How do I loop through a row while using a cursor - sql

Question

create table ranks (
rank varchar(20)
);
create table people (
name varchar(20)
);
insert into people values('Sam', 'Bob', 'Tim');
declare cursor c1 is (select substr(name, -1) from people)
begin
for i in c1
loop
update ranks
set rank = 'S'
where i = 'S';
end loop;
end;
Hello, I am trying to use the last letter of the people table to decide who gets the S rank, but it isn't working. I keep getting - expression is of wrong type - error. Please help.

Answer 1

Data model looks wrong. That should be only one table with two columns.
SQL> CREATE TABLE people
2 (
3 name VARCHAR2 (20),
4 RANK VARCHAR2 (1)
5 );
Table created.
SQL> INSERT INTO people (name) VALUES ('Sam');
1 row created.
SQL> INSERT INTO people (name) VALUES ('Bob');
1 row created.
SQL> INSERT INTO people (name) VALUES ('Tim');
1 row created.
SQL> COMMIT;
Commit complete.
SQL> SELECT * FROM people;
NAME RANK
-------------------- -----
Sam
Bob
Tim
SQL>
Then, you don't need PL/SQL - a simple UPDATE will do. However, code you posted doesn't make much sense either - substr(name, -1) selects the last letter; nobody has a name that ends with an S so - no rows will ever be updated (at least, not for sample data). That's why I modified it to use the 1st letter.
SQL> UPDATE people
2 SET RANK = 'S'
3 WHERE SUBSTR (name, 1, 1) = 'S';
1 row updated.
SQL> SELECT * FROM people;
NAME R
-------------------- -
Sam S
Bob
Tim
SQL>
If it has to be PL/SQL (because you're learning it), then you'd
SQL> ROLLBACK;
Rollback complete.
SQL> BEGIN
2 FOR cur_r IN (SELECT name FROM people)
3 LOOP
4 UPDATE people
5 SET RANK = 'S'
6 WHERE name = cur_r.name
7 AND SUBSTR (name, 1, 1) = 'S';
8 END LOOP;
9 END;
10 /
PL/SQL procedure successfully completed.
SQL> SELECT * FROM people;
NAME RANK
-------------------- ----
Sam S
Bob
Tim
SQL>

Answer 2

insert into people values('Sam', 'Bob', 'Tim');
Will fail as you only have one column and not three. You want to either use multiple inserts:
insert into people (name) values('Sam');
insert into people (name) values('Bob');
insert into people (name) values('Tim');
Or, use an INSERT ... SELECT ...
insert into people (name)
SELECT 'Sam' FROM DUAL UNION ALL
SELECT 'Bob' FROM DUAL UNION ALL
SELECT 'Tim' FROM DUAL;
Then you want something like:
begin
for i in (select substr(name, -1) AS last_character from people)
loop
update ranks
set rank = 'S'
where i.last_character = 'S';
end loop;
end;
/
But that can be simplified to get rid of the cursor and use a single UPDATE statement:
UPDATE ranks
SET rank = 'S'
WHERE EXISTS(
SELECT 1
FROM people
WHERE name LIKE '%S'
);
But neither of those will do anything as:
The ranks table contains zero rows.
None of the people have a name ending in S.
If you fix both of those then you will just end up updating every row in the ranks table as there is no relationship between a person and a rank.
db<>fiddle here

Answer 3

A simplified version of using cursors is to declare the cursor and then use it
multiple times if required later. This way opening similar dataset in multiple
cursors can be avoided. A good practice to develop when working on code
intensive procedures.
SQL> Declare
cur_people is SELECT name FROM people;
BEGIN
FOR cur_r IN cur_people
LOOP
UPDATE people
SET RANK = 'S'
WHERE name = cur_r.name
AND SUBSTR (name, 1, 1) = 'S';
END LOOP;
END;
/