Fix calculation of certain aggregates from expressions when no

matching features exist

Eg sum and count should return 0 in this case rather than
null

src/core/qgsaggregatecalculator.cpp

@@ -101,7 +101,7 @@ QVariant QgsAggregateCalculator::calculate( QgsAggregateCalculator::Aggregate ag
       //no matching features
       if ( ok )
         *ok = true;
-      return QVariant();
+      return defaultValue( aggregate );
     }
 
     if ( context )
@@ -497,6 +497,42 @@ QVariant QgsAggregateCalculator::concatenateStrings( QgsFeatureIterator& fit, in
   return result;
 }
 
+QVariant QgsAggregateCalculator::defaultValue( QgsAggregateCalculator::Aggregate aggregate ) const
+{
+  // value to return when NO features are aggregated:
+  switch ( aggregate )
+  {
+      // sensible values:
+    case Count:
+    case CountDistinct:
+    case CountMissing:
+    case Sum:
+      return 0;
+
+    case StringConcatenate:
+      return ""; // zero length string - not null!
+
+      // undefined - nothing makes sense here
+    case Min:
+    case Max:
+    case Mean:
+    case Median:
+    case StDev:
+    case StDevSample:
+    case Range:
+    case Minority:
+    case Majority:
+    case FirstQuartile:
+    case ThirdQuartile:
+    case InterQuartileRange:
+    case StringMinimumLength:
+    case StringMaximumLength:
+    case GeometryCollect:
+      return QVariant();
+  }
+  return QVariant();
+}
+
 QVariant QgsAggregateCalculator::calculateDateTimeAggregate( QgsFeatureIterator& fit, int attr, QgsExpression* expression,
     QgsExpressionContext* context, QgsDateTimeStatisticalSummary::Statistic stat )
 {

src/core/qgsaggregatecalculator.h

@@ -169,6 +169,8 @@ class CORE_EXPORT QgsAggregateCalculator
                                QgsExpressionContext* context, bool* ok = nullptr );
     static QVariant concatenateStrings( QgsFeatureIterator& fit, int attr, QgsExpression* expression,
                                         QgsExpressionContext* context, const QString& delimiter );
+
+    QVariant defaultValue( Aggregate aggregate ) const;
 };
 
 #endif //QGSAGGREGATECALCULATOR_H

tests/src/python/test_qgsaggregatecalculator.py

@@ -386,6 +386,36 @@ def testExpression(self):
         self.assertTrue(ok)
         self.assertEqual(val, 5)
 
+    def testExpressionNoMatch(self):
+        """ test aggregate calculation using an expression with no features """
+
+        # no features
+        layer = QgsVectorLayer("Point?field=fldint:integer", "layer", "memory")
+
+        # sum
+        agg = QgsAggregateCalculator(layer)
+        val, ok = agg.calculate(QgsAggregateCalculator.Sum, 'fldint * 2')
+        self.assertTrue(ok)
+        self.assertEqual(val, 0)
+
+        # count
+        agg = QgsAggregateCalculator(layer)
+        val, ok = agg.calculate(QgsAggregateCalculator.Count, 'fldint * 2')
+        self.assertTrue(ok)
+        self.assertEqual(val, 0)
+
+        # count distinct
+        agg = QgsAggregateCalculator(layer)
+        val, ok = agg.calculate(QgsAggregateCalculator.CountDistinct, 'fldint * 2')
+        self.assertTrue(ok)
+        self.assertEqual(val, 0)
+
+        # count missing
+        agg = QgsAggregateCalculator(layer)
+        val, ok = agg.calculate(QgsAggregateCalculator.CountMissing, 'fldint * 2')
+        self.assertTrue(ok)
+        self.assertEqual(val, 0)
+
     def testStringToAggregate(self):
         """ test converting strings to aggregate types """