Group by count zero rows not displaying - sql

Question

I want to count the amount of rows of every componistId. When I run the following SQL statement it works fine:
SELECT C.componistId, COUNT(*)
FROM Componist C LEFT JOIN Stuk S ON S.componistId = C.componistId
GROUP BY C.componistId
Now I want only the rows where stukNrOrigineel is null
SELECT C.componistId, COUNT(*)
FROM Componist C LEFT JOIN Stuk S ON S.componistId = C.componistId
WHERE S.stuknrOrigineel IS NULL
GROUP BY C.componistId
But when I do this, all the rows with a result of 0 disappear. Only the rows with at least 1 row are displayed. How can I make this work?

Answer 1

You need to include the condition in the on clause:
SELECT C.componistId, COUNT(C.componistId)
FROM Componist C LEFT JOIN
Stuk S
ON S.componistId = C.componistId AND
S.stuknrOrigineel IS NULL
GROUP BY C.componistId;
Note: I changed the COUNT() to count from the second table. This is normally what you want when combining LEFT JOIN with COUNT().
On some databases, I think the above might not quite work as expected (the question is whether the condition on S is evaluated before or after the LEFT JOIN). This should always work:
SELECT C.componistId, COUNT(s.componistId)
FROM Componist C LEFT JOIN
(SELECT S.* FROM Stuk S WHERE S.stuknrOrigineel IS NULL
) s
ON S.componistId = C.componistId AND
GROUP BY C.componistId;
Another generic solution is move the condition into the aggregation function:
SELECT C.componistId,
SUM(CASE WHEN S.stuknrOrigineel IS NULL THEN 1 ELSE 0 END)
FROM Componist C LEFT JOIN
Stuk S
ON S.componistId = C.componistId
GROUP BY C.componistId;