Is There A Way To Create A Lock

We have transaction table, and we want to make sure that when we query the balance in an account that another process isn't able to enter a transaction on the same account until we are done with our transaction.

This is a simplified version of the current code:

BEGIN TRANSACTION

SELECT @CurrentBalance = dbo.GetAccountBalanceAsOfDate(@AccountId, GETDATE())

IF (@Debit <= @CurrentBalance)
BEGIN
INSERT INTO dbo.Transactions() VALUES()
END

COMMIT TRANSACTION