create ordering for php table without doing two queries to the database

$q4 = "SELECT order_id FROM menu WHERE category_id = $cat_id";
        $r4 = mysqli_query($dbc, $q4);

        $num_items = mysqli_num_rows($r4)+1;



        $q5 = "INSERT INTO menu (category_id, order_id, item, price, date_added, name) VALUES ('$cat_id', '$num_items', '$item', $开发者_JAVA技巧price, NOW(), $name)";
        $r5 = mysqli_query($dbc, $q5);

As of right now im doing one initial query to get the total amount of rows in the table so I can add the right order_id to the newly inserted row.

Basically, I want to retain the 1,2,3,4 so I the user can edit the order themselves

Is there a way to do this in one query? Or perhaps you guys know a better method?

Thanks

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A. using composite primary key on category_id, order_id, and build an auto increment on that, like

create table menu
(
  category_id int(10) unsigned default 0,
  order_id int(10) unsigned auto_increment,
  item int(10) unsigned default 0,
  price double(10, 2) unsigned default 0,
  date_added datetime,
  name varchar(255),
  primary key(category_id, order_id)
);

B: use a sub-query during insert, like

insert into menu
(category_id, item, price, date_added, name, order_id)
select 
  $category_id as category_id, 
  $item as item, 
  $price as price, 
  now() as date_added, 
  $name as name,
  (select max(order_id) from menu where category_id=$category_id)+1 as order_id
from dual;

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You should set AUTO_INCREMENT on your order_id. Then MySQL will automatically assign the next value to order_id as long as you don't explicitly specify one.

If you haven't, you should also set order_id to be unique.