What type of Join to use?

Question

I've got a core table and and 3 tables that extend the 'core' table in different ways.

I'm working with MLS data and I have a 'common' table that contains information common to all mls listings and then a table that has specifically "residential" information, one for "commercial",etc... I have been using mls number to join a single table when I know a listing when the property type is known, but for searching I want to join all of them and have the special fields available for search criteria (not simply searching the common table).

What type of join will give me a dataset that will contain all listings (including the extended fields in the idx tables) ?

For each Common table record there is a single corresponding record in ONLY ONE of the idx tables.

                     ___________
                    |           |
                    |  COMMON   |
                    |           |
                    |___________|
                         _|_   
                          |
       ___________________|_____________________
     _|_                 _|_                   _|_
 _____|_____         _____|______           ____|______ 
|           |       |            |         |           |
|   IDX1    |       |   IDX2     |         |   IDX3    |
|           |       |            |         |           |
|___________|       |____________|         |___________|

Answer 1

If you want everything in one row, you can use something like this format. Basically it gives you all the "Common" fields, then the other fields if there is a match otherwise NULL :

SELECT  Common.*,
        Idx1.*,
        Idx2.*,
        Idx3.*
FROM Common
LEFT JOIN Idx1
    ON Idx1.MLSKey = Common.MLSKey
LEFT JOIN Idx2
    ON Idx2.MLSKey = Common.MLSKey  
LEFT JOIN Idx3
    ON Idx3.MLSKey = Common.MLSKey

Bear in mind it's better to list out fields than to use the SELECT * whenever possible...

Also I'm assuming MySQL syntax is the same as SQL Server, which is what I use.

Answer 2

I have a similar set up of tables where the table 'jobs' is the core table.

I have this query that selects certain elements from each of the other 2 tables:

SELECT jobs.frequency, twitterdetails.accountname, feeds.feed
FROM jobs
JOIN twitterdetails ON twitterdetails.ID = jobs.accountID
JOIN feeds ON jobs.FeedID = feeds.FeedID 
WHERE jobs.username ='".$currentuser."';");

So, as you can see, no specific JOIN, but the linking fields defined. You'd probably just need an extra JOIN line for your set up.

Answer 3

Ugly solution / poor attempt / may have misunderstood question:

SELECT common.*,IDX1.field,NULL,NULL FROM COMMON 
LEFT JOIN IDX1 ON COMMON.ID = IDX1.ID
WHERE TYPE="RESIDENTIAL"
UNION ALL
SELECT common.*,NULL,IDX2.field,NULL FROM COMMON 
LEFT JOIN IDX2 ON COMMON.ID = IDX2.ID
WHERE TYPE="RESIDENTIAL"
UNION ALL
SELECT common.*,NULL,NULL,IDX3.field FROM COMMON
LEFT JOIN IDX3 ON COMMON.ID = IDX3.ID
WHERE TYPE="INDUSTRIAL"

Answer 4

Orbit is close. Use inner join, not left join. You don't want common to show up in the join if it does not have a row in idx.

You MUST union 3 queries to get the proper results assuming each record in common can only have 1 idx table. Plug in "NULL" to fill in the columns that each idx table is missing so they can be unioned.

BTW your table design is good.