You first have to resolve a fundamental problem of not having a unique identifier for each item in the stock table. I suggest an integer surrogate primary key for your stock table if you have to allow duplicate item numbers among different suppliers.

Then use the group by as Sergey suggested but group by the unique ID or by whatever field you want to aggregate counts and or sums for.

>I use following statement to select the records I want :
>
>SELE a.po,a.item,a.qty,b.price,a.qty*b.price AS amt ;
> FROM purchase a,stock b ;
> WHERE a.item = b.item
>
>However, when stock table has a item that supplied by 2 supplier, then my output will go wrong, for eg :
>
>purchase table
>po item qty
>123 a 10
>
>stock table
>item supplier price
>a abc 10
>a bcd 9
>
>
>then I get the result
>
>123,a,10,10,100
>123,a,10,9,90
>
>How can I solve this problem by single SQL SELECT statement.
>
>Thank In advance.