update using for loop in plsql - oracle

Question

i'm having problem updating and insert into below column. Please advise on this.
This is the input
depnto extra comm
----------------------------
20 300 NULL
20 300 400
20 NULL NULL
20 500 NULL
This is the expected output
depnto Extra comm
---------------------
20 300 300
20 300 400
20 NULL NULL
20 500 500
I need to update comm column with extra column on below conditions.
If comm Is null then extra value is updated to comm.
If comm Is not null, no need to update,
If both are null, leave as null,
if comm column has a value no need to overwrite.
My program is below. Even I need to keep track which are rows are updated and to which value in another table.
PROCEDURE (dept_id )
AS
BEGIN
FOR r IN (SELECT *
FROM emp
WHERE comm IS NULL AND extra IS NOT NULL AND deptno = dept_id)
LOOP
UPDATE emp
SET comm = extra
WHERE comm IS NULL AND extra IS NOT NULL AND deptno = dept_id;
INSERT INTO changed_comm (deptno, oldval, newval)
VALUES (dept_id, r.comm, r.extra);
END LOOP;
EXCEPTION
WHEN NO_DATA_FOUND
THEN
NULL;
END;
please provide some opinion on above. Its not inserting correctly.

Answer 1

You do not need FOR LOOP, just a single UPDATE does the work:
UPDATE emp
SET comm = extra
WHERE comm IS NULL AND extra IS NOT NULL;
Here is a demo: http://www.sqlfiddle.com/#!4/aacc3/1
--- EDIT ----
I didn't notice, that in the expected output deptno 10 was updated to 20, to update deptno an another query is needed:
UPDATE emp
SET deptno = 20
WHERE deptno = 10;
---- EDIT -----
If you want to insert changed values to the other table, try a procedure with RETURNING..BULK COLLECT and FORALL:
CREATE OR REPLACE PROCEDURE pro_cedure( p_dept_id number )
IS
TYPE changed_table_type IS TABLE OF changed%ROWTYPE;
changed_buff changed_table_type;
BEGIN
SELECT deptno, comm, extra BULK COLLECT INTO changed_buff
FROM emp
WHERE comm IS NULL AND extra IS NOT NULL AND deptno = p_dept_id
FOR UPDATE;
UPDATE emp
SET comm = extra
WHERE comm IS NULL AND extra IS NOT NULL AND deptno = p_dept_id;
FORALL i IN 1 .. changed_buff.count
INSERT INTO changed VALUES changed_buff( i );
END;
/
The procedure should work if you are not going to process huge number of records in a one call (more than 1000 ... or maximum a few thousands). If one dept_id can contain ten thousands and more rows, then this procedure might be slow, becasue it will consume a huge amount of PGA memory. In such a case, an another approach with bulk collectiong in chunks is required.
-- EDIT --- how to store sequence values -------
I assume that the table changed has 4 columns, like this:
CREATE TABLE "TEST"."CHANGED"
( "DEPTNO" NUMBER,
"OLDVAL" NUMBER,
"NEWVAL" NUMBER,
"SEQ_NEXTVAL" NUMBER
) ;
and we will store sequence values in the seq_nextval column.
In such a case the procedure might look like this:
create or replace
PROCEDURE pro_cedure( p_dept_id number )
IS
TYPE changed_table_type IS TABLE OF changed%ROWTYPE;
changed_buff changed_table_type;
BEGIN
SELECT deptno, comm, extra, sequence_name.nextval
BULK COLLECT INTO changed_buff
FROM emp
WHERE comm IS NULL AND extra IS NOT NULL AND deptno = p_dept_id
FOR UPDATE;
UPDATE emp
SET comm = extra
WHERE comm IS NULL AND extra IS NOT NULL AND deptno = p_dept_id;
FORALL i IN 1 .. changed_buff.count
INSERT INTO changed VALUES changed_buff( i );
END;
--- EDIT --- version with cursor for small sets of data -----
Yes, for small sets of data bulk collecting doesn't give significant increase of the speed, and plain cursor with for..loop is sufficient in such a case.
Below is an example how tu use the cursor together with update, notice the FOR UPDATE clause, it is required when we plan to update a record fetched from the cursor using WHERE CURRENT OF clause.
This time a sequence value is evaluated within the INSERT statement.
create or replace
PROCEDURE pro_cedure( p_dept_id number )
IS
CURSOR mycursor IS
SELECT deptno, comm, extra
FROM emp
WHERE comm IS NULL AND extra IS NOT NULL
AND deptno = p_dept_id
FOR UPDATE;
BEGIN
FOR emp_rec IN mycursor
LOOP
UPDATE emp
SET comm = extra
WHERE CURRENT OF mycursor;
INSERT INTO changed( deptno, oldval, newval, seq_nextval)
VALUES( emp_rec.deptno, emp_rec.comm,
emp_rec.extra, sequence_name.nextval );
END LOOP;
END;

Answer 2

BEGIN
FOR person IN (SELECT A FROM EMP WHERE B IN (SELECT B FROM ustom.cfd_180518) )
LOOP
--dbms_output.put_line(person.A);
UPDATE custom.cfd_180518 SET c = person.a;
END LOOP;
END;