Make `sum()` nullable

sqlparser/lib/src/analysis/types/graph/relationships.dart

@@ -91,6 +91,13 @@ class CopyAndCast extends TypeRelation implements DirectedRelation {
   final Typeable other;
   final CastMode cast;
   final bool dropTypeHint;
+  final bool makeNullable;
 
-  CopyAndCast(this.target, this.other, this.cast, {this.dropTypeHint = false});
+  CopyAndCast(
+    this.target,
+    this.other,
+    this.cast, {
+    this.dropTypeHint = false,
+    this.makeNullable = false,
+  });
 }

sqlparser/lib/src/analysis/types/graph/type_graph.dart

@@ -112,8 +112,12 @@ class TypeGraph {
           _copyType(resolved, t, other);
         }
       } else if (edge is CopyAndCast) {
-        _copyType(resolved, t, edge.target,
-            this[t]!.cast(edge.cast, edge.dropTypeHint));
+        var copied = this[t]!.cast(edge.cast, edge.dropTypeHint);
+        if (edge.makeNullable) {
+          copied = copied.withNullable(true);
+        }
+
+        _copyType(resolved, t, edge.target, copied);
       } else if (edge is MultiSourceRelation) {
         // handle many-to-one changes, if all targets have been resolved or
         // lax handling is enabled.

sqlparser/lib/src/analysis/types/resolving_visitor.dart

@@ -549,10 +549,18 @@ class TypeResolver extends RecursiveVisitor<TypeExpectation, void> {
         // ignore: dead_code
         throw AssertionError(); // required so that this switch compiles
       case 'sum':
+        checkArgumentCount(1);
+
+        // The result of `sum()` is `NULL` if there are no input rows.
+        session._addRelation(CopyAndCast(
+          e,
+          params.first,
+          CastMode.numeric,
+          dropTypeHint: true,
+          makeNullable: true,
+        ));
         session._addRelation(
-            CopyAndCast(e, params.first, CastMode.numeric, dropTypeHint: true));
-        session._addRelation(DefaultType(e, defaultType: _realType));
-        nullableIfChildIs();
+            DefaultType(e, defaultType: _realType.withNullable(true)));
         return null;
       case 'lower':
       case 'ltrim':

sqlparser/test/analysis/errors/invalid_parameter_count_test.dart

@@ -15,4 +15,14 @@ void main() {
           message: 'iif expects 3 arguments, got 2.'),
     ]);
   });
+
+  test('sum', () {
+    final engine = SqlEngine();
+    final result = engine.analyze('SELECT sum(1, 2, 3)');
+
+    result.expectError(
+      '1, 2, 3',
+      type: AnalysisErrorType.invalidAmountOfParameters,
+    );
+  });
 }

sqlparser/test/analysis/types2/misc_cases_test.dart

@@ -53,7 +53,8 @@ const Map<String, ResolvedType?> _types = {
   "SELECT 'a' -> ? = 'b'": ResolvedType(type: BasicType.text, nullable: false),
   "SELECT 'a' ->> ? = 'b'": ResolvedType(type: BasicType.text, nullable: false),
   'SELECT MAX(id, ?) FROM demo': ResolvedType(type: BasicType.int),
-  'SELECT SUM(id = 2) = ? FROM demo': ResolvedType(type: BasicType.int),
+  'SELECT SUM(id = 2) = ? FROM demo':
+      ResolvedType(type: BasicType.int, nullable: true),
   "SELECT unixepoch('now') = ?":
       ResolvedType(type: BasicType.int, nullable: true, hints: [IsDateTime()]),
   "SELECT datetime('now') = ?":