PL/SQL counting by looping? - sql

Question

I have an assignment asking me to rewrite this PL/SQL code I wrote for a previous assignment:
DECLARE
-- Variables used to count a, b, c, d, and f grades:
na integer := 0;
nb integer := 0;
nc integer := 0;
nd integer := 0;
nf integer := 0;
BEGIN
select count(*) into na
from gradeReport1
where grade = 'A';
select count(*) into nb
from gradeReport1
where grade = 'B';
select count(*) into nc
from gradeReport1
where grade = 'C';
select count(*) into nd
from gradeReport1
where grade = 'D';
select count(*) into nf
from gradeReport1
where grade = 'F';
if na > 0 then
DBMS_OUTPUT.PUT_LINE('There are total ' || na || ' A''s');
else
DBMS_OUTPUT.PUT_LINE('There are no As');
end if;
if nb > 0 then
DBMS_OUTPUT.PUT_LINE('There are total ' || nb || ' B''s');
else
DBMS_OUTPUT.PUT_LINE('There are no Bs');
end if;
if nc > 0 then
DBMS_OUTPUT.PUT_LINE('There are total ' || nc || ' C''s');
else
DBMS_OUTPUT.PUT_LINE('There are no Cs');
end if;
if nd > 0 then
DBMS_OUTPUT.PUT_LINE('There are total ' || nd || ' D''s');
else
DBMS_OUTPUT.PUT_LINE('There are no Ds');
end if;
if nf > 0 then
DBMS_OUTPUT.PUT_LINE('There are total ' || nf || ' F''s');
else
DBMS_OUTPUT.PUT_LINE('There are no Fs');
end if;
END;
All it does is search a table I made called gradeReport that stores studentID's and associates them with a grade. The PL/SQL counts all instances of a grade A through F. The question wants me to rewrite this solution using looping and VARRAYS. Could anyone give me a hint to help get the ball rolling for me? I've only been using PL/SQL for a few weeks and don't have much more than a basic understanding of the syntax so I'm completely lost and have no idea where to start.
Not looking for any answers here, just some ideas.
Thank You

Answer 1

How about starting with the doc. http://docs.oracle.com/database/122/LNPLS/plsql-control-statements.htm#LNPLS004

Answer 2

DECLARE
-- Need to ensure the array size will hold all the grades
TYPE grade_tab IS VARRAY(200) OF gradeReport1.grade%TYPE;
-- variable used to store the grades:
t_grades grade_tab;
-- Variables used to count a, b, c, d, and f grades:
na INTEGER;
nb INTEGER;
nc INTEGER;
nd INTEGER;
nf INTEGER;
BEGIN
-- Store the grades in an array:
SELECT grade
BULK COLLECT INTO t_grades
FROM gradeReport1
WHERE grade IN ( 'A', 'B', 'C', 'D', 'F' );
-- Loop through the grades and count how many of each:
FOR i IN 1 .. t_grades.COUNT LOOP
IF t_grades(i) = 'A' THEN na := na + 1;
ELSIF t_grades(i) = 'B' THEN nb := nb + 1;
ELSIF t_grades(i) = 'C' THEN nc := nc + 1;
ELSIF t_grades(i) = 'D' THEN nd := nd + 1;
ELSIF t_grades(i) = 'F' THEN nf := nf + 1;
END IF;
END LOOP;
-- Output grade counts
END;
/
However, a much simpler solution would be to do the counting in a single SQL query (although this doesn't meet the assessment's requirements of using a VARRAY):
DECLARE
-- Variables used to count a, b, c, d, and f grades:
na INTEGER;
nb INTEGER;
nc INTEGER;
nd INTEGER;
nf INTEGER;
BEGIN
SELECT COUNT( CASE grade WHEN 'A' THEN 1 END ),
COUNT( CASE grade WHEN 'B' THEN 1 END ),
COUNT( CASE grade WHEN 'C' THEN 1 END ),
COUNT( CASE grade WHEN 'D' THEN 1 END ),
COUNT( CASE grade WHEN 'F' THEN 1 END )
INTO na,
nb,
nc,
nd,
nf
FROM gradeReport1;
-- Output grade counts...
END;
/

Answer 3

Edit: as the requirement is specifically for varrays, see replies by AmmoQ and MTO. As they both point out, though, you'd be unlikely to need arrays for this type of task in practice, and even if you did, you would use a nested table or an associative array and not a varray.
You'll want a Cursor FOR loop, along the lines of
for r in (
select grade from gradereport1
)
loop
...
end loop;
In real code you'd probably make that a group by query and have SQL do the counting for you.
Then just conditionally increment the counters in the loop depending in the value of r.grade.
You can rationalise all of the if statements for reporting the totals by writing a procedure that takes a grade and a total, as the logic is the same for all of them.
procedure showgrade
( p_grade gradereport1.grade%type
, p_count integer )
is
begin
...
end showgrade;
I'll leave the details as an exercise.
Just for fun, here is another approach, using arrays and looping (but not a varray - they really are a bit useless):
declare
type gradereport_tt is table of pls_integer index by gradereport.grade%type;
gradecounts gradereport_tt;
g gradereport.grade%type;
begin
-- Initialise counts:
gradecounts('A') := 0;
gradecounts('B') := 0;
gradecounts('C') := 0;
gradecounts('D') := 0;
gradecounts('E') := 0;
gradecounts('F') := 0;
-- Count grades:
for r in (
select grade from gradereport
)
loop
gradecounts(r.grade) := gradecounts(r.grade) +1;
end loop;
-- Report counts:
g := gradecounts.first;
while g is not null loop
dbms_output.put_line(g || ': ' || gradecounts(g));
g := gradecounts.next(g);
end loop;
end;
btw there is no need to put brackets after if as in some other languages, unless the condition contains a mixture of and and or conditions that need separating.
There is also no need to write anything in uppercase. It's quite common and Steven Feuerstein does it all the time, but they had this debate in the HTML/CSS world and settled on lowercase for readability. And if you are going to have an uppercase rule, at least use it consistently. Your code example has end if; but END; not to mention Select (which I've fixed). Some people seem to be able to read code like this without it driving them nuts, but I'm afraid I'm not one of them.

Answer 4

set SERVEROUTPUT ON
declare
type number_array is VARRAY(5) OF integer;
total integer :=0;
i number :=1;
begin
numbers :=number_array(14,45,67,89,21);
arr_size := numbers.count;
FOR i in 1..arr_size loop
total :=total+numbers(i);
end loop;
dbms_output.put_line('total-' || total);
end;
here I want to count the number_array elements, but I can't get the correct answers. what is the problem with this?

Answer 5

A solution using VARRAYs could look like that:
DECLARE
type chararray IS VARRAY(6) OF CHAR(1);
type numarray IS VARRAY(6) OF INTEGER;
grades chararray;
cnt numarray;
BEGIN
select grade, count(*)
bulk collect into grades, cnt
from gradeReport1
group by grade
order by grade;
for i in 1..grades.count loop
DBMS_OUTPUT.PUT_LINE('There are total ' || cnt(i) || ' ' ||grades(i)||'s');
end loop;
END;
/
But honestly, it's pointless to use VARRAYs in that case. Just use a cursor loop:
BEGIN
for c in ( select grade, count(*) cnt
from gradeReport1
group by grade
order by grade ) loop
DBMS_OUTPUT.PUT_LINE('There are total ' || c.cnt || ' ' ||c.grade||'s');
end loop;
END;
/
Finding the missing marks (those with a count of 0) is a bit more difficult, though.