Question

This is my query:

SELECT UID, COUNT( * ) *100 AS Points
FROM  `Visited` 
WHERE UID =  '25'
UNION ALL 
SELECT UID, COUNT( * ) *1000 AS Points
FROM  `Sites` 
WHERE UID =  '25'
UNION ALL 
SELECT UID, COUNT( * ) *1000 AS Points
FROM  `userTags` 
WHERE UID =  '25'

When I run it, and one of the subqueries finds no results I get:

#1048 - Column 'UID' cannot be null

I don’t really understand the problem, running the subquery that gives no results alone works alright and displays:

UID  | POINTS
NULL    0 

However the big query won’t unite it for some reason. What could be the reason?

—- EDIT

Also if the three subqueries give results the query works fine. The only problem is where there are no results in one of them.

Answer 1

It looks like there are no matching rows for UID = 25, however COUNT(*) still returns a row saying there are 0 matching rows. You can either remove the row by using an outer select where UID is not null, or more simply replace the NULL, like this:

SELECT UID, COUNT( * ) *100 AS Points
FROM  `Visited` 
WHERE UID =  '25'
UNION ALL 
SELECT ifnull(UID, '25'), COUNT( * ) *1000 AS Points
FROM  `Sites` 
WHERE UID =  '25'
UNION ALL 
SELECT UID, COUNT( * ) *1000 AS Points
FROM  `userTags` 
WHERE UID =  '25';

Of course, you would replace the '25' with $uid for running from your code.

Do the same for the other tables too if you think there’s a chance there are no matching rows