Question

I am writing a stock market trading game base on yahoo using php/mysql & it’s almost done. Last part I’m struggling is calculating the profit & loss

Lets say have these as transaction
Starting balance = $5000

> order_id stock_id  transaction qty_shares cost  user_id
> 1        1234      B           100        1000  1
> 2        1234      B           50         450   1
> 3        1234      S           80         640   1
> 4        1234      S           30         300   1

I’m lost trying to come up with an algorithm or a mysql query to calculate the profit & loss

Added after reading the 2 replies posted below
Additional Note :
Please take note that in this scenerio 150 shares was purchased & only 110 shares was sold & all was purchased & sold at different prices

Answer 1

SELECT SUM(qty_shares*cost) AS out, user_id, stock_id, order_id
FROM table
WHERE transaction = 'B'
GROUP BY user_id, stock_id, order_id

Then:

SELECT SUM(qty_shares*cost) AS in, user_id, stock_id
FROM table
WHERE transaction = 'S'
GROUP BY user_id, stock_id, order_id

PHP:

$Margin = ($query2->in - $query1->out);
$Balance += $Margin;

It could all be done on a single query, but if speed is more important then this could be faster than subqueries.

EDIT:
For average profit/loss per share:

SELECT AVG(qty_shares/cost) AS price_per_share, transaction, user_id, stock_id
FROM table
GROUP BY transaction,user_id,stock_id

This should return the average price per share for each transaction type, per stock type per user. so the transaction S row is average price sold for, and transaction B row is average price bought for. The difference between them is average profit or loss.

SELECT 
SUM(
qty_shares *
    (
      SELECT AVG(A.qty_shares/A.cost) AS price_per_share
      FROM table AS A
      WHERE A.transaction = table.transaction
      AND   A.user_id = table.user_id
      AND   A.stock_id = table.stock_id
    )
), user_id, stock_id, order_id
FROM table
WHERE transaction = 'B'
GROUP BY user_id, stock_id, order_id

That query should return the number of shares sold per row multiplied by the current average price per share, giving you the average margin per transaction with the sold margin being negative. You may find the query slow though. I’d recommend using this information as a starting point and finding the best way for your application.