Support SQL statements that return no results (#222)

Adds support for statements such as insert, update etc which do not
return any rows.

`engine.execute` is deprecated and so execution has been updated to use
`connection.exec_driver_sql` as-per:


https://docs.sqlalchemy.org/en/14/core/connections.html#sqlalchemy.engine.Engine.execute

langchain/sql_database.py

@@ -66,6 +66,14 @@ class SQLDatabase:
         return "\n".join(tables)
 
     def run(self, command: str) -> str:
-        """Execute a SQL command and return a string of the results."""
+        """Execute a SQL command and return a string representing the results.
-        result = self._engine.execute(command).fetchall()
+
+        If the statement returns rows, a string of the results is returned.
+        If the statement returns no rows, an empty string is returned.
+        """
+        with self._engine.connect() as connection:
+            cursor = connection.exec_driver_sql(command)
+            if cursor.returns_rows:
+                result = cursor.fetchall()
                 return str(result)
+        return ""

tests/integration_tests/chains/test_sql_database.py

@@ -28,3 +28,20 @@ def test_sql_database_run() -> None:
     output = db_chain.run("What company does Harrison work at?")
     expected_output = " Harrison works at Foo."
     assert output == expected_output
+
+
+def test_sql_database_run_update() -> None:
+    """Test that update commands run successfully and returned in correct format."""
+    engine = create_engine("sqlite:///:memory:")
+    metadata_obj.create_all(engine)
+    stmt = insert(user).values(user_id=13, user_name="Harrison", user_company="Foo")
+    with engine.connect() as conn:
+        conn.execute(stmt)
+    db = SQLDatabase(engine)
+    db_chain = SQLDatabaseChain(llm=OpenAI(temperature=0), database=db)
+    output = db_chain.run("Update Harrison's workplace to Bar")
+    expected_output = " Harrison's workplace has been updated to Bar."
+    assert output == expected_output
+    output = db_chain.run("What company does Harrison work at?")
+    expected_output = " Harrison works at Bar."
+    assert output == expected_output

tests/unit_tests/test_sql_database.py

@@ -47,3 +47,17 @@ def test_sql_database_run() -> None:
     output = db.run(command)
     expected_output = "[('Harrison',)]"
     assert output == expected_output
+
+
+def test_sql_database_run_update() -> None:
+    """Test commands which return no rows return an empty string."""
+    engine = create_engine("sqlite:///:memory:")
+    metadata_obj.create_all(engine)
+    stmt = insert(user).values(user_id=13, user_name="Harrison")
+    with engine.connect() as conn:
+        conn.execute(stmt)
+    db = SQLDatabase(engine)
+    command = "update user set user_name='Updated' where user_id = 13"
+    output = db.run(command)
+    expected_output = ""
+    assert output == expected_output