fix: SQL ELSE clause should be implicitly NULL when omitted (#19714)

crates/polars-sql/src/sql_expr.rs

@@ -919,7 +919,7 @@ impl SQLExprVisitor<'_> {
             }
             let else_res = match else_result {
                 Some(else_res) => self.visit_expr(else_res)?,
-                None => polars_bail!(SQLSyntax: "ELSE expression is required"),
+                None => lit(Null), // ELSE clause is optional; when omitted, it is implicitly NULL
             };
             if let Some(operand_expr) = operand {
                 let first_operand_expr = self.visit_expr(operand_expr)?;

py-polars/tests/unit/sql/test_conditional.py

@@ -36,6 +36,24 @@ def test_case_when() -> None:
         }
 
 
+@pytest.mark.parametrize("else_clause", ["ELSE NULL ", ""])
+def test_case_when_optional_else(else_clause: str) -> None:
+    df = pl.DataFrame(
+        {
+            "a": [1, 2, 3, 4, 5, 6, 7],
+            "b": [7, 6, 5, 4, 3, 2, 1],
+            "c": [3, 4, 0, 3, 4, 1, 1],
+        }
+    )
+    query = f"""
+        SELECT
+          AVG(CASE WHEN a <= b THEN c {else_clause}END) AS conditional_mean
+          FROM self
+    """
+    res = df.sql(query)
+    assert res.to_dict(as_series=False) == {"conditional_mean": [2.5]}
+
+
 def test_control_flow(foods_ipc_path: Path) -> None:
     nums = pl.LazyFrame(
         {