Send whole column to the function and check row by row - sql

Question

I want send whole columns to my function! Something like function min() or max(), is it possible?
How to check query results row by row? I wrote something like:
CREATE OR REPLACE FUNCTION gowno.kiki(temppp INTEGER ) RETURNS INTEGER AS
$$
DECLARE
val INTEGER := 0;
i tyczka%ROWTYPE;
BEGIN
FOR i IN (SELECT adres FROM tyczka)
LOOP
RETURN CAST(i.adres AS INTEGER);
IF CAST(i.adres AS INTEGER) > val THEN
val = CAST(i.adres_ AS INTEGER);
END IF;
END LOOP;
RETURN val;
END
$$
LANGUAGE 'plpgsql'
For example, I have something like the following table. And I want to calculate the differences between the field in column poziom_wody where id is the same.

Answer 1

The syntax of your function would work like this:
CREATE OR REPLACE FUNCTION foo(temppp int)
RETURNS INTEGER AS
$func$
DECLARE
val int := 0;
i tyczka;
BEGIN
FOR i IN
SELECT * FROM tyczka
LOOP
RETURN i.adres::int;
IF i.adres::int > val THEN
val := i.adres::int;
END IF;
END LOOP;
RETURN val;
END
$func$ LANGUAGE plpgsql;
Depending on your table definition this could be further simplified.
Depending on what you want to achieve exactly, there is probably a superior set-based approach. Looping is typically a measure of last resort in a relational database.
There are occasions where it's the best one, but people coming from procedural languages tend to over-use loops.
Added example
To get the difference between the maximum and minimum poziom_wody with the same id_rzeki:
SELECT max(poziom_wody) - min(poziom_wody) AS diff
FROM tbl
WHERE id_rzeki = 2;
This works for any number of rows.