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

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
;

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.

There are two different ways of creating a database in PostgreSQL. One is from the command line. The other is from the PostgreSQL console.

Command Line

In order to be able to create a database from the command line, you must first change to a user with rights to create a database. As you have shown with the \du command above, the user with those rights is postgres.

root@eric-desktop:/home/eric# su postgres
postgres@eric-desktop:/home/eric$ createdb prismatest2
postgres@eric-desktop:/home/eric$

As you can see, the creatdb command does not give any feedback. You need to confirm the creation by listing the current databases:

psql -U postgres -l
                                                 List of databases
    Name     |  Owner   | Encoding |   Collate   |    Ctype    | ICU Locale | Locale Provider |   Access privileges   

-------------+----------+----------+-------------+-------------+------------+-----------------+-----------------------

 postgres    | postgres | UTF8     | en_US.UTF-8 | en_US.UTF-8 |            | libc            | 

 prismatest  | postgres | UTF8     | en_US.UTF-8 | en_US.UTF-8 |            | libc            | 

 prismatest2 | postgres | UTF8     | en_US.UTF-8 | en_US.UTF-8 |            | libc            | 

PostgreSQL Console

Another possibility is to use the PostgreSQL console. To enter the console:

root@eric-desktop:/home/eric# sudo -u postgres psql psql (15.7 (Debian 15.7-0+deb12u1)) Type "help" for help.

postgres=# 
postgres=# CREATE DATABASE "prismatest3";
CREATE DATABASE
postgres=# 

To confirm the creation of the database, list the databases:

postgres=# \l

                                                 List of databases
    Name     |  Owner   | Encoding |   Collate   |    Ctype    | ICU Locale | Locale Provider |   Access privileges   

-------------+----------+----------+-------------+-------------+------------+-----------------+-----------------------

 postgres    | postgres | UTF8     | en_US.UTF-8 | en_US.UTF-8 |            | libc            | 

 prismatest  | postgres | UTF8     | en_US.UTF-8 | en_US.UTF-8 |            | libc            | 

 prismatest2 | postgres | UTF8     | en_US.UTF-8 | en_US.UTF-8 |            | libc            | 

 prismatest3 | postgres | UTF8     | en_US.UTF-8 | en_US.UTF-8 |            | libc            | 

Hi,

Every connection in postgresql needs to connect to an existing database, so trying to connect to a not existing data will error in any tool.

An usual solution is to connect to template1, check of the db exist, create it if not, then connect to it.