fix: Correct SUM() evaluation for empty result sets

Author: Taras Omelianchuk

src/Processor/Expression/FunctionEvaluator.php

@@ -372,7 +372,8 @@ private static function sqlSum(
         $sum = 0;
 
         if (!$result->rows) {
-            if ($expr instanceof FunctionExpression) {
+            $isQueryWithoutFromClause = empty($result->columns);
+            if ($expr instanceof FunctionExpression && $isQueryWithoutFromClause) {
                 return self::evaluate($conn, $scope, $expr, [], $result);
             }
 

tests/EndToEndTest.php

@@ -2,9 +2,6 @@
 namespace Vimeo\MysqlEngine\Tests;
 
 use PDOException;
-use Vimeo\MysqlEngine\Parser\Token;
-use Vimeo\MysqlEngine\Query\Expression\ColumnExpression;
-use Vimeo\MysqlEngine\TokenType;
 
 class EndToEndTest extends \PHPUnit\Framework\TestCase
 {
@@ -13,6 +10,29 @@ public function tearDown() : void
         \Vimeo\MysqlEngine\Server::reset();
     }
 
+    public function testSumWithEmptyResultSetReturnsNull()
+    {
+        $sql = "
+           SELECT
+                SUM(
+                  IF(
+                    id > 1,
+                    0,
+                    1
+                  )
+                )
+            FROM
+              video_game_characters
+            WHERE
+              id > 100;
+           ";
+
+        $pdo = self::getConnectionToFullDB();
+        $query= $pdo->query($sql);
+
+        $this->assertNull($query->fetchColumn());
+    }
+
     public function testSelectEmptyResults()
     {
         $pdo = self::getConnectionToFullDB();