postgresql insert into if not exists code example

Example 1: if not exists insert sql

IF NOT EXISTS (SELECT * FROM EmailsRecebidos 
                   WHERE De = @_DE
                   AND Assunto = @_ASSUNTO
                   AND Data = @_DATA)
   BEGIN
       INSERT INTO EmailsRecebidos (De, Assunto, Data)
       VALUES (@_DE, @_ASSUNTO, @_DATA)
   END

Example 2: mysql insert into if not exists

INSERT INTO table_name (firstname, lastname)
SELECT 'NEW FIRSTNAME', 'NEW LASTNAME'
FROM DUAL
WHERE NOT EXISTS(
    SELECT 1
    FROM table_name
    WHERE firstname = 'NEW FIRSTNAME' AND lastname = 'NEW LASTNAME'
)
LIMIT 1;