SQL & PL/SQL

I use a parameter inside an SQL Plus Script to set a value for the creation of different environments.

DEFINE _env = '&1'

The parameter &1 is used. However a NULL value is allowed. But if this parameter is not set then sql*plus prompts to enter something for &1. How can we avoid that?

sql plus call

sqlplus user/password@database @test_parameter.sql

SQL> exit

Verbunden mit:
Oracle Database 11g Enterprise Edition Release 11.2.0.1.0 - 64bit Production
With the Partitioning, Oracle Label Security, OLAP, Data Mining,
Oracle Database Vault and Real Application Testing options

"Geben Sie einen Wert f³r 1 ein:"

SQL> prompt Env&_env.
Env
SQL>

I'm looking for an easy way to avoid the message <font color="red">"Enter a value for 1:"</font> if no parameter is added.
If a parameter is added, then this message does not appear.

sqlplus user/password@database @test_parameter.sql _TST

sample script

-- set environment 
------------------
-- Allowed values are 
-- "_DEV" = Development 
-- "_TST" = Test
-- "" = Production)
set verify off

DEFINE _env = '&1'

begin 
  if '&_env' is null 
    or regexp_like ('&_env', '_[[:alnum:]]{3}') then 
     null; --ok
  else
    raise_application_error(-20000,'Parameter 1 (''&_env'') has wrong value. Must start with _ and only 3 letters or be empty.' );
  end if;
end;
/

set echo on

prompt env&_env.

Edited by: Sven W. on Mar 15, 2012 4:40 PM