Question

I have this simple MySQL statement:

 SELECT 
   ((AVG(q1) + AVG(q8) + AVG(q15)) / 3 ) AS Res  
 FROM tresults 
 WHERE id = '1' AND date = 'MARCH2010' AND q25 = '1'
 GROUP BY q25

Now, if there are no rows with the date MARCH2010 then the query returns zero results (which is correct) but I’d like it to return a row – even if the result is NULL.

Answer 1

You can just select a single row as a constant, and then left join it to your result set:

select l.*, r.* 
from (select "your constant" as constant) as l 
left join ( 
  SELECT 
     ((AVG(q1) + AVG(q8) + AVG(q15)) / 3 ) AS Res  
   FROM tresults 
   WHERE id = '1' AND date = 'MARCH2010' AND q25 = '1'
   GROUP BY q25
) as r on 1

How this works:

• select "your constant" as constant always returns a single row
• left join always returns all of the rows in the left table at least once
• if the right table has no rows, then the entire left table is extended with a bunch of null columns, and the result has one row
• if the right table has n rows, the result has n rows that each have an additional “your constant” column