SQL Isolation level - sql

Question

We are using session isolation serializable in our application. The intended behavior is that when a user is going insert a new row, it should-should check for the presence of the row with the same key and update the same if row found. But I have found multiple rows created for the same key in SQL server. Is this issue with isolation or the way we are handling the case?
Following is the code I am using,
private int getNextNumber(String objectName, Connection sqlConnection) throws SQLException {
// TODO Auto-generated method stub
int number = 0;
try{
sqlConnection.setTransactionIsolation(Connection.TRANSACTION_SERIALIZABLE);
System.out.println("##### Transaction isolation set : " + sqlConnection.getTransactionIsolation());
Statement stmt = sqlConnection.createStatement();
ResultSet rs = stmt.executeQuery("select * from [dbo].[db] where DocumentNumber = '" + objectName.toString() + "' FOR UPDATE");
while(rs.next()) {
printNumber = rs.getInt("PrintNumber");
}
System.out.println("#### Print number found from sql is : " + printNumber);
if(printNumber == 0) {
printNumber = printNumber + 1;
stmt.execute("INSERT INTO [dbo].[db] (number, DocumentNumber) VALUES (1 ,'" + objectName.toString() + "')");
} else {
number = number + 1;
stmt.execute("UPDATE [dbo].[db] SET Number =" + number + " WHERE DocumentNumber ='" + objectName.toString() + "'");
}
//sqlConnection.commit();
}catch(Exception e) {
sqlConnection.rollback();
e.printStackTrace();
} finally {
sqlConnection.commit();
}
return number;
}
Thanks,
Kishor Koli

Answer 1

It's an issue with the way your database is set up. You need a unique constraint to enforce uniqueness. You can check at insert time all you like but a unique constraint is the only way it's going to work 100% so it's just a waste of time selecting before inserting in the hope you'll prevent a duplicate. Insert, catch the exception/error or proceed.