TST: Add tests for if_exists keyword argument in df.to_gbq (#30447)

Author: charlesdong1991

pandas/tests/io/test_gbq.py

@@ -195,3 +195,37 @@ def test_roundtrip(self):
             dialect="standard",
         )
         assert result["num_rows"][0] == test_size
+
+    @pytest.mark.parametrize(
+        "if_exists, expected_num_rows",
+        [("append", 300), ("fail", 200), ("replace", 100)],
+    )
+    def test_gbq_if_exists(self, if_exists, expected_num_rows):
+        # GH 29598
+        destination_table = DESTINATION_TABLE + "2"
+
+        test_size = 200
+        df = make_mixed_dataframe_v2(test_size)
+
+        df.to_gbq(
+            destination_table,
+            _get_project_id(),
+            chunksize=None,
+            credentials=_get_credentials(),
+        )
+
+        df.iloc[:100].to_gbq(
+            destination_table,
+            _get_project_id(),
+            if_exists=if_exists,
+            chunksize=None,
+            credentials=_get_credentials(),
+        )
+
+        result = pd.read_gbq(
+            f"SELECT COUNT(*) AS num_rows FROM {destination_table}",
+            project_id=_get_project_id(),
+            credentials=_get_credentials(),
+            dialect="standard",
+        )
+        assert result["num_rows"][0] == expected_num_rows