Compilation error Oracle SQL - sql

Question

I am trying to learn how to create functions on Oracle. But I when I create my simple function it says that it was create but with compilation error.
This is my code:
create or replace function show_speed
(in_nplate varchar2)
return number
is
ret number(4);
begin
select speed into ret from cars where in_nplate = nplate;
return ret;
end show_speed;
Now I what do this query,
select nplate, speed, show_speed(nplate) from cars;
It has to show all the plates of all cars and their speed 2 times, one for the attribute and second from the function.
I get this error:
exact fetch returns more than requested number of rows. T
he error it show in the line: "select speed into...."
I was looking on internet but I don't find my mistake.

Answer 1

The error text
exact fetch returns more than requested number of rows.
Told you that your table cars has two or more duplicates line.
For example: imagin that you table contain the follow data.
car_____________speed_______in_plate
toyota | 70 | plate_one
shevrolette | 60 | plate_two
volvo | 80 | plate_two
when you execute your select without function it will return all lines, but in function you stick the result of select in one variable. And if select in fuction will return to you two lines then it will throw exception, because Oracle doesn't manage to stick two lines for one variable.
So, you have two different ways: throw throw exception from you function like that:
create or replace function show_speed
(in_nplate varchar2)
return number
is
ret number(4);
begin
select speed into ret from cars where in_nplate = nplate;
return ret;
exception when too_many_rows than
return ret;
end show_speed;
Or another way: process exception with for .. loop
create or replace function show_speed
(in_nplate varchar2)
return number
is
ret varchar(4);
begin
for x in (select speed from cars where in_nplate = nplate)
loop
ret:= ret||','||x.speed;
end loop;
return ret;
end show_speed;
but this varians is not compatible with your expectings.