GroupedSelection: an aggregate over a group without rows returns null

max()/min()/sum() answered 0 for a group with no rows, which is a value the data
never contained and which the plain Selection does not report either - it returns
null there, like the SQL aggregate functions do for an empty set. count() keeps
returning 0, it casts the result.

Author: dg

src/Database/Table/GroupedSelection.php

@@ -136,7 +136,7 @@ public function aggregation(string $function, ?string $groupFunction = null): mi
 			}
 		}
 
-		return 0;
+		return null; // the group has no rows, which is what the aggregate function would return for them
 	}
 
 

tests/Database/Explorer/Explorer.aggregation.phpt

@@ -80,6 +80,19 @@ test('filtering groups by related count', function () use ($explorer) {
 	], $bookTags);
 });
 
+test('aggregation of a group without rows', function () use ($explorer) {
+	$aggregates = [];
+	foreach ($explorer->table('author') as $author) {
+		$books = $author->related('book');
+		$aggregates[$author->name] = [$books->count('*'), $books->max('id'), $books->min('id'), $books->sum('id')];
+	}
+
+	// Geek has no books, so the aggregate functions have nothing to return but null; only count() is a number
+	Assert::same([0, null, null, null], $aggregates['Geek']);
+	Assert::same([2, 4, 3, 7], $aggregates['David Grudl']);
+});
+
+
 test('nested group by and having', function () use ($explorer) {
 	$bookTags = [];
 	foreach ($explorer->table('author') as $author) {