How to fetch one - one row from table - plsql

Question

I have a table called stud_ans_sheet.
I want to run a loop
which would every time fetch next values from table.
It's a PL/SQL program and I am doing it with a cursor.
This is my program,
but it has a lot of errors:
set serveroutput on;
declare
cursor c_stud is select stud_no,ans from stud_ans_sheet;
v_stud c_stud%rowtype;
v_no stud_ans_sheet.stud_no%type;
answer varchar(10);
i number(3);
v_corr stud_ans_sheet.corr_ans%type;
v_wrong stud_ans_sheet.wrong_ans%type;
v_unattempt stud_ans_sheet.unattempt_ans%type;
score number(5,2);
v_ans varchar(10);
str1 varchar(40);
str2 varchar(40);
nval stud_ans_sheet.stud_no%type;
total number(5,2);
begin
answer:='AACCABAABD';
open c_stud;
loop
fetch c_stud into v_stud;
exit when c_stud%notfound;
for i in 1..10
loop
nval:= select seq.nextval from stud_ans_sheet.stud_no;
select stud_no,ans into v_no,v_ans from stud_ans_sheet where stud_no=nval;
str2:=substr(v_ans,i,1);
str1:=substr(answer,i,1);
if(str2=str1) then
update stud_ans_sheet
set corr_ans=v_corr+1;
elsif(str2='E') then
update stud_ans_sheet
set unattempt_ans=v_unattempt+1;
else
update stud_ans_sheet
set wrong_ans=v_wrong+1;
end if;
end loop;
update stud_ans_sheet
set score=corr_ans-wrong_ans*0.25+unattempt_ans;
end loop;
close c_stud;
end;
/

Answer 1

This line doesn't really make sense:
nval:= select seq.nextval from stud_ans_sheet.stud_no;
There is no variable := select ... construction in PL/SQL, and even if there was, stud_ans_sheet.stud_no isn't a table so you can't select from it, and if you could it's unclear what you want the sequence for anyway.
If you just want to copy the record attribute v_stud.stud_no into the local variable nval then you can just use:
nval := v_stud.stud_no;
but then the question is why copy it at all, when you can just use v_stud.stud_no directly?
It is generally best to avoid the verbose declare-open-fetch construction for cursors when all you need to do is loop through them. Instead, you can just use:
for r in (
select stud_no, ans from stud_ans_sheet
)
loop
I can't see why you are re-querying stud_ans_sheet in the loop just to get back the same row you already have. If it's something to do with your update logic, I couldn't tell what that was supposed to do.
The update statements in your code will update every row in the table every time, because they don't have any where clause.
In PL/SQL, if conditions are terminated by then instead of being enclosed in brackets as in some other languages, so for example,
if (str2 = str1) then
can be decluttered into:
if str2 = str1 then
The construction for i in 1..10 implicitly declares i with its scope as the loop, so the other i you declared at the top is a different variable and not used.
You are allowed to use spaces for readability, so
update stud_ans_sheet
set score=corr_ans-wrong_ans*0.25+unattempt_ans
where ...
can be written more clearly as
update stud_ans_sheet
set score = corr_ans - wrong_ans * 0.25 + unattempt_ans
where ...;
Probably you should focus on one error at a time rather than dumping the whole thing here for review.

Answer 2

I have changed some of your code, try this:
SET SERVEROUTPUT ON;
DECLARE
ANSWER VARCHAR (10);
V_CORR STUD_ANS_SHEET.CORR_ANS%TYPE;
V_WRONG STUD_ANS_SHEET.WRONG_ANS%TYPE;
V_UNATTEMPT STUD_ANS_SHEET.UNATTEMPT_ANS%TYPE;
STR1 VARCHAR (40);
STR2 VARCHAR (40);
BEGIN
ANSWER := 'AACCABAABD';
FOR R_STUD IN (SELECT STUD_NO, ANS
FROM STUD_ANS_SHEET)
LOOP
V_CORR := 0;--It's better to have only one update
V_WRONG := 0;
V_UNATTEMPT := 0;
FOR I IN 1 .. 10
LOOP
STR2 := SUBSTR (R_STUD.ANS, I, 1);
STR1 := SUBSTR (ANSWER, I, 1);
IF (STR2 = STR1)
THEN
V_CORR := V_CORR + 1;
ELSIF (STR2 = 'E')
THEN
V_UNATTEMPT := V_UNATTEMPT + 1;
ELSE
V_WRONG := V_WRONG + 1;
END IF;
END LOOP;
UPDATE STUD_ANS_SHEET
SET SCORE = V_CORR- V_WRONG* 0.25 + V_UNATTEMPT,
CORR_ANS = V_CORR,
WRONG_ANS = V_WRONG,
UNATTEMPT_ANS = V_UNATTEMPT
WHERE R_STUD.STUD_NO = STUD_NO;
END LOOP;
END;
/

Answer 3

SET SERVEROUTPUT ON;
DECLARE
aswer varchar (10);
v_corr number(3);
v_wrong number (3);
v_unattempt number (3);
str1 VARCHAR (10);
str2 VARCHAR (10);
BEGIN
answer:= 'AACCABAABD';
FOR v_stud in c_stud
LOOP
v_corr:= 0;
v_wrong:= 0;
v_unattempt := 0;
FOR I IN 1 .. 10
LOOP
str2 := SUBSTR (v_stud.ans, i, 1);
str1 := SUBSTR (answer, i, 1);
IF (str2 = str1)
THEN
v_corr := v_corr + 1;
ELSIF (str2 = 'E')
THEN
v_unattempt := v_unattempt + 1;
else
v_wrong:= v_wrong + 1;
END IF;
END LOOP;
UPDATE stud_ans_sheet
SET
corr_ans= v_corr,
wrong_ans = v_wrong,
unattempt_ans=v_unattempt , score = v_corr- v_wrong* 0.25,
WHERE stud_no= v_stud.stud_no;
END LOOP;
END;
/