more complex sql chain (#619)

add a more complex sql chain that first subsets the necessary tables

langchain/sql_database.py

@@ -50,7 +50,8 @@ class SQLDatabase:
         """Return string representation of dialect to use."""
         return self._engine.dialect.name
 
-    def _get_table_names(self) -> Iterable[str]:
+    def get_table_names(self) -> Iterable[str]:
+        """Get names of tables available."""
         if self._include_tables:
             return self._include_tables
         return set(self._all_tables) - set(self._ignore_tables)
@@ -58,9 +59,19 @@ class SQLDatabase:
     @property
     def table_info(self) -> str:
         """Information about all tables in the database."""
+        return self.get_table_info()
+
+    def get_table_info(self, table_names: Optional[List[str]] = None) -> str:
+        """Get information about specified tables."""
+        all_table_names = self.get_table_names()
+        if table_names is not None:
+            missing_tables = set(table_names).difference(all_table_names)
+            if missing_tables:
+                raise ValueError(f"table_names {missing_tables} not found in database")
+            all_table_names = table_names
         template = "Table '{table_name}' has columns: {columns}."
         tables = []
-        for table_name in self._get_table_names():
+        for table_name in all_table_names:
             columns = []
             for column in self._inspector.get_columns(table_name, schema=self._schema):
                 columns.append(f"{column['name']} ({str(column['type'])})")