Select where null only if a non-null value doesn't exist? - mysql

Question

I have a simple user preferences table that looks like this:
id | user_id | preference_name | preference_value
What makes this table unique though is if the user_id field is null, it means it is the default value for that preference. I'm trying to get all the preferences for a user and use the default value only if an actual value hasn't been specified for that user.
So basically I need to:
SELECT * FROM user_preferences WHERE user_id = {userIdVar} OR user_id IS NULL;
BUT, I want to throw out a user_id is null result if there is another row in the result set with the same preference_name and a value for user_id.
Is there a way to do this with a single SQL query or should I do this in code?

Answer 1

Use NOT EXISTS:
SELECT up1.*
FROM user_preferences up1
WHERE ( NOT EXISTS(SELECT 1
FROM user_preferences up2
WHERE user_id = {userIdVar})
AND user_id IS NULL )
OR ( user_id = {userIdVar} );

Answer 2

There are various ways you can do this, but if all preferences have a default value, or you have a complete list of preferences somewhere else, I would do it like this:
select
default_preferences.preference_name,
coalesce(
real_user_preferences.preference_value,
default_preferences.preference_value) as preference_value
from
(select * from user_preferences where user_id is null)
as default_preferences
left join
(select * from user_preferences where user_id = #user_id)
as real_user_preferences
on
real_user_preferences.preference_name = default_preferences.preference_name
You've tagged your question both MySQL and SQL Server, I don't know which dialect you're looking for. I know SQL Server accepts this syntax, but it might need some tweaking for MySQL.
Edit: funkwurm points out that subqueries make this likely to perform poorly on MySQL. If that turns out to be a problem, it can be rewritten without subqueries as
select
default_preferences.preference_name,
coalesce(
real_user_preferences.preference_value,
default_preferences.preference_value) as preference_value
from
user_preferences as default_preferences
left join
user_preferences as real_user_preferences
on
real_user_preferences.preference_name = default_preferences.preference_name
and real_user_preferences.user_id = #user_id
where
default_preferences.user_id is null
Edit 2: if there are preferences that do not have a default value, the first version can be modified to use full join instead of left join, and take preference_name from either the defaults or the user-specific preferences, just like preference_value. However, the second version is not so easily modified.

Answer 3

COALESCE returns the first non null values of the params provided: http://dev.mysql.com/doc/refman/5.0/en/comparison-operators.html#function_coalesce
So if you grab the set of default preferences and JOIN them with the users preferences, you can use the COALESCE in your columns to populate the correct values.

Answer 4

This should work to select the first row that is either NULL or set the the user_id variable where the user_id variable is preffered if both are set and then shows every preference_name only once.
SELECT
*
FROM
(
SELECT
*
FROM
user_preferences
WHERE
user_id = {userIdVar} OR
user_id IS NULL
ORDER BY
CASE WHEN user_id IS NULL THEN 1 ELSE 0 END
) sub_query
GROUP BY
preference_name
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