LEFT JOIN but with WHERE criteria, rows getting lost - mysql

Question

I have a simple database with three tables. In the database I have a table for users of my system, a table for applications to a competition, and an intermediary table that allows me to track which users have selected which applications to view.
Table 1 = users (user_id, username, first, last, etc...)
Table 2 = applications (application_id, company_name, url, etc...)
Table 3 = picks (pick_id, user_id, application_id, picked)
I am trying to write an SQL query that will show all the applications that have been submitted and if any individual application has been selected by a user will show that it has been "picked" (1=picked, 0=not picked).
So for user_id = 1 I'd like to see:
Column Names (application_id, company_name, picked)
1, Foo, 1
2, Bar, 1
3, Alpha, Null
4, Beta, Null
I tried it with the following query:
SELECT applications.application_id, applications.company_name, picks.picked
FROM applications
LEFT JOIN picks ON applications.application_id = picks.application_id
ORDER BY applications.application_id ASC
Which is returning this:
1, Foo, 1
1, Foo, 1
2, Bar, null
3, Alpha, null
4, Beta, null
I have a second user (user_id = 2) that also picked application 1 ("Foo") which I know is returning the second row.
Then I tried to limit the scope by specifying user_id = 1 here:
SELECT applications.application_id, applications.company_name, picks.picked
FROM applications
LEFT JOIN picks ON applications.application_id = picks.application_id
WHERE user_id = 1
ORDER BY applications.application_id ASC
Now I'm only getting:
1, Foo, 1
Any suggestions on how I can get what I'm looking for? Again, ideally for a single user I'd like to see:
Column Names (application_id, company_name, picked)
1, Foo, 1
2, Bar, 1
3, Alpha, Null
4, Beta, Null

Answer 1

You have a so-called join table in your database schema. In your case it's called picks. This allows you to create a many-to-many relationship between your users and applications.
To use that join table correctly you need to join all three tables. These queries are easier to write if you use table aliases (applications AS a, etc.)
SELECT a.application_id, a.company_name, p.picked, u.user_id, u.username
FROM applications AS a
LEFT JOIN picks AS p ON a.application_id = p.application_id
LEFT JOIN users AS u ON p.user_id = u.user_id
ORDER BY a.application_id, u.user_id
This will give you a list of all applications with the users who have made them. If no users are related to an application, the LEFT JOIN operations will retain the application row and you'll see NULL values for columns from the picks and users table.
Now, if you add a WHERE p.something = something or u.something = something clause to this query in an attempt to narrow down the presentation, it has the effect of converting the LEFT JOIN clauses into INNER JOIN clauses. That is, you won't retain the applications rows that don't have matching rows in the other tables.
If you want to retain those unmatched rows in your result set, put the condition in the first ON clause instead of the WHERE clause, like so.
SELECT a.application_id, a.company_name, p.picked, u.user_id, u.username
FROM applications AS a
LEFT JOIN picks AS p ON a.application_id = p.application_id AND p.user_id = 1
LEFT JOIN users AS u ON p.user_id = u.user_id
ORDER BY a.application_id, u.user_id
Edit Many join tables like your picks table are set up with a composite primary key, in your example (application_id, user_id). That ensures just one row per possible relationship between the tables being joined. In your case you have the potential for multiple such rows.
To use only the most recent of those rows (the one with the highest pick_id) takes a little more work. You need a subquery (virtual table) to extract it, and to retrieve the appropriate value of picked so your query works. So now things get interesting.
SELECT MAX(pick_id) AS pick_id,
application_id, user_id
FROM picks
GROUP BY application_id, user_id
retrieves the unique relationship pair. That is good. But next we have to fetch the picked column detail value from those rows. That takes another join, using the MAX value of pick_id, like so
SELECT q.application_id, q.user_id, r.picked
FROM (
SELECT MAX(pick_id) AS pick_id,
application_id, user_id
FROM picks
GROUP BY application_id, user_id
) AS q
JOIN picks AS r ON q.pick_id = r.pick_id
So, we need to substitute this little virtual table (subquery) in place of the pick AS p table in the original query. That looks like this.
SELECT a.application_id, a.company_name, p.picked, u.user_id, u.username
FROM applications AS a
LEFT JOIN (
SELECT q.application_id, q.user_id, r.picked
FROM (
SELECT MAX(pick_id) AS pick_id,
application_id, user_id
FROM picks
GROUP BY application_id, user_id
) AS q
JOIN picks AS r ON q.pick_id = r.pick_id
) AS p ON a.application_id = p.application_id AND p.user_id = 1
LEFT JOIN users AS u ON p.user_id = u.user_id
ORDER BY a.application_id, u.user_id
Some developers prefer to create VIEW objects for subqueries like the one here, rather than creating a club sandwich of a query like this one. It's not called Structured Query Language on a foolish whim, eh? These subqueries sometimes can be elements of a structure.