Server programming can mean a few different things. Server programming is not just writing server functions. There are many other things you can do in the server which can be considered programming.
For more complex tasks you can define your own types, operators, and casts from one type to another, letting you actually compare apples and oranges.
As shown in the next example, you can define the type, fruit_qty, for fruit-with-quantity and then teach PostgreSQL to compare apples and oranges, say to make one orange to be worth 1.5 apples and convert apples to oranges:
postgres=# CREATE TYPE FRUIT_QTY as (name text, qty int);
postgres=# SELECT '("APPLE", 3)'::FRUIT_QTY;
fruit_quantity
----------------
(APPLE,3)
(1 row)
CREATE FUNCTION fruit_qty_larger_than(left_fruit FRUIT_QTY,
right_fruit FRUIT_QTY)
RETURNS BOOL
AS $$
BEGIN
IF (left_fruit.name = 'APPLE' AND right_fruit.name = 'ORANGE')
THEN
RETURN left_fruit.qty > (1.5 * right_fruit.qty);
END IF;
IF (left_fruit.name = 'ORANGE' AND right_fruit.name = 'APPLE' )
THEN
RETURN (1.5 * left_fruit.qty) > right_fruit.qty;
END IF;
RETURN left_fruit.qty > right_fruit.qty;
END;
$$
LANGUAGE plpgsql;
postgres=# SELECT fruit_qty_larger_than('("APPLE", 3)'::FRUIT_QTY,'("ORANGE", 2)'::FRUIT_QTY);
fruit_qty_larger_than
-----------------------
f
(1 row)
postgres=# SELECT fruit_qty_larger_than('("APPLE", 4)'::FRUIT_QTY,'("ORANGE", 2)'::FRUIT_QTY);
fruit_qty_larger_than
-----------------------
t
(1 row)
CREATE OPERATOR > (
leftarg = FRUIT_QTY,
rightarg = FRUIT_QTY,
procedure = fruit_qty_larger_than,
commutator = >
);
postgres=# SELECT '("ORANGE", 2)'::FRUIT_QTY > '("APPLE", 2)'::FRUIT_QTY;
?column?
----------
t
(1 row)
postgres=# SELECT '("ORANGE", 2)'::FRUIT_QTY > '("APPLE", 3)'::FRUIT_QTY;
?column?
----------
f
(1 row)