Attempting to write PostgreSQL 9.0 function - function

Question

I am attempting to write a function in PostgreSQL 9.0. This will eventually be used in a new aggregate function, but one step at a time.
Here is what I have so far:
create or replace function encstate(text,text) returns text as $$
DECLARE
oldstate alias for $1;
arg alias for $2;
BEGIN
IF length(oldstate)>0 then
select 'Encrypted';
else if
select '';
end if;
END;
$$ language sql strict immutable;
(I know I'm not yet using the $2 argument.)
The result is:
ERROR: syntax error at or near "alias"
LINE 3: oldstate alias for $1;
When I remove the DECLARE block and just refer to the arguments as $1 etc in the body, the result is:
ERROR: syntax error at or near "if"
LINE 3: if length($1)>0 then
As far as I can tell, what I have matches examples found on the web, except I could find no examples of functions with an if-statement, so I have no idea what I'm doing wrong. Any help would be appreciated.

Answer 1

I would suggest doing this as an SQL function:
create or replace function encstate(text,text) returns text as $$
SELECT CASE WHEN length($1)>0 then 'Encrypted' ELSE '' END;
$$ language sql strict immutable;
You could also do what you did with the other, but change sql to plpgsql. My suggestion though is that what you can do in an SQL function you should do in one usually. You will get better performance and the planner can do more with it.

Answer 2

If you want a SQL function:
create or replace function encstate(text, text) returns text as $$
select case
when length($1) > 0 then 'Encrypted'
else ''
end
;
$$ language sql strict immutable;
SQL has no variables or control structures as it is not procedural, it is declarative. If you want procedural features then use a plpgsql function:
create or replace function encstate(text, text) returns text as $$
DECLARE
oldstate alias for $1;
arg alias for $2;
BEGIN
IF length(oldstate) > 0 then
return 'Encrypted';
else
return '';
end if;
END;
$$ language plpgsql strict immutable;

Answer 3

SQL
CREATE OR REPLACE FUNCTION encstate(oldstate text, arg text)
RETURNS text LANGUAGE SQL IMMUTABLE AS
$func$
SELECT CASE WHEN $1 <> '' THEN 'Encrypted' ELSE '' END
$func$
PL/pgSQL
CREATE OR REPLACE FUNCTION encstate(oldstate text, arg text)
RETURNS text LANGUAGE plpgsql IMMUTABLE AS
$func$
BEGIN
IF oldstat <> '' THEN
RETURN 'Encrypted';
ELSE
RETURN '';
END IF;
END
$func$;
Major points
The expression length(x) > 0 (x being text) only excludes '' and NULL.
Use the 100 % equivalent expression x <> ''. Does the same simpler and faster, regardless of whether the function is declared STRICT or not.
Don't use plpgsql ALIAS if you don't have to. It's only there for compatibility and to rename pre-determined parameter names. The manual actively discourages its use for other purposes. I never use it. Named parameters are available since version 8.1. Simpler, better.
In SQL functions you can refer to parameter names (instead of positional parameters ($1, $2, ..) since PostgreSQL 9.2. It's still a good idea to name parameters even before that, for documentation.
I suspect you do not want to declare this function STRICT (synonym: RETURNS NULL ON NULL INPUT). Like the synonym implies, that returns NULL on (any) NULL input. Seems like you want an empty string ('') instead.
There is also a performance implication:
Function executes faster without STRICT modifier?