what is this sql query?

Hi
I cannot figure out this query:

SELECT symbol,SUM(IF(t_type='sell',-shares,shares)) AS shares,
                                                           MAX(price) AS maxprice, MIN(price) AS minprice
                                                           FROM transactions
                                                          WHERE completed=0
                                                          GROUP BY symbol
                                                          ORDER BY shares

Code (markup):

My problem is "-shares"; what does it mean?
how about this: IF(t_type='sell',-shares,shares)
t_type and shares are table fields
Thanks for your help

 

- shares means it is adding negative count, so the "shares" column will show correct figure.

 

SUM(IF(t_type='sell',-shares,shares)) AS shares

gives you the total number of shares held after subtracting all the sales for that stock symbol

Thanks

Lavee