diff --git a/target_redshift/sinks.py b/target_redshift/sinks.py
index 0949e30..ac59c1b 100644
--- a/target_redshift/sinks.py
+++ b/target_redshift/sinks.py
@@ -126,23 +126,23 @@ def process_batch(self, context: dict) -> None:
             self.path = Path(self.config["temp_dir"]) / self.file
             self.path.parent.mkdir(parents=True, exist_ok=True)
             self.object = f'{self.config["s3_key_prefix"]} / {self.file}'
-            self._bulk_insert_records(
+            self.bulk_insert_records(
                 table=temp_table,
                 records=context["records"],
                 cursor=cursor,
             )
             self.logger.info(f'merging {len(context["records"])} records into {table}')  # noqa: G004
             # Merge data from temp table to main table
-            self._upsert(
+            self.upsert(
                 from_table=temp_table,
                 to_table=table,
                 join_keys=self.key_properties,
                 cursor=cursor,
             )
             # clean_resources
-        self._clean_resources()
+        self.clean_resources()
 
-    def _bulk_insert_records(  # type: ignore[override]
+    def bulk_insert_records(  # type: ignore[override]
         self,
         table: sqlalchemy.Table,
         records: Iterable[dict[str, Any]],
@@ -164,14 +164,14 @@ def _bulk_insert_records(  # type: ignore[override]
         Returns:
             True if table exists, False if not, None if unsure or undetectable.
         """
-        self._write_csv(records)
+        self.write_csv(records)
         msg = f'writing {len(records)} records to s3://{self.config["s3_bucket"]}/{self.object}'
         self.logger.info(msg)
-        self._copy_to_s3()
-        self._copy_to_redshift(table, cursor)
+        self.copy_to_s3()
+        self.copy_to_redshift(table, cursor)
         return True
 
-    def _upsert(
+    def upsert(
         self,
         from_table: sqlalchemy.Table,
         to_table: sqlalchemy.Table,
@@ -200,16 +200,21 @@ def _upsert(
             join_predicates.append(from_table_key == to_table_key)
 
         join_condition = sqlalchemy.and_(*join_predicates)
-        merge_sql = f"""
-            MERGE INTO {self.connector.quote(str(to_table))}
-            USING {self.connector.quote(str(from_table))}
-            ON {join_condition}
-            REMOVE DUPLICATES
-            """
-        cursor.execute(merge_sql)
-        return None
+        if len(join_keys) > 0:
+            sql = f"""
+                MERGE INTO {self.connector.quote(str(to_table))}
+                USING {self.connector.quote(str(from_table))}
+                ON {join_condition}
+                REMOVE DUPLICATES
+                """
+        else:
+            sql = f"""
+                INSERT INTO {self.connector.quote(str(to_table))}
+                SELECT * FROM {self.connector.quote(str(from_table))}
+                """  # noqa: S608
+        cursor.execute(sql)
 
-    def _write_csv(self, records: list[dict]) -> None:
+    def write_csv(self, records: list[dict]) -> None:
         """Write records to a local csv file.
 
         Parameters
@@ -255,7 +260,7 @@ def _write_csv(self, records: list[dict]) -> None:
             )
             writer.writerows(records)
 
-    def _copy_to_s3(self) -> None:
+    def copy_to_s3(self) -> None:
         """Copy the csv file to s3."""
         try:
             _ = self.s3_client.upload_file(
@@ -264,7 +269,7 @@ def _copy_to_s3(self) -> None:
         except ClientError:
             self.logger.exception()
 
-    def _copy_to_redshift(self, table: sqlalchemy.Table, cursor: Cursor) -> None:
+    def copy_to_redshift(self, table: sqlalchemy.Table, cursor: Cursor) -> None:
         """Copy the s3 csv file to redshift."""
         copy_credentials = f"IAM_ROLE '{self.config['aws_redshift_copy_role_arn']}'"
 
@@ -288,7 +293,7 @@ def _copy_to_redshift(self, table: sqlalchemy.Table, cursor: Cursor) -> None:
         """
         cursor.execute(copy_sql)
 
-    def _parse_timestamps_in_record(
+    def parse_timestamps_in_record(
         self,
         record: dict,
         schema: dict,
@@ -333,7 +338,7 @@ def _parse_timestamps_in_record(
                     )
                 record[key] = date_val
 
-    def _clean_resources(self) -> None:
+    def clean_resources(self) -> None:
         """Remove local and s3 resources."""
         Path.unlink(self.path)
         if self.config["remove_s3_files"]: