Question

I am basically having the exact same problem as here:

SQL View: Join tables without causing the data to duplicate on every row?

Except on that question he was using SQL, and I am using mysql. I am wondering if the same query is possible in mysql. If so, I may have the wrong syntax?

I am trying to do something like

select a.name as account_Name, 
   p.description as property_DESCRIPTION, 
   p.address as property_ADDRESS, 
   null as vehicles_DESCRIPTION,
   null as vehicles_MAKE, 
   null as vehicles_MODEL
from Accounts a
    inner join Properties p
        on a.accountid = p.accountid
UNION ALL   
select a.name as account_Name, 
   null as property_DESCRIPTION, 
   null as property_ADDRESS, 
   v.description as vehicles_DESCRIPTION,
   v.make as vehicles_MAKE, 
   v.model as vehicles_MODEL
from Accounts a
    inner join vehicles v
        on a.accountid = v.accountid

Here is my actual code:

SELECT user.first_name, user.last_name, upi.image_id, NULL AS friends.friend_user_id FROM user 
INNER JOIN user_profile_images as upi ON (user.user_id = upi.user_id) 
UNION 
SELECT user.first_name, user.last_name, NULL AS upi.image_id, friends.friend_user_id FROM user 
INNER JOIN friends ON (user.user_id = friends.user_id) 
WHERE user.user_id = '$profile_id'

where I have 3 tables: user, user_profile_images, and friends. Both user_profile_images and friends are related to the user through the user_id. So a user can have multiple profile images as well as multiple friend entries. I can post the table diagrams if it doesnt make sense. But what I want is basically a view of all the info, with fields NULL if they don’t apply to the overall view.

If I do the query with 2 tables, either with user and user_profile_images, or user and friends, I get the desired results, but adding the third table gives me duplicate rows.

Answer 1

The solution, as @MarcB suggests, is to use UNION rather than UNION ALL.

However, I have a question for you – why use the UNION at all? The following is equivalent, except that if (say) account 1 has one property and one vehicle, instead of getting:

account_Name   property_DESCRIPTION   vehicles_MAKE
  account1          property1             NULL
  account1           NULL                vehicle1

You’ll get

account_Name   property_DESCRIPTION   vehicles_MAKE
  account1          property1            vehicle1

Query:

SELECT a.name as account_Name, 
   p.description as property_DESCRIPTION, 
   p.address as property_ADDRESS, 
   v.description as vehicles_DESCRIPTION,
   v.make as vehicles_MAKE, 
   v.model as vehicles_MODEL
FROM Accounts a
LEFT JOIN Properties p
        on a.accountid = p.accountid
LEFT JOIN vehicles v
        on a.accountid = v.accountid
WHERE p.description IS NOT NULL AND v.make IS NOT NULL

Note – the last line (IS NOT NULL for both p and v) simulates the ‘accounts table’ part of the INNER JOIN and makes sure that only accounts with at least a property OR a vehicle are shown. Substitute the id columns of p and v there.