Need help understanding Cursor for loop - oracle

Question

I am trying to write a code for every stock value that is $75 or more
add a "*" in the STK_FLAG column. my error repots are: Error report -
ORA-06550: line 15, column 21: PLS-00201: identifier 'STK_FLG' must be
declared ORA-06550: line 15, column 5: PL/SQL: SQL Statement ignored
ORA-06550: line 23, column 7: PL/SQL: ORA-00904: "STK_FLG": invalid
identifier ORA-06550: line 17, column 5: PL/SQL: SQL Statement ignored
06550. 00000 - "line %s, column %s:\n%s"
*Cause: Usually a PL/SQL compilation error.
*Action:
Blockquote
SET SERVEROUTPUT ON
DECLARE
CURSOR CURR
IS
SELECT STK_FLAG
FROM MM_MOVIE
WHERE MOVIE_VALUE * MOVIE_QTY >= 75
FOR UPDATE;
BEGIN
OPEN CURR;
LOOP
FETCH CURR INTO STK_FLG;
UPDATE
MM_MOVIE
SET
STK_FLG= '*'
WHERE
CURRENT OF CURR;
EXIT
WHEN CURR%notfound;
END LOOP;
Commit;
CLOSE CURR;
END;
/

Answer 1

You didn't declare a cursor variable (so there's nowhere you can put values returned by the cursor). Don't name it as column name; use a prefix, such as v_ or l_ or whichever you want.
Furthermore, in UPDATE you referenced a non-existent column. Cursor suggests its name is stk_flag, not stk_flg
Therefore, code that might be OK is
DECLARE
CURSOR curr IS
SELECT stk_flag
FROM mm_movie
WHERE movie_value * movie_qty >= 75
FOR UPDATE;
l_stk_flag mm_movie.stk_flag%TYPE; --> this
BEGIN
OPEN curr;
LOOP
FETCH curr INTO l_stk_flag;
EXIT WHEN curr%NOTFOUND;
UPDATE mm_movie
SET stk_flag = '*' --> this
WHERE CURRENT OF curr;
END LOOP;
COMMIT;
CLOSE curr;
END;
/

Answer 2

Why use a pl/sql anonymous block? Even if there is an "external" requirement for the functionality wrapped into pl/sql why use a cursor and loop? Using code that might be OK (from #Littlefoot) you retrieve a single column meeting your condition, iterate the resulting record set fetching that column but otherwise do nothing with it, and update a single row on every iteration of the loop with a literal value. SQL is designed for processing entire sets of rows at a time. Your processing can be done in a single update statement. Assuming there is an external requirement for a pl/sql block your code reduces to:
BEGIN
UPDATE mm_movie
SET stk_flag = '*'
WHERE movie_value * movie_qty >= 75;
COMMIT;
END;
Take away: When working with SQL stop thinking in terms of iterating (loops). Instead think of the commonality (set) of all objects to be processed. A different way of looking at problems and their corresponding solutions to be sure. Getting used to thinking that way will take some time, but in the long run your SQL and procedures will greatly improve because of it. Both in performance and clarity.