Question

I am working on an inventory php/mysql application.

I have a main table (inv_main) which will hold the equipmentID and the userID whom actually holds this equipment.
I then have a equiptment table (inv_equip) which holds information about the piece of equipment as well as who ordered the product, and who it was ordered for, both linking back to userIDs.

I am trying to return a result of the equipmentID, ownerName, orderedByName, orderedForName

When I run the following query I get NULLs for orderedByName, and OrderedForName which should both return names as those fields have IDs and are not null.

SELECT inv_equip.tech_id, inv_user.user_lname as ownedByName, 
orderFor.user_lname as orderForName, orderFrom.user_lname as orderFromName
FROM inv_main
LEFT JOIN inv_equip ON inv_main.main_equip_id = inv_equip.equip_id 
LEFT JOIN inv_user ON inv_main.main_user_id = inv_user.user_id 
LEFT JOIN inv_user as orderFor ON inv_equip.equip_ordered_for = inv_user.user_id  
LEFT JOIN inv_user as orderFrom ON inv_equip.equip_ordered_from = inv_user.user_id   
WHERE (inv_main.main_equip_id = 26)

This is what I am getting for my output

tech_id                ownedByName  orderForName    orderFromName
TCH20110305_1299355914  admin     NULL            NULL

Now if i run this query I am getting the correct ownedByname again but the orderForName return i record for each user in my user table. The difference is I am not specifying to return a certain id but all in the db.

SELECT inv_equip.tech_id, inv_user.user_lname AS ownedByName, 
orderFor.user_lname AS orderForName, orderFrom.user_lname AS orderFromName
FROM inv_main
LEFT JOIN inv_equip ON inv_main.main_equip_id = inv_equip.equip_id
LEFT JOIN inv_user ON inv_main.main_user_id = inv_user.user_id
LEFT JOIN inv_user AS orderFor ON inv_equip.equip_ordered_for = inv_user.user_id
LEFT JOIN inv_user AS orderFrom ON inv_equip.equip_ordered_from = inv_user.user_id

I am getting the following results

                           OwnedBy OrderedFor   OrderedBy   
TCH20110304_1299257155  hucker  admin       NULL
TCH20110304_1299257155  hucker  hucker      NULL
TCH20110304_1299257155  hucker  beatty          NULL
TCH20110304_1299257155  hucker  Frank2          NULL
TCH20110304_1299257245  beatty  admin           admin
TCH20110304_1299257245  beatty  admin           hucker
TCH20110304_1299257245  beatty  admin           beatty

Where as I am looking for a sql query which results in

itemID  OwnedBy OrderedFor   OrderedBy   
 x      Hucker  Beatty       Frank

Any tips on how I could correct my join logic to return the names of the respected userID with only one record?

Answer 1

Your 2nd and 3rd joins specify an on condition against the 1st join:

LEFT JOIN inv_user 
ON        inv_main.main_user_id = inv_user.user_id 
LEFT JOIN inv_user as orderFor 
ON        inv_equip.equip_ordered_for = inv_user.user_id  
LEFT JOIN inv_user as orderFrom 
ON        inv_equip.equip_ordered_from = inv_user.user_id 

Try modifying the code so each join uses it’s own table:

LEFT JOIN inv_user 
ON        inv_main.main_user_id = inv_user.user_id 
LEFT JOIN inv_user as orderFor 
ON        inv_equip.equip_ordered_for = **orderFor**.user_id  
LEFT JOIN inv_user as orderFrom 
ON        inv_equip.equip_ordered_from = **orderFrom**.user_id 

The ** are just there for emphasis.