MySQL Function, must bring back a row - mysql

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

Answer 2

I'm not absolutely positive, but this case statement might work. I can't test it atm.
Case When
(SELECT
Count(*)
FROM tresults
WHERE id = '1' AND date = 'MARCH2010' AND q25 = '1'
GROUP BY q25) > 0
Then
SELECT
((AVG(q1) + AVG(q8) + AVG(q15)) / 3 ) AS Res
FROM tresults
WHERE id = '1' AND date = 'MARCH2010' AND q25 = '1'
GROUP BY q25
Else
SELECT null
End

Answer 3

SELECT
((AVG(q1) + AVG(q8) + AVG(q15)) / 3 ) AS Res
FROM tresults
WHERE id = '1' AND date = 'MARCH2010' AND q25 = '1'
GROUP BY q25
UNION ALL
SELECT NULL AS Res
FROM dual
WHERE NOT EXISTS (
SELECT
((AVG(q1) + AVG(q8) + AVG(q15)) / 3 ) AS Res
FROM tresults
WHERE id = '1' AND date = 'MARCH2010' AND q25 = '1'
GROUP BY q25
)
Will always return one row beacuse of second part.