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    <p>WHERE works directly on the rows, that's also the reason why you can't specify column aliases in WHERE clause. It would cause an unknown column error.</p> <p>You could use HAVING or a subquery. In both cases the result will be calculated for all rows, then filtered.</p> <p><strong>UPDATE:</strong></p> <p>You have to filter by distance, right? So you can't filter rows first, then calculate distance. But what you can do, is to use the spatial extension. Then you'll have spatial indexes which could make your queries pretty fast, I guess (never had anything to do with things like this).</p> <p>Read more about it <a href="http://dev.mysql.com/doc/refman/5.0/en/spatial-extensions.html" rel="nofollow">here</a>.</p> <p><strong>UPDATE 2:</strong></p> <p>Actually I don't get what your question is at all. You want to order by something you have to calculate first. So what's the question? Will it be fast? I don't know. All I can tell is, that no index can be used since MySQL does not support indexes on calculated columns. Probably a temporary table to sort whatever data you're getting will be used, too, depends on how much data it is. I'd suggest <strong>try things out</strong>. </p> <p>Just do </p> <pre><code>SELECT coordA, coordB FROM whereever ORDER BY coordA - coordB </code></pre> <p>and that's it. When you have problems then, ask again. </p> <p>To filter later</p> <pre><code>SELECT * FROM ( SELECT coordA, coordB, coordA - coordB AS distance FROM whereever ORDER BY distance ) sq WHERE distance &gt; $foo </code></pre> <p>When that's too slow play around with indexes and/or the extension mentioned earlier. </p>
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    1. POMySQL which is done first - filtering the WHERE conditions or calculating fields?
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    1. POMySQL which is done first - filtering the WHERE conditions or calculating fields?
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    1. COOk, thanks. so would you say that fetching the data with the WHERE conditions with one query, then using a second query to work out the distances and order the results based on the first query would be the best way of going about this (performance wise)?
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    2. CO@ollie Updated my answer.
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    3. COI don't have to filter by distance like the article I linked to - I was a little unclear - I need to order by distance but there are also other criteria that need to be met. If I could filter by these other criteria first then it would save working out the distance for entries which will never be included right? In theory, I could then filter by distance later if I needed to but on the smaller sample. (Added a couple of sentences to the original question to clarify)
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