Stop transaction if bank account not found

[philirving]

I want to NOT complete the transaction if there is no record with AcctNum="1001". For instance:
begin transaction; update Accounts set Bal=Bal-250 where AcctNum="1001"; update Accounts set Bal=Bal+250 where AcctNum="2002"; commit transaction

When I do this (in SQLite) AcctNum 2002 gets the 250 even if there is no record AcctNum="1001". How can I stop the transaction, and perhaps show an error message? 

Very simple table to test with: create table [Accounts] ( [AcctNum] char[4] primary key not null, [Bal] real ) ; insert into Accounts values ("2002",500.0); insert into Accounts values ("2005",250.0) ; 

Thanks in advance.

[niche]

https://stackoverflow.com/questions/6833867/close-kill-transaction