Обсуждение: a select statement that sometimes joins

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:

SELECTmessages.msg_id,message_attachments.attachment_idFROM messages,message_attachmentsWHERE 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"

(Let's further assume that a message will have only one attachment).

Is this possible? Anyone like to share an example? Much thanks.
 -mark

http://mark.stosberg.com/

Mark Stosberg wrote: > > > >Here's a situation I've run into more than once with SQL: > >I want to select all the rows
ina table that match a criteria, where one >of the criteria is possibly having a related entry in a second table. For
>myexample, lets say I have table named 'messages' and another named >'message_attachments'. The former has a primary
keyof msg_id, the latter >also contains msg_id, and has an attachment_id as it's primary key. > >This statement shows
meall 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
thatsays: "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.

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Oliver Elphick                                Oliver.Elphick@lfix.co.uk
Isle of Wight                              http://www.lfix.co.uk/oliver
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