 'Jones' and 'Miller'. Depending on whether that's what you want or not, you will need to use a subselect to make sure you get all of the authors. But I'm not sure if I can come up with the correct subselect at the moment :-)

In reply to Re: Many-to-many relationships in databases by Corion
in thread Many-to-many relationships in databases : SOLVED by CountZero

Title:
Use:  <p> text here (a paragraph) </p>
and:  <code> code here </code>
to format your post, it's "PerlMonks-approved HTML":


  • Posts are HTML formatted. Put <p> </p> tags around your paragraphs. Put <code> </code> tags around your code and data!
  • Titles consisting of a single word are discouraged, and in most cases are disallowed outright.
  • Read Where should I post X? if you're not absolutely sure you're posting in the right place.
  • Please read these before you post! —
  • Posts may use any of the Perl Monks Approved HTML tags:
    a, abbr, b, big, blockquote, br, caption, center, col, colgroup, dd, del, details, div, dl, dt, em, font, h1, h2, h3, h4, h5, h6, hr, i, ins, li, ol, p, pre, readmore, small, span, spoiler, strike, strong, sub, summary, sup, table, tbody, td, tfoot, th, thead, tr, tt, u, ul, wbr
  • You may need to use entities for some characters, as follows. (Exception: Within code tags, you can put the characters literally.)
            For:     Use:
    & &amp;
    < &lt;
    > &gt;
    [ &#91;
    ] &#93;
  • Link using PerlMonks shortcuts! What shortcuts can I use for linking?
  • See Writeup Formatting Tips and other pages linked from there for more info.