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POHow do I employ user variables to select x records for each value of a given column?
 

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  1. POHow do I employ user variables to select x records for each value of a given column?
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    <p><strong>EDIT: I've discovered this problem only appears when I use phpMyAdmin to execute the query, and not when I execute the same query using PHP's mysql_query(). (My webhost does not provide command-line access.) Specifically, when run on a table of 65k+ rows, phpMyAdmin appears to hang, but mysql_query() returns the expected result.</strong></p> <hr> <p>I have a table: products (product_id, shop_id, date)</p> <p>For each shop_id in the table, the following query retrieves the 10 rows with the most recent dates</p> <p><pre><code> set @num := 0, @shop_id := NULL;</p> <p>select shop_id, date from ( select shop_id, date, @num := if(@shop_id = shop_id, @num + 1, 1) as row_number, @shop_id := shop_id as dummy from products order by shop_id, date DESC ) as x where x.row_number &lt;= 10; </pre></code></p> <p>For example, if there are 10 distinct shop_ids, and each of these shop_ids appears in at least 10 rows, the query will return 100 rows, 10 per shop_id. This query works fine.</p> <p>In addition to the date and shop_id, I would also like to select the product_id for each row. However while the following query works fine up to abut 55k rows, when I try 65k rows, MySQL appears to hang indefinitely (at least the phpMyAdmin interface fails to return a page of results):</p> <p><pre><code> set @num := 0, @shop_id := NULL;</p> <p>select product_id, shop_id, date from ( select product_id, shop_id, date, @num := if(@shop_id = shop_id, @num + 1, 1) as row_number, @shop_id := shop_id as dummy from products order by shop_id, date DESC ) as x where row_number &lt;= 10; </pre></code></p> <p>I believe my tenuous grasp of user variables is to blame, but I cannot figure out what I am doing wrong.</p>
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    1. PTQuestion
    1. USjela
    1. USjela
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    1. COdid you switch `@shop_id := 0` to `@shop_id := NULL` on purpose?
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      1. POHow do I employ user variables to select x records for each value of a given column?
      1. USGalz
    2. CO@Galz oops, it occurred to me at some point that there might be a shop_id of 0, and thus it would make sense not to initialize @shop_id with that value. In order to avoid confusion, I will edit the original query so that they both use NULL. This change does not affect the result of the query.
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      1. POHow do I employ user variables to select x records for each value of a given column?
      1. USjela
    3. COIt turns out that employing the following JOIN provides the correct result: `set @num := 0, @shop_id := NULL; select p.product_id,x.shop_id, x.date from ( select shop_id, date, @num := if(@shop_id = shop_id, @num + 1, 1) as row_number, @shop_id := shop_id as dummy from products order by shop_id, date DESC ) as x join products p on x.shop_id = p.shop_id and s.date = p.date where x.row_number <= 10; ` however I do not yet understand why my original syntax is incorrect, and will consider that explanation to be the answer to the question.
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      1. POHow do I employ user variables to select x records for each value of a given column?
      1. USjela
 

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