feat(rust,python,cli): add SQL engine support for IFNULL function

crates/polars-sql/src/functions.rs

@@ -316,18 +316,28 @@ pub(crate) enum PolarsSQLFunctions {
     /// SELECT UPPER(column_1) from df;
     /// ```
     Upper,
-    /// SQL 'nullif' function
-    /// Returns NULL if two expressions are equal, otherwise returns the first
-    /// ```sql
-    /// SELECT NULLIF(column_1, column_2) from df;
-    /// ```
-    NullIf,
+
+    // ----
+    // Conditional functions
+    // ----
     /// SQL 'coalesce' function
     /// Returns the first non-null value in the provided values/columns
     /// ```sql
     /// SELECT COALESCE(column_1, ...) from df;
     /// ```
     Coalesce,
+    /// SQL 'ifnull' function
+    /// If an expression value is NULL, return an alternative value.
+    /// ```sql
+    /// SELECT IFNULL(string_col, 'n/a') from df;
+    /// ```
+    IfNull,
+    /// SQL 'nullif' function
+    /// Returns NULL if two expressions are equal, otherwise returns the first
+    /// ```sql
+    /// SELECT NULLIF(column_1, column_2) from df;
+    /// ```
+    NullIf,
 
     // ----
     // Aggregate functions
@@ -581,9 +591,10 @@ impl PolarsSQLFunctions {
             "radians" => Self::Radians,
 
             // ----
-            // Comparison functions
+            // Conditional functions
             // ----
             "coalesce" => Self::Coalesce,
+            "ifnull" => Self::IfNull,
             "nullif" => Self::NullIf,
 
             // ----
@@ -701,15 +712,19 @@ impl SQLFunctionVisitor<'_> {
                     }))
                 }),
                 _ => {
-                    polars_bail!(InvalidOperation:"Invalid number of arguments for Round: {}",function.args.len());
+                    polars_bail!(InvalidOperation:"Invalid number of arguments for Round: {}", function.args.len());
                 },
             },
 
             // ----
-            // Comparison functions
+            // Conditional functions
             // ----
-            NullIf => self.visit_binary(|l, r: Expr| when(l.clone().eq(r)).then(lit(LiteralValue::Null)).otherwise(l)),
             Coalesce => self.visit_variadic(coalesce),
+            IfNull => match function.args.len() {
+                2 => self.visit_variadic(coalesce),
+                _ => polars_bail!(InvalidOperation:"Invalid number of arguments for IfNull: {}", function.args.len())
+            },
+            NullIf => self.visit_binary(|l, r: Expr| when(l.clone().eq(r)).then(lit(LiteralValue::Null)).otherwise(l)),
 
             // ----
             // String functions

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

@@ -1161,28 +1161,35 @@ def test_sql_nullif_coalesce(foods_ipc_path: Path) -> None:
         {
             "x": [1, None, 2, 3, None, 4],
             "y": [5, 4, None, 3, None, 2],
-            "z": [3, 4, None, 3, None, None],
+            "z": [3, 4, None, 3, 6, None],
         }
     )
-
     res = pl.SQLContext(df=nums).execute(
         """
         SELECT
-        COALESCE(x,y,z) as "coal",
-        NULLIF(x,y) as "nullif x_y",
-        NULLIF(y,z) as "nullif y_z",
-        COALESCE(x, NULLIF(y,z)) as "both"
+          COALESCE(x,y,z) as "coalsc",
+          NULLIF(x, y) as "nullif x_y",
+          NULLIF(y, z) as "nullif y_z",
+          IFNULL(x, y) as "ifnull x_y",
+          IFNULL(y,-1) as "inullf y_z",
+          COALESCE(x, NULLIF(y,z)) as "both"
         FROM df
         """,
         eager=True,
     )
 
     assert res.to_dict(as_series=False) == {
-        "coal": [1, 4, 2, 3, None, 4],
+        "coalsc": [1, 4, 2, 3, 6, 4],
         "nullif x_y": [1, None, 2, None, None, 4],
         "nullif y_z": [5, None, None, None, None, 2],
+        "ifnull x_y": [1, 4, 2, 3, None, 4],
+        "inullf y_z": [5, 4, -1, 3, -1, 2],
         "both": [1, None, 2, 3, None, 4],
     }
+    for null_func in ("IFNULL", "NULLIF"):
+        # both functions expect only 2 arguments
+        with pytest.raises(InvalidOperationError):
+            pl.SQLContext(df=nums).execute(f"SELECT {null_func}(x,y,z) FROM df")
 
 
 def test_sql_order_by(foods_ipc_path: Path) -> None: