Simulate CREATE DATABASE IF NOT EXISTS for PostgreSQL?

If you don't care about the data, you can drop database first and then recreate it:

DROP DATABASE IF EXISTS dbname;
CREATE DATABASE dbname;

Restrictions

You can ask the system catalog pg_database - accessible from any database in the same database cluster. The tricky part is that CREATE DATABASE can only be executed as a single statement. The manual:

CREATE DATABASE cannot be executed inside a transaction block.

So it cannot be run directly inside a function or DO statement, where it would be inside a transaction block implicitly. SQL procedures, introduced with Postgres 11, cannot help with this either.

Workaround from within psql

You can work around it from within psql by executing the DDL statement conditionally:

SELECT 'CREATE DATABASE mydb'
WHERE NOT EXISTS (SELECT FROM pg_database WHERE datname = 'mydb')\gexec

The manual:

\gexec

Sends the current query buffer to the server, then treats each column of each row of the query's output (if any) as a SQL statement to be executed.

Workaround from the shell

With \gexec you only need to call psql once:

echo "SELECT 'CREATE DATABASE mydb' WHERE NOT EXISTS (SELECT FROM pg_database WHERE datname = 'mydb')\gexec" | psql

You may need more psql options for your connection; role, port, password, ... See:

• Run batch file with psql command without password

The same cannot be called with psql -c "SELECT ...\gexec" since \gexec is a psql meta‑command and the -c option expects a single command for which the manual states:

command must be either a command string that is completely parsable by the server (i.e., it contains no psql-specific features), or a single backslash command. Thus you cannot mix SQL and psql meta-commands within a -c option.

Workaround from within Postgres transaction

You could use a dblink connection back to the current database, which runs outside of the transaction block. Effects can therefore also not be rolled back.

Install the additional module dblink for this (once per database):

• How to use (install) dblink in PostgreSQL?

Then:

DO
$do$
BEGIN
   IF EXISTS (SELECT FROM pg_database WHERE datname = 'mydb') THEN
      RAISE NOTICE 'Database already exists';  -- optional
   ELSE
      PERFORM dblink_exec('dbname=' || current_database()  -- current db
                        , 'CREATE DATABASE mydb');
   END IF;
END
$do$;

Again, you may need more psql options for the connection. See Ortwin's added answer:

• Simulate CREATE DATABASE IF NOT EXISTS for PostgreSQL?

Detailed explanation for dblink:

• How do I do large non-blocking updates in PostgreSQL?

You can make this a function for repeated use.

another alternative, just in case you want to have a shell script which creates the database if it does not exist and otherwise just keeps it as it is:

psql -U postgres -tc "SELECT 1 FROM pg_database WHERE datname = 'my_db'" | grep -q 1 || psql -U postgres -c "CREATE DATABASE my_db"

I found this to be helpful in devops provisioning scripts, which you might want to run multiple times over the same instance.

For those of you who would like an explanation:

-c = run command in database session, command is given in string
-t = skip header and footer
-q = silent mode for grep 
|| = logical OR, if grep fails to find match run the subsequent command