SQL indexing on lines from joined table on each record from the original

Question

I have two tables.

Orders (o)

id | buyer  
1  | Joe  
2  | Ann  
3  | Sue  

And Order_Items (oi)

id | master_record_id | stock number | qty  
1  | 1                | 1234         | 1  
2  | 1                | 7890         | 1  
3  | 1                | 4987         | 1  
4  | 2                | 1234         | 1  
5  | 2                | 7890         | 1  
6  | 2                | 4987         | 1  
7  | 3                | 1234         | 1  
8  | 3                | 7890         | 1  
9  | 3                | 4987         | 1  

Table oi is dependable on orders by oi.master_record_id. Every record (id) in table oi is a unique one (there is no storing of which item this is on an order) so for the example I have 3 orders each with 3 lines and in table oi I have 9 records with ids 1 to 9.

My question is what query should I use to be able to export the orders (a line for each ordered item) where each oi has an index based on the order it is connected to:

Example output:

o.id | o.buyer | Required: Item on Order | oi.stock_number
1    | Joe     | 1                       | 1234
1    | Joe     | 2                       | 7890
1    | Joe     | 3                       | 4987
2    | Ann     | 1                       | 1234
2    | Ann     | 2                       | 7890
2    | Ann     | 3                       | 4987
3    | Sue     | 1                       | 1234
3    | Sue     | 2                       | 7890
3    | Sue     | 3                       | 4987

Many Thanks,

Dani

Answer 1

In MySQL, you can use "user variables" to generate row numbers. Generate row numbers for your oi records and join with your Orders table to get your results. Here is a good blog post on how to achieve row numbering in MySQL.

Answer 2

SELECT id,buyer,Item_on_Order,stock_number
FROM
(SELECT O.`id` as id,
       O.`buyer` as buyer,
       (@row_num := IF(@category=O.`id`,@row_num+1,1)) as Item_on_Order,
       OI.`stock_number` as stock_number,
       @category := O.`id` AS Temp
FROM
Orders O
INNER JOIN Order_Items OI ON O.`id` = OI.`master_record_id`)T

Hope this helps.