Column is invalid in the select list when selecting count of children rows

Question

I have a parent table A(id,fld1,fld2), and child table B(id,parentId,fld1, fld2)

I want to select A.* and the count of the children rows

my query is:

SELECT A.*, COUNT(B.parentId)
INNER JOIN A ON B.parentId = A.id
GROUP BY B.parentId

but I'm getting:

Column 'A.id' is invalid in the select list because it is not contained in either an aggregate function or the GROUP BY clause.

Answer 1

Here's another way:

SELECT A.*, B.cnt
FROM A
CROSS APPLY (SELECT COUNT(*) As cnt
             FROM B
             WHERE B.parentId = A.id
            ) As B

Answer 2

Old fashion (correlated subquery):

SELECT A.*, (SELECT COUNT(*) As cnt
             FROM B
             B.parentId = A.id
            ) As cnt
FROM A