Postgres IF on variable - postgresql

Question

I'm in the process of learning postgres, I've already found a work around to this problem but I wanted to ask the community if something like this is even possible, maybe my syntax is just off.
DO $$ BEGIN
IF :MODIFYBY IS NOT NULL THEN
UPDATE User SET ModifyBy = :MODIFYBY WHERE UserId = :USERID;
UPDATE Profile SET ModifyBy = :MODIFYBY WHERE UserId = :USERID;
END IF;
END $$;
Receiving
syntax error at or near ":"
as :MODIFYBY is a parameter to this sql.
How can I test if a parameter is null?
Note: Running on PostgreSQL 9.6
Update:
It is possible my terminology is not correct. The full sql statement is this
BEGIN;
UPDATE User
SET Email = :EMAIL
,ModifyDate = now() at time zone 'utc'
WHERE
UserId = :USERID;
UPDATE Profile
SET FirstName = :FIRSTNAME
,LastName = :LASTNAME
,ModifyDate = now() at time zone 'utc'
WHERE
UserId = :USERID;
DO $$ BEGIN
IF :MODIFYBY IS NOT NULL THEN
UPDATE User SET ModifyBy = :MODIFYBY WHERE UserId = :USERID;
UPDATE Profile SET ModifyBy = :MODIFYBY WHERE UserId = :USERID;
END IF;
END $$;
COMMIT;
I added the DO $$ BEGIN and END $$; to get the IF statement to work...

Answer 1

I think your problem is with the use of : parameters in a function doesn't use it.
This is an example of a function using IF
CREATE OR REPLACE FUNCTION traffic.check_distance(
int_route_source_id bigint,
num_distance_geo numeric)
RETURNS boolean AS
$BODY$
DECLARE
bol_route_error boolean = false;
num_distance_rto numeric;
BEGIN
-- CALCULATE ROUTE DISTANCE
SELECT INTO num_distance_rto
....
--RAISE DISTANCE ALARM
IF num_distance_rto > 3.5 * num_distance_geo THEN
UPDATE traffic.Route_Sources
SET
IsValid = FALSE,
result = '3.5x MUY LARGO'
WHERE
route_source_id = int_route_source_id;
bol_route_error = true;
END IF;
RETURN bol_route_error;
END;
$BODY$
LANGUAGE plpgsql VOLATILE
COST 100;
ALTER FUNCTION traffic.check_distance(bigint, numeric)
OWNER TO postgres;