fix(comparison): wrap isnull equality check in parens (#8366)

We wrap arguments to a sge.Equals in parens if the inputs are
themselves binary operations. There are other operations we will have
to wrap -- for now I'm just adding isNull to the list, but I imagine
this will grow, possibly on a backend-by-backend basis.

I'm going to investigate the mssql and oracle failures but they are
unrelated to this PR (they fail the added test with or without the parentheses)

ibis/backends/base/sqlglot/compiler.py

@@ -321,6 +321,8 @@ class SQLGlotCompiler(abc.ABC):
         ops.IntervalSubtract,
     )
 
+    NEEDS_PARENS = BINARY_INFIX_OPS + (ops.IsNull,)
+
     def __init__(self) -> None:
         self.agg = AggGen(aggfunc=self._aggregate)
         self.f = FuncGen()
@@ -1192,7 +1194,7 @@ def visit_Aggregate(self, op, *, parent, groups, metrics):
 
     @classmethod
     def _add_parens(cls, op, sg_expr):
-        if isinstance(op, cls.BINARY_INFIX_OPS):
+        if isinstance(op, cls.NEEDS_PARENS):
             return paren(sg_expr)
         return sg_expr
 

ibis/backends/impala/tests/snapshots/test_sql/test_is_parens/isnull/out.sql

@@ -3,4 +3,8 @@ SELECT
   `t0`.`b`
 FROM `table` AS `t0`
 WHERE
-  `t0`.`a` IS NULL = `t0`.`b` IS NULL
+  (
+    `t0`.`a` IS NULL
+  ) = (
+    `t0`.`b` IS NULL
+  )

ibis/backends/tests/test_generic.py

@@ -1846,3 +1846,15 @@ def test_select_mutate_with_dict(backend):
 
     expr = t.select({"a": ibis.literal(1.0)}).limit(1)
     backend.assert_frame_equal(result, expected)
+
+
+@pytest.mark.broken(["mssql", "oracle"], reason="incorrect syntax")
+def test_isnull_equality(con, backend, monkeypatch):
+    monkeypatch.setattr(ibis.options, "default_backend", con)
+    t = ibis.memtable({"x": ["a", "b", None], "y": ["c", None, None], "z": [1, 2, 3]})
+    expr = t.mutate(out=t.x.isnull() == t.y.isnull()).order_by("z").select("out")
+    result = expr.to_pandas()
+
+    expected = pd.DataFrame({"out": [True, False, True]})
+
+    backend.assert_frame_equal(result, expected)