1. 用find_in_set方法查询
2. Db::table('think_user')
3.    // ->where('find_in_set(:id,sids)',['id'=>$id])
4.       ->where("FIND_IN_SET(3,category_id)")
       ->select();

 ->where("FIND_IN_SET(值,参数名字)")

多条件查询

if ($cateId) {
    $where['category_id'] = $cateId;
}
if ($second) {
    $where['second_category_id'] = $second;
}
if ($tagId) {
    $where[] = ['EXP',Db::raw("FIND_IN_SET($tagId,tags_id)")];
}

修改 

->update(['del_users' => Db::raw("concat(del_users,',$user_id')")]);