MySQL Query - getting missing records when using group-by

Question

I have a query :

select score, count(1) as \'NumStudents\' from testresults where testid = \'mytestid\'
group by score order by score
         


        

Answer 1

The most obvious way would be to create a table named "Scores" and left outer join your table to it.

SELECT s.score, COUNT(1) AS scoreCount
FROM score AS s
LEFT OUTER JOIN testScores AS ts
ON s.score = ts.score
GROUP BY s.score

If you don't want to create the table, you could use

SELECT
1 as score, SUM(CASE WHEN ts.score = 1 THEN 1 ELSE 0 END) AS scoreCount,
2 as score, SUM(CASE WHEN ts.score = 2 THEN 1 ELSE 0 END) AS scoreCount,
3 as score, SUM(CASE WHEN ts.score = 3 THEN 1 ELSE 0 END) AS scoreCount,
4 as score, SUM(CASE WHEN ts.score = 4 THEN 1 ELSE 0 END) AS scoreCount,
... 10 as score, SUM(CASE WHEN ts.score = 10 THEN 1 ELSE 0 END) AS scoreCount
FROM testScores AS ts