返回最后一条执行的SQL语句.
$database->select("account", [ "user_name", "email"], [ "user_id[ 20]);$database->insert("account", [ "user_name" => "foo", "email" => "foo@bar.com"]);echo $database->last_query();// INSERT INTO "account" ("user_name", "email") VALUES ('foo', 'foo@bar.com')
