I hv a table on server where in i have a field say TNO varchar2(10) which is auto-number with data T001, T002, T003 and so on...

Now problem is when 2 user adds data 1 can insert and other gets error may b coz both read same data nd try 2 insert same record...How can i achieve this ?? is there any lock or smthing tat can help me or any kind of help will be appreciated.....

Thanks in advance...