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POPostgres - Multiple joins is causing my query to return incorrect data
 

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  1. POPostgres - Multiple joins is causing my query to return incorrect data
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    <p>I have three tables in my database. Their schema is basically:</p> <pre><code>books: id | book_title books_tags: id | book_id | tag_id books_votes: id | book_id | vote </code></pre> <p>The idea is to be able to search for books and filter by given tags (716 and 101, in this case). The <code>total_votes</code> is used in the ORDER BY caluse.</p> <pre><code>SELECT books.id AS books_id, sum(book_votes.vote) AS total_votes FROM books JOIN -- works fine without this join book_votes ON books.id = book_votes.book_id JOIN books_tags ON books.id = books_tags.book_id WHERE books_tags.tag_id IN (716, 101) GROUP BY books.id HAVING count(books.id) = 2 </code></pre> <p>The tag filtering, by itself, works great. I can add as many tag ids to the IN clause as I wish and it will continue to filter the results to only show books with those tags. Perfect.</p> <p>The problem occurs when I add in the second JOIN to the <code>books_votes</code> table. This join doesn't produce any errors it just causes the query to return the wrong data - Like it's ignoring the tag ids.</p> <p>What's wrong with having the second join?</p> <p>EDIT:</p> <p>Here's the dumps from the tables:</p> <pre><code>books: id | book_title ----+----------------- 1 | first 2 | second 3 | third book 4 | fourth book 5 | fifth 6 | sixth book books_tags: id | book_id | tag_id ----+---------+-------- 1 | 1 | 293 2 | 1 | 32 3 | 1 | 370 4 | 2 | 101 5 | 2 | 357 6 | 3 | 554 7 | 3 | 808 8 | 3 | 716 9 | 3 | 101 10 | 4 | 787 11 | 4 | 808 12 | 4 | 322 13 | 5 | 787 17 | 6 | 716 18 | 6 | 554 19 | 6 | 101 books_votes: id | book_id | vote ----+---------+------ 2 | 2 | 1 3 | 3 | 1 4 | 4 | 1 7 | 4 | 1 8 | 2 | 1 11 | 5 | 1 12 | 5 | 1 13 | 1 | 1 </code></pre> <p>Here's the data returned, from the query I posted, when I leave out the second join (to books_votes):</p> <pre><code> book_id --------- 6 3 </code></pre> <p>As you can see, the correct books are returned. Books 6 and 3 have been tagged with the ids 716 and 101.</p> <p>Here's what's returned when I run the query with the books_votes table joined:</p> <pre><code> book_id | total_votes ---------+------------- 3 | 2 2 | 3 </code></pre>
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