What is `AS $$` in sql functions?

Question

CREATE FUNCTION one() RETURNS integer AS $$
    SELECT 1 AS result;
$$ LANGUAGE SQL;

why i cant write like this:

CREATE FUNCTION one() RETURNS integer
    SELECT 1 AS result;

• what is AS $$?
• what is $$ LANGUAGE SQL?
• what is $$?

Answer 1

In PostgreSQL it is a feature called dollar-quoting, which allows you to include a body of text without escaping the single quotes

Using it as

CREATE OR REPLACE FUNCTION hello_world(param_your_name text)
RETURNS text AS
$$
SELECT 'Hello world. My name is ' || param_your_name || '.';
$$
language sql STRICT;

which is easier to read than

CREATE OR REPLACE FUNCTION hello_world(param_your_name text)
RETURNS text AS
'
SELECT ''Hello world. My name is '' || param_your_name || ''.'';
'
language sql STRICT;

Read more here - https://www.postgresonline.com/journal/archives/376-Dollar-quoting-for-escaping-single-quotes.html