How Do I use an Else If case in a Function Block - postgresql

Question

I am trying to use a function to update a column on a table by creating a function that will do it.
I am trying to create a customer tier based on purchases made by each customer. I know the following code will update my column when I run the command by itself:
update email_campaign_detailed
set customer_tier = 'standard'
where num_dvd_rented <=20;
However I want to use a function to do this so I can input the data and have it auto-rank.
this is the code I'm using that isn't working:
Create or replace function customer_tier(num_dvds_rented bigint)
Returns character varying (50)
Language plpgsql
As
$$
Declare
Begin
case
when num_dvd_rented <=20 then update email_campaign_detailed set customer_tier ='standard'
when num_dvd_rented >= 21 and <= 39 then update email_campaign_detailed set customer_tier ='silver'
else update email_campaign_detailed set customer_tier ='gold'
End case;
end;
$$
I get a syntax error at the second "when" statement.
I cannot for the life of me figure out what I need to do.
Any help or insight is deeply appreciated.
Thanks!

Answer 1

You don't need a stored procedure when using the case-expression. Because case is an expression, you can just use it in your sql-statement (like any other expression):
update email_campaign_detailed
set
customer_tier = case
when num_dvd_rented <= 20 then 'standard'
when num_dvd_rented >= 21 and <= 39 then 'silver'
else 'gold'
end
where num_dvd_rented <= 20;
==== EDIT ===
Your stored procedure doesn't work because you can't have an expression (an update statement) within your when branches.
I suppose you want to use the num_dvds_rented parameter in the stored procedure for your where statement?
create or replace function customer_tier(num_dvds_rented bigint)
language plpgsql
As
$$
declare
begin
update email_campaign_detailed
set
customer_tier = case
when num_dvd_rented <= 20 then 'standard'
when num_dvd_rented >= 21 and <= 39 then 'silver'
else 'gold'
end
where num_dvd_rented <= num_dvds_rented;
end;
$$
Also, your stored procedure doesn't really return anything -- so you don't need the returns... in the declaration.
Now, if your goal is to do something like this:
update email_campaign_detailed
set customer_tier = customer_tier(num_dvd_rented)
where num_dvd_rented <= 20;
then your procedure might look like this:
create or replace function customer_tier(num_dvds_rented bigint)
returns character varying (50)
language plpgsql
As
$$
declare
begin
return case
when num_dvds_rented <= 20 then 'standard'
when num_dvds_rented >= 21 and <= 39 then 'silver'
else 'gold'
end;
end;
$$

Answer 2

Proof you can have expressions in the CASE branches. You just need to terminate then with a ;.
CREATE OR REPLACE FUNCTION public.case_test()
RETURNS void
LANGUAGE plpgsql
AS $function$
DECLARE
int_val integer;
BEGIN
CASE WHEN 1 = 1 THEN
select 1 into int_val;
ELSE
select 3 into int_val;
END CASE;
RAISE NOTICE 'Val is %', int_val;
END;
$function$
;
select case_test();
NOTICE: Val is 1
case_test
-----------