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    <p>You are mixing implicit joins with explicit joins. That is allowed, but you need to be aware of how to do that properly.</p> <p>The thing is, explicit joins (the ones that are implemented using the <code>JOIN</code> keyword) take precedence over implicit ones (the 'comma' joins, where the join condition is specified in the <code>WHERE</code> clause).</p> <p>Here's an outline of your query:</p> <pre><code>SELECT … FROM a, b LEFT JOIN dkcd ON … WHERE … </code></pre> <p>You are probably expecting it to behave like this:</p> <pre><code>SELECT … FROM (a, b) LEFT JOIN dkcd ON … WHERE … </code></pre> <p>that is, the combination of tables <code>a</code> and <code>b</code> is joined with the table <code>dkcd</code>. In fact, what's happening is</p> <pre><code>SELECT … FROM a, (b LEFT JOIN dkcd ON …) WHERE … </code></pre> <p>that is, as you may already have understood, <code>dkcd</code> is joined specifically against <code>b</code> and only <code>b</code>, then the result of the join is combined with <code>a</code> and filtered further with the <code>WHERE</code> clause. In this case, any reference to <code>a</code> in the <code>ON</code> clause is invalid, <code>a</code> is unknown at that point. That is why you are getting the error message.</p> <p>If I were you, I would probably try to rewrite this query, and one possible solution might be:</p> <pre><code>SELECT DISTINCT a.maxa, b.mahuyen, a.tenxa, b.tenhuyen, ISNULL(dkcd.tong, 0) AS tongdkcd FROM phuongxa a INNER JOIN quanhuyen b ON LEFT(a.maxa, 2) = b.mahuyen LEFT OUTER JOIN ( SELECT maxa, COUNT(*) AS tong FROM khaosat WHERE CONVERT(datetime, ngaylap, 103) BETWEEN 'Sep 1 2011' AND 'Sep 5 2011' GROUP BY maxa ) AS dkcd ON dkcd.maxa = a.maxa WHERE a.maxa &lt;&gt; '99' ORDER BY a.maxa </code></pre> <p>Here the tables <code>a</code> and <code>b</code> are joined first, then the result is joined to <code>dkcd</code>. Basically, this is the same query as yours, only using a different syntax for one of the joins, which makes a great difference: the reference <code>a.maxa</code> in the <code>dkcd</code>'s join condition is now absolutely valid.</p> <p>As @Aaron Bertrand has correctly noted, you should probably qualify <code>maxa</code> with a specific alias, probably <code>a</code>, in the <code>ORDER BY</code> clause.</p>
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    1. COORDER BY maxa is still ambiguous, no? Also I'd be careful with 'Sep 1 2011' as a date, won't work with different language/regional settings.
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    2. CO@Aaron: Agree about `ORDER BY maxa`, thanks. As for the dates, I believe that's how the OP has chosen to specify them in their environment.
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    3. CO"explicit joins... take precedence over implicit ones" -- can you provide a citation for this, please? e.g. is that defined in SQL Standards or is it a product feature? Thanks.
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