连贯操作join用于连接表,等同于SQL语句中的join。
$model->join('left','table1 as a','a.id=1')->select();
生成的SQL语句:`select * from `ap_` left join table1 as a on a.id=1`

$model->join('left','table1 as a','a.id=1')->select();
生成的SQL语句:`select * from `ap_` left join table1 as a on a.id=1`