Simulate Create Database If Not Exists For Postgresql

Simulate CREATE DATABASE IF NOT EXISTS for PostgreSQL?

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.

Postgres database create if not exists

No but you could query the pg_catalog.pg_database table to see if it exists.

Why is the CREATE DATABASE command not working?

You have to terminate SQL commands with ;.

In your case:

CREATE DATABASE test;

what is the equivalent of CREATE VIEW IF NOT EXISTS in postresql

You could use CREATE OR REPLACE:

CREATE OR REPLACE VIEW is similar, but if a view of the same name already exists, it is replaced. The new query must generate the same columns that were generated by the existing view query (that is, the same column names in the same order and with the same data types), but it may add additional columns to the end of the list. The calculations giving rise to the output columns may be completely different.

CREATE OR REPLACE VIEW complete_user_profile ...

PostgreSQL Table exists but python says it doesn't

You should try this.

import psycopg2
# Connect to the Database Server, now with the created DB
try:
    connection = psycopg2.connect(user="postgres", password="postgres", host="127.0.0.1", port="5432", database="dashboard")
except (Exception, psycopg2.Error) as error:
    print("Connection not established", error)


# Check if test_run Table Exists
cursor = connection.cursor()
cursor.execute("SELECT EXISTS(SELECT * FROM information_schema.tables WHERE table_name='test_run')")
if bool(cursor.fetchone()[0]):
    print('test_run table exists. Moving On.')
else:
    print('test_run does not exist. Creating the Table now.')
    cursor.execute("CREATE TABLE test_run (run_id serial PRIMARY KEY, date date, status varchar(255), "
               "total_time integer, project_name varchar(255));")
    connection.commit()

Create New Database and a Schema in new database using Sql Script in PostgreSQL

Try:

\connect dezdaz.

You did not do CREATE DATABASE "DEZDAZ" so the database name is folded to lower case dezdaz per Identifiers. Looks like \connect is doing "DEZDAZ" and not finding the upper case version of the name. This is called out in src/bin/psql/command.c:

/*
 * Read and interpret an argument to the \connect slash command.
...

/*
         * Ideally we should treat the arguments as SQL identifiers.  But for
         * backwards compatibility with 7.2 and older pg_dump files, we have to
         * take unquoted arguments verbatim (don't downcase them). For now,
         * double-quoted arguments may be stripped of double quotes (as if SQL
         * identifiers).  By 7.4 or so, pg_dump files can be expected to
         * double-quote all mixed-case \connect arguments, and then we can get rid
         * of OT_SQLIDHACK.
         */

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