Function returns max of whole table instead of someting particular - PLSQL - sql

Question

I got a PLSQL question related to my output.
Suppose i have a table like this:
Rates:
Company | Country | Level
AA | US | 5
BB | UK | 4
CC | FRANCE | 2
DD | FRANCE | 3
EE | US | 4
FF | UK | 5
I need to create a FUNCTION that gets as parameter the country and provides me it's maximal level.
this is what i wrote:
CREATE OR REPLACE
FUNCTION getMaxLevel(Country VARCHAR2)
RETURN NUMBER
IS
MaxLevel NUMBER;
BEGIN
SELECT max(R.Level) INTO MaxLevel
FROM Rates R
WHERE R.country=Country;
RETURN MaxLevel;
END getMaxLevel;
/
/*CALL TO CHECK*/
DECLARE
X NUMBER;
BEGIN
X:=getMaxLevel('FRANCE');
dbms_output.put_line(X);
END;
and my output is the maximal rate of whole Rates table which here for instance is 6, but i needed to get 3.
Will be thankful for finding my bug :)
Thanks!

Answer 1

You problem is the parameter name. I would recommend prefixing it with something like in:
CREATE OR REPLACE
FUNCTION getMaxLevel(
in_Country VARCHAR2
)
RETURN NUMBER IS
MaxLevel NUMBER;
BEGIN
SELECT max(R.Level) INTO MaxDegree
FROM Rates R
WHERE R.country = in_Country;
RETURN MaxLevel;
END getMaxLevel;
/
You think that Country in your expression:
R.country = Country
is referring to the parameter. But that is not how SQL scoping rules work. It is referring to R.country -- hence the non-sensical result.

Answer 2

You should give your parameter another name that the name of the column in the table, otherwise the condition in the WHERE clause is ambiguous.
In condition R.country = Country, the database thinks that the second Country refers to the (unqualified) column name, not to the parameter. This condition is always true (unless Country is null), and the query ends up returning the maximum level from the whole table.
CREATE OR REPLACE
FUNCTION getMaxLevel(pCountry VARCHAR2)
RETURN NUMBER
IS
MaxLevel NUMBER;
BEGIN
SELECT max(R.Level) INTO MaxLevel
FROM Rates R
WHERE R.country = pCountry;
RETURN MaxLevel;
END getMaxLevel;
/
/*CALL TO CHECK*/
DECLARE
X NUMBER;
BEGIN
X:=getMaxLevel('FRANCE');
dbms_output.put_line(X);
END;
Side note: there is a typo in the query, you meant SELECT .. INT0 MaxLevel instead of SELECT .. INT0 MaxDegree.

Answer 3

You forgot tо prefix the function parameter, this is the name of the function itself.
Rename the column level in the table (e.g. lvl), it is the key word.
Consider this working example:
create or replace function getMaxLevel (Country varchar2) return number is
begin
for r in (
select max (r.Lvl) MaxLevel
from Rates r
where R.country=getMaxLevel.Country
) loop return r.MaxLevel;
end loop;
raise program_error;
end getMaxLevel;
/
var x number
exec :x := getMaxLevel ('FRANCE');
X
-
3