Re: a select statement that sometimes joins

Mark Stosberg wrote:
>
>
>
>Here's a situation I've run into more than once with SQL:
>
>I want to select all the rows in a table that match a criteria, where one
>of the criteria is possibly having a related entry in a second table. For
>my example, lets say I have table named 'messages' and another named
>'message_attachments'. The former has a primary key of msg_id, the latter
>also contains msg_id, and has an attachment_id as it's primary key.
>
>This statement shows me all the messages that also have attachments:
>
>SELECT
> messages.msg_id,
> message_attachments.attachment_id
> FROM messages,message_attachments
> WHERE messages.msg_id = message_attachments.msg_id;
>
>But I want a statement that says: "Show me all the messages, and include
>information about an attachment if they have one"

SELECT m.msg_id, a.attachment_id
FROM messages AS m
LEFT OUTER JOIN message-attachments AS a
ON m.msg_id = a.msg_id;

This requires 7.1 for the LEFT OUTER JOIN. In 7.0.3 you could do it
with a UNION.

--
Oliver Elphick Oliver(dot)Elphick(at)lfix(dot)co(dot)uk
Isle of Wight http://www.lfix.co.uk/oliver
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