MySQL 5.7

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CREATE TABLE `executed_tests` (
    `id` INTEGER AUTO_INCREMENT NOT NULL,
    `user_id` INTEGER NOT NULL,
    `test_id` INTEGER NOT NULL,
    `start_date` DATE NOT NULL,
    `completed_date` DATE,
    `_helper` CHAR(11) AS (IFNULL(`completed_date`, -1)),
    PRIMARY KEY (`id`),
    UNIQUE(`user_id`, `test_id`, `_helper`)
);
insert into executed_tests (user_id, test_id, start_date) values (1, 2, '1981-08-24');
select * from executed_tests;


id
user_id
test_id
start_date
completed_date
_helper


1
1
2
1981-08-24
null
-1


insert into executed_tests (user_id, test_id, start_date) values (1, 2, '1981-08-24');
Duplicate entry '1-2--1' for key 'user_id'

select * from executed_tests;


id
user_id
test_id
start_date
completed_date
_helper


1
1
2
1981-08-24
null
-1


insert into executed_tests (user_id, test_id, start_date, completed_date) values (1, 2, '1981-08-24', '1981-08-24');
select * from executed_tests;


id
user_id
test_id
start_date
completed_date
_helper


1
1
2
1981-08-24
null
-1

3
1
2
1981-08-24
1981-08-24
1981-08-24


insert into executed_tests (user_id, test_id, start_date, completed_date) values (1, 2, '1981-08-24', '1981-08-24');
Duplicate entry '1-2-1981-08-24' for key 'user_id'

select * from executed_tests;


id
user_id
test_id
start_date
completed_date
_helper


1
1
2
1981-08-24
null
-1

3
1
2
1981-08-24
1981-08-24
1981-08-24