Create PostgreSQL ROLE (user) if it doesn't exist

Question

How do I write an SQL script to create a ROLE in PostgreSQL 9.1, but without raising an error if it already exists?

The current script simply has:

CREATE ROLE my_user LOGIN PASSWORD 'my_password';

This fails if the user already exists. I'd like something like:

IF NOT EXISTS (SELECT * FROM pg_user WHERE username = 'my_user')
BEGIN
    CREATE ROLE my_user LOGIN PASSWORD 'my_password';
END;

... but that doesn't work - IF doesn't seem to be supported in plain SQL.

I have a batch file that creates a PostgreSQL 9.1 database, role and a few other things. It calls psql.exe, passing in the name of an SQL script to run. So far all these scripts are plain SQL and I'd like to avoid PL/pgSQL and such, if possible.

Answer 1

Simplify in a similar fashion to what you had in mind:

DO
$do$
BEGIN
   IF NOT EXISTS (
      SELECT FROM pg_catalog.pg_roles  -- SELECT list can be empty for this
      WHERE  rolname = 'my_user') THEN

      CREATE ROLE my_user LOGIN PASSWORD 'my_password';
   END IF;
END
$do$;

(Building on @a_horse_with_no_name's answer and improved with @Gregory's comment.)

Unlike, for instance, with CREATE TABLE there is no IF NOT EXISTS clause for CREATE ROLE (up to at least pg 12). And you cannot execute dynamic DDL statements in plain SQL.

Your request to "avoid PL/pgSQL" is impossible except by using another PL. The DO statement uses plpgsql as default procedural language. The syntax allows to omit the explicit declaration:

DO [ LANGUAGE lang_name ] code
...
lang_name
The name of the procedural language the code is written in. If omitted, the default is plpgsql.