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POUnderstanding how JOIN works when 3 or more tables are involved. [SQL]
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copied!<p>I wonder if anyone can help improve my understanding of JOINs in SQL. [If it is significant to the problem, I am thinking MS SQL Server specifically.]</p> <p>Take 3 tables A, B [A related to be by some A.AId], and C [B related to C by some B.BId]</p> <p>If I compose a query e.g</p> <pre><code>SELECT * FROM A JOIN B ON A.AId = B.AId </code></pre> <p>All good - I'm sweet with how this works.</p> <p>What happens when Table C (Or some other D,E, .... gets added)</p> <p>In the situation</p> <pre><code>SELECT * FROM A JOIN B ON A.AId = B.AId JOIN C ON C.BId = B.BId </code></pre> <p>What is C joining to? - is it that B table (and the values in the B table?) Or is it some other temporary result set that is the result of the A+B Join that the C table is joined to?</p> <p>[The implication being not all values that are in the B table will necessarily be in the temporary result set A+B based on the join condition for A,B]</p> <p>A specific (and fairly contrived) example of why I am asking is because I am trying to understand behaviour I am seeing in the following:</p> <pre><code>Tables Account (AccountId, AccountBalanceDate, OpeningBalanceId, ClosingBalanceId) Balance (BalanceId) BalanceToken (BalanceId, TokenAmount) Where: Account->Opening, and Closing Balances are NULLABLE (may have opening balance, closing balance, or none) Balance->BalanceToken is 1:m - a balance could consist of many tokens </code></pre> <p>Conceptually, Closing Balance of a date, would be tomorrows opening balance</p> <p>If I was trying to find a list of all the opening and closing balances for an account</p> <p>I might do something like</p> <pre><code>SELECT AccountId , AccountBalanceDate , Sum (openingBalanceAmounts.TokenAmount) AS OpeningBalance , Sum (closingBalanceAmounts.TokenAmount) AS ClosingBalance FROM Account A LEFT JOIN BALANCE OpeningBal ON A.OpeningBalanceId = OpeningBal.BalanceId LEFT JOIN BALANCE ClosingBal ON A.ClosingBalanceId = ClosingBal.BalanceId LEFT JOIN BalanceToken openingBalanceAmounts ON openingBalanceAmounts.BalanceId = OpeningBal.BalanceId LEFT JOIN BalanceToken closingBalanceAmounts ON closingBalanceAmounts.BalanceId = ClosingBal.BalanceId GROUP BY AccountId, AccountBalanceDate </code></pre> <p>Things work as I would expect until the last JOIN brings in the closing balance tokens - where I end up with duplicates in the result.</p> <p>[I can fix with a DISTINCT - but I am trying to understand why what is happening is happening]</p> <p>I have been told the problem is because the relationship between Balance, and BalanceToken is 1:M - and that when I bring in the last JOIN I am getting duplicates because the 3rd JOIN has already brought in BalanceIds multiple times into the (I assume) temporary result set.</p> <p>I know that the example tables do not conform to good DB design</p> <p>Apologies for the essay, thanks for any elightenment :)</p> <p>Edit in response to question by Marc</p> <p>Conceptually for an account there should not be duplicates in BalanceToken for An Account (per AccountingDate) - I think the problem comes about because 1 Account / AccountingDates closing balance is that Accounts opening balance for the next day - so when self joining to Balance, BalanceToken multiple times to get opening and closing balances I think Balances (BalanceId's) are being brought into the 'result mix' multiple times. If it helps to clarify the second example, think of it as a daily reconciliation - hence left joins - an opening (and/or) closing balance may not have been calculated for a given account / accountingdate combination.</p>

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