将符合条件的id查询出来，结果是一个一位数组

$classid = M("product_sort")->where("pid = $pid")->getField("id",true);

ThinkPHP in 查询

//将一维数组转换为字符串，用,号隔开

$classid1 = implode(",",$classid);

//原生的in查询方法

/*$pro = M("product")->query("SELECT * FROM `lc_product` where pid IN(SELECT id FROM `lc_product_sort` WHERE pid = $pid)");*/

 $pro = selects("product",array("pid"=>array("in","$classid1")),15,"",15);