fix: Preserve sort with empty group {} (#5635)

Co-authored-by: Claude <[email protected]>

Author: max-sixty

prqlc/prqlc/src/semantic/resolver/flatten.rs

@@ -77,7 +77,11 @@ impl PlFold for Flattener {
                     }
                     TransformKind::Group { by, pipeline } => {
                         let sort_undone = self.sort_undone;
-                        self.sort_undone = true;
+                        // Only mark sort as undone if there's an actual partition.
+                        // Empty group {} should preserve sort (fixes #5100).
+                        if !matches!(by.kind, ExprKind::Tuple(ref fields) if fields.is_empty()) {
+                            self.sort_undone = true;
+                        }
 
                         let input = self.fold_expr(*t.input)?;
 

prqlc/prqlc/tests/integration/sql.rs

@@ -6720,3 +6720,24 @@ fn test_group_with_only_sort() {
       employees AS a
     ");
 }
+
+#[test]
+fn test_group_empty_preserves_sort() {
+    // Issue #5100: Empty group {} should preserve inner sort.
+    assert_snapshot!(compile(r###"
+    from foo
+    group {} (
+        sort a
+        take 1
+    )
+    "###).unwrap(), @r"
+    SELECT
+      *
+    FROM
+      foo
+    ORDER BY
+      a
+    LIMIT
+      1
+    ");
+}