Creating an Oracle User if it doesn't already exist

The IF NOT EXISTS syntax available in SQL Server, is not available in Oracle.

In general, Oracle scripts simply execute the CREATE statement, and if the object already exist, you'll get an error indicating that, which you can ignore. This is what all the standard Oracle deployment scripts do.

However, if you really want to check for existence, and only execute if object doesn't exist, thereby avoiding the error, you can code a PL/SQL block. Write a SQL that checks for user existence, and if it doesn't exist, use EXECUTE IMMEDIATE to do CREATE USER from the PL/SQL block.

An example of such a PL/SQL block might be:

declare
userexist integer;
begin
  select count(*) into userexist from dba_users where username='SMITH';
  if (userexist = 0) then
    execute immediate 'create user smith identified by smith';
  end if;
end;
/

You need to write a pl/sql block. See an example here

You can check if the user exists in the all_users table using some pl/sql code like:

SELECT count(*) INTO v_count_user
FROM all_users
WHERE username = 'Kyle'

and then use v_count_user in an IF condition to conditionally execute the create user statement.

From the previous answers, it is clear that if not exists is not supported in Oracle. To clarify which error(s) are thrown by Oracle when attempting to create an already existing user (and as a bonus, when attempting to drop a non existing user):

drop user foo;
ORA-01918: user 'foo' does not exist

create user existing_user IDENTIFIED BY existing_user;
ORA-01920: user name 'existing_user' conflicts with another user or role name

The statements above were executed on Oracle Database 11g Enterprise Edition Release 11.2.0.1.0 - 64bit Production