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POHow to select just 1 child table item for each parent record?
 

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  1. POHow to select just 1 child table item for each parent record?
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    <p>I have a table of lists and a table of list items. I want to formulate a query to select just one list item for each item in the lists table. Here's some simple data to illustrate my question:</p> <pre><code> 'lists' table id updated share --- ---------- ----- 1 2013-07-11 1 2 2013-07-13 0 3 2013-07-15 1 4 2013-07-14 0 5 2013-07-14 1 'list_items' table id l_id description sort likes -- ---- ----------- ---- ----- 1 1 hello 0 3 2 1 goodbye 0 0 3 1 thanks 0 4 4 2 ok 0 0 5 3 love 0 2 6 3 hate 1 1 7 4 celebrate 0 0 8 5 party 0 1 9 5 summer 1 5 10 5 winter 2 2 </code></pre> <p>Now say I want to get the first item from each shared list (share = 1). By first I mean if the list items were sorted by 'sort'.</p> <p>The expected result based on the above data would be:</p> <pre><code>lists.id id l_id description sort likes -------- -- ---- ----------- ---- ----- 1 1 1 hello 0 3 3 5 3 love 0 2 5 8 5 party 0 1 </code></pre> <p><strong>Update:</strong> I struggled to get my head around the solutions provided by peterm and hims056 and while kayla's solution looked more like something I could follow it didn't return the correct results. Taking ideas from these solutions I had a crack at it myself and came up with</p> <pre><code> SELECT * FROM ( SELECT lists.id AS listid, lists.share, list_items.* FROM list_items, lists WHERE lists.id = l_id AND lists.share = 1 ORDER BY sort) q GROUP BY q.listid </code></pre> <p>This seems to work but as peterm points out, the values for the columns in select clause that are not part of group by clause may be ambiguous.</p> <p>I though someone would come up with a solution using <code>LIMIT</code> as that was the way I was thinking about doing it first. You can return the list ids which allow sharing simply by:</p> <pre><code>SELECT lists.id FROM lists WHERE share = 1 </code></pre> <p>and for a given list id you can return the top list item by:</p> <pre><code>SELECT lists.id AS listid, lists.share, list_items.* FROM list_items, lists WHERE lists.id = l_id AND lists.id = 1 ORDER BY sort LIMIT 1 </code></pre> <p>But is there a way to put these 2 statements together to return top list item for each list that allows sharing?</p>
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