MySQL 8.0

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CREATE TABLE table__(customer_id INT ,transaction_date date,income INT)
INSERT INTO table__ VALUES(1,now(),100),(1,now(),101),(1,now(),102),(1,now(),103),
(2,now(),100),(2,now(),101),(2,now(),102)
ALTER TABLE table__ ADD id BIGINT UNSIGNED AUTO_INCREMENT,
  ADD PRIMARY KEY (id);
  ALTER TABLE table__ MODIFY COLUMN id BIGINT FIRST;
ALTER TABLE `table__` ADD UNIQUE `unique_index`(`customer_id`, `transaction_date`, `income`);
SELECT * FROM table__


id
customer_id
transaction_date
income


1
1
2022-08-09
100

2
1
2022-08-09
101

3
1
2022-08-09
102

4
1
2022-08-09
103

5
2
2022-08-09
100

6
2
2022-08-09
101

7
2
2022-08-09
102


INSERT INTO table__ VALUES(1,1,now(),100)
Duplicate entry '1' for key 'table__.PRIMARY'

id	customer_id	transaction_date	income
1	1	2022-08-09	100
2	1	2022-08-09	101
3	1	2022-08-09	102
4	1	2022-08-09	103
5	2	2022-08-09	100
6	2	2022-08-09	101
7	2	2022-08-09	102