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POWhat's happening in this MYSQL query with a left join and a group by (on the wrong column)?
 

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  1. POWhat's happening in this MYSQL query with a left join and a group by (on the wrong column)?
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    <p>I have the following tables:</p> <pre><code>create temporary table Items (item_id int, item_name varchar(10)); create temporary table ItemRating (item_id int, rating int); </code></pre> <p>With the following data:</p> <pre><code>insert into Items (item_id, item_name) values (1,'Item 1'),(2,'Item 2'),(3,'Item 3'),(4,'Item 4'),(5,'Item 5'); insert into ItemRating values (1,9),(1,6),(3,10); </code></pre> <p>And I run the following query:</p> <pre><code>select i.item_id, i.item_name, avg(ir.rating) from Items i left join ItemRating ir ON ir.item_id = i.item_id group by ir.item_id; </code></pre> <p>This is the result I get:</p> <pre><code>+---------+-----------+----------------+ | item_id | item_name | avg(ir.rating) | +---------+-----------+----------------+ | 2 | Item 2 | NULL | | 1 | Item 1 | 7.5000 | | 3 | Item 3 | 10.0000 | +---------+-----------+----------------+ </code></pre> <p>Now, I fully understand that the query is written wrong, what I want is to be doing a group by on i.item_id. But I don't understand the behavior. Why does MYSQL display item_id 2 in the results, but not 4 or 5? I would actually expect to see only items 1 &amp; 3 because they're the only ones with a corresponding record in ItemRating.</p> <p>So, can anyone explain to me what MYSQL is doing here?</p>
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    1. USJohn
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      1. POWhat's happening in this MYSQL query with a left join and a group by (on the wrong column)?
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      1. VTUpMod
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      1. POWhat's happening in this MYSQL query with a left join and a group by (on the wrong column)?
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      1. POWhat's happening in this MYSQL query with a left join and a group by (on the wrong column)?
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    1. COJust for sanity can you check your Items table def has five items in it?
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      1. POWhat's happening in this MYSQL query with a left join and a group by (on the wrong column)?
      1. USDave Walker
 

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