> What do you meant by "opening balance"? Is it Initial Stock Balance or Opening Balance of partcular period?
> If the answer is "Opening Balance of partcular period", I would like to know how could you get this beside of doing something like closing stock ?

There is nothing magic about "closing"; closing usually involves posting a few adjusting entries at the end of a period.

The opening balance for a given period is the same as the closing balance of the previous period. In a "real-time" stock application (the only ones worth developing, IMO), transactions are posted as they incur so the current balance is always available. "Accounts" maintain balances and activity by period, so the opening/closing balances are always readily available. If your system doesn't follow this model, then I cannot answer your question.

>What Happend if some of the transactions purged? It may get the wrong result.

In any application the follows GAAP (Generally Accepted Accounting Practices), including Inventory, one NEVER "purges" transactions (for up to 7 years, anyway). One ONLY makes "adjustments".