[sql] PostgreSQL IF statement

How can I do such query in Postgres?

IF (select count(*) from orders) > 0
THEN
  DELETE from orders
ELSE 
  INSERT INTO orders values (1,2,3);

From the docs

IF boolean-expression THEN
    statements
ELSE
    statements
END IF;

So in your above example the code should look as follows:

IF select count(*) from orders > 0
THEN
  DELETE from orders
ELSE 
  INSERT INTO orders values (1,2,3);
END IF;

You were missing: END IF;

Just to help if anyone stumble on this question like me, if you want to use if in PostgreSQL, you use "CASE"

select 
    case
        when stage = 1 then 'running'
        when stage = 2 then 'done'
        when stage = 3 then 'stopped'
    else 
        'not running'
    end as run_status from processes

You could also use the the basic structure for the PL/pgSQL CASE with anonymous code block procedure block:

DO $$ BEGIN
    CASE
        WHEN boolean-expression THEN
          statements;
        WHEN boolean-expression THEN
          statements;
        ...
        ELSE
          statements;
    END CASE;
END $$;

References:

1. http://www.postgresql.org/docs/current/static/sql-do.html
2. https://www.postgresql.org/docs/current/static/plpgsql-control-structures.html