Unique constraint violated message on inserting code below - oracle

Question

I am writing this code which throws an error in primary key:
DECLARE CURSOR A1 AS
SELECT
TRANS_DET_ID,
(SELECT MAX (NVL(TRANS_DET_DET_ID, 0) + 1)
FROM PROD_OPERATIONS_RATE) DET_ID,
OPER_CODE, ART_CODE, RATE, FROM_DATE, CLOSE_IND
FROM
PROD_OPERATIONS_RATE
WHERE
TRANS_DET_ID = 1
AND OPER_CODE = 1
AND RATE = 2.3005;
a1_var A1%ROWTYPE;
BEGIN
OPEN A1;
LOOP
FETCH A1
INTO a1_var;
EXIT WHEN A1%NOTFOUND;
INSERT INTO PROD_OPERATIONS_RATE (
TRANS_DET_ID,TRANS_DET_DET_ID,OPER_CODE,ART_CODE,RATE,FROM_DATE,CLOSE_IND)
VALUES (1,a1_var.DET_ID,1,a1_var.ART_CODE,2.50,DATE '2022-05-01','N');
END LOOP;
CLOSE A1;
COMMIT;
END;
I want to insert data same table throw conditions met, it throws an error of unique constraint on column TRANS_DET_DET_ID which is the primary key. What am I doing wrong? Please can anyone help me with this? Regards

Answer 1

Unique (primary) key value which is calculated as MAX + 1 is almost always wrong. Switch to a sequence.
Find MAX trans_det_det_id value:
SELECT MAX (trans_det_det_id) max_id FROM PROD_OPERATIONS_RATE;
Create sequence as max_id + 1 (I put a dummy value of 1000; you'd use what query actually returns):
CREATE SEQUENCE seq START WITH 1000;
Now, use the sequence in your PL/SQL script:
DECLARE
CURSOR A1 IS
SELECT TRANS_DET_ID,
--(SELECT MAX (NVL (TRANS_DET_DET_ID, 0) + 1)
-- FROM PROD_OPERATIONS_RATE) DET_ID,
OPER_CODE,
ART_CODE,
RATE,
FROM_DATE,
CLOSE_IND
FROM PROD_OPERATIONS_RATE
WHERE TRANS_DET_ID = 1
AND OPER_CODE = 1
AND RATE = 2.3005;
a1_var A1%ROWTYPE;
BEGIN
OPEN A1;
LOOP
FETCH A1 INTO a1_var;
EXIT WHEN A1%NOTFOUND;
INSERT INTO PROD_OPERATIONS_RATE (TRANS_DET_ID,
TRANS_DET_DET_ID,
OPER_CODE,
ART_CODE,
RATE,
FROM_DATE,
CLOSE_IND)
VALUES (1,
seq.NEXTVAL, -- a1_var.DET_ID,
1,
a1_var.ART_CODE,
2.50,
DATE '2022-05-01',
'N');
END LOOP;
CLOSE A1;
COMMIT;
END;
By the way, if there's no particular reason for doing it slowly in a loop, use an ordinary INSERT INTO statement (SQL, not PL/SQL), it'll be much faster:
INSERT INTO prod_operations_rate (trans_det_id,
trans_det_det_id,
oper_code,
art_code,
rate,
from_date,
close_ind)
SELECT trans_det_id,
seq.NEXTVAL,
oper_code,
art_code,
rate,
from_date,
close_ind
FROM prod_operations_rate
WHERE trans_det_id = 1
AND oper_code = 1
AND rate = 2.3005;