Implement basic COALESCE functionality

dask_sql/physical/rel/logical/aggregate.py

@@ -457,7 +457,18 @@ def _collect_aggregations(
                 filter_backend_col = None
 
             try:
-                aggregation_function = self.AGGREGATION_MAPPING[aggregation_name]
+                # This unifies CPU and GPU behavior by ensuring that performing a
+                # sum on a null column results in null and not 0
+                if aggregation_name == "sum" and isinstance(df._meta, pd.DataFrame):
+                    aggregation_function = AggregationSpecification(
+                        dd.Aggregation(
+                            name="custom_sum",
+                            chunk=lambda s: s.sum(min_count=1),
+                            agg=lambda s0: s0.sum(min_count=1),
+                        )
+                    )
+                else:
+                    aggregation_function = self.AGGREGATION_MAPPING[aggregation_name]
             except KeyError:
                 try:
                     aggregation_function = context.schema[schema_name].functions[

dask_sql/physical/rex/core/call.py

@@ -573,6 +573,25 @@ def overlay(self, s, replace, start, length=None):
         return s
 
 
+class CoalesceOperation(Operation):
+    def __init__(self):
+        super().__init__(self.coalesce)
+
+    def coalesce(self, *operands):
+        result = None
+        for operand in operands:
+            if is_frame(operand):
+                # Check if frame evaluates to nan or NA
+                if len(operand) == 1 and not operand.isnull().all().compute():
+                    return operand if result is None else result.fillna(operand)
+                else:
+                    result = operand if result is None else result.fillna(operand)
+            elif not pd.isna(operand):
+                return operand if result is None else result.fillna(operand)
+
+        return result
+
+
 class ExtractOperation(Operation):
     def __init__(self):
         super().__init__(self.extract)
@@ -1059,6 +1078,7 @@ class RexCallPlugin(BaseRexPlugin):
         "substr": SubStringOperation(),
         "substring": SubStringOperation(),
         "initcap": TensorScalarOperation(lambda x: x.str.title(), lambda x: x.title()),
+        "coalesce": CoalesceOperation(),
         "replace": ReplaceOperation(),
         # date/time operations
         "extract": ExtractOperation(),

tests/integration/test_rex.py

@@ -355,6 +355,65 @@ def test_null(c):
     assert_eq(df, expected_df)
 
 
+@pytest.mark.parametrize("gpu", [False, pytest.param(True, marks=pytest.mark.gpu)])
+def test_coalesce(c, gpu):
+    df = dd.from_pandas(
+        pd.DataFrame({"a": [1, 2, 3], "b": [np.nan] * 3}), npartitions=1
+    )
+    c.create_table("df", df, gpu=gpu)
+
+    df = c.sql(
+        """
+        SELECT
+            COALESCE(3, 5) as c1,
+            COALESCE(NULL, NULL) as c2,
+            COALESCE(NULL, 'hi') as c3,
+            COALESCE(NULL, NULL, 'bye', 5/0) as c4,
+            COALESCE(NULL, 3/2, NULL, 'fly') as c5,
+            COALESCE(SUM(b), 'why', 2.2) as c6,
+            COALESCE(NULL, MEAN(b), MEAN(a), 4/0) as c7
+        FROM df
+        """
+    )
+
+    expected_df = pd.DataFrame(
+        {
+            "c1": [3],
+            "c2": [np.nan],
+            "c3": ["hi"],
+            "c4": ["bye"],
+            "c5": ["1"],
+            "c6": ["why"],
+            "c7": [2.0],
+        }
+    )
+
+    assert_eq(df, expected_df, check_dtype=False)
+
+    df = c.sql(
+        """
+        SELECT
+            COALESCE(a, b) as c1,
+            COALESCE(b, a) as c2,
+            COALESCE(a, a) as c3,
+            COALESCE(b, b) as c4
+        FROM df
+        """
+    )
+
+    expected_df = pd.DataFrame(
+        {
+            "c1": [1, 2, 3],
+            "c2": [1, 2, 3],
+            "c3": [1, 2, 3],
+            "c4": [np.nan] * 3,
+        }
+    )
+
+    assert_eq(df, expected_df, check_dtype=False)
+    c.drop_table("df")
+
+
 def test_boolean_operations(c):
     df = dd.from_pandas(pd.DataFrame({"b": [1, 0, -1]}), npartitions=1)
     df["b"] = df["b"].apply(

tests/unit/test_queries.py

@@ -37,11 +37,8 @@
     69,
     70,
     72,
-    75,
     77,
-    78,
     80,
-    84,
     86,
     87,
     88,