Mysql 'insert if not exists', preventing 'overlap' - mysql

Question

My general question seems to be answered by https://stackoverflow.com/a/3025332/3650835, but I do not fully understand it after reading the MySql docs and am wondering if my solution will work as I expect, and also wonder if LIMIT 1 is needed.
Goal: to ensure that, for a given user_id, start and end never "overlap". As an example:
test_table
user_id start end
4 1 5
4 6 13
4 11 17 --> NOT allowed, bc 11 <= 13
2 1 9 --> allowed, user_id is different
My current solution
/* this should not insert anything, as it would cause an "overlap" of start
and end, based on row 2 having end = 13 */
INSERT INTO `test_table` (user_id, start, end)
SELECT '4', '11', '17' FROM DUAL
WHERE NOT EXISTS (SELECT * FROM `test_table`
WHERE user_id = '4' AND end >= '11')
LIMIT 1;
Does the WHERE NOT EXISTS section mean "only do this insert...if this following select returns nothing"?
Also, there was the following comment in the linked solution, but I do not understand why based on the MySql docs this would be true. If true, I could remove Limit 1 from my solution:
If you use "from dual" on line 2 instead of "from table", then you
don't need the "limit 1" clause
Thanks for your time.
Edit: here is all the sql for testing/setup:
CREATE TABLE `test_table`(
`id` INT PRIMARY KEY AUTO_INCREMENT,
`user_id` INT,
`start` INT,
`end` INT
);
INSERT INTO `test_table` (user_id, start, end)
SELECT '4', '1', '5' FROM DUAL
WHERE NOT EXISTS (SELECT * FROM `test_table`
WHERE user_id = '4' AND end >= '1')
LIMIT 1;
INSERT INTO `test_table` (user_id, start, end)
SELECT '2', '1', '9' FROM DUAL
WHERE NOT EXISTS (SELECT * FROM `test_table`
WHERE user_id = '2' AND end >= '1')
LIMIT 1;
INSERT INTO `test_table` (user_id, start, end)
SELECT '4', '6', '13' FROM DUAL
WHERE NOT EXISTS (SELECT * FROM `test_table`
WHERE user_id = '4' AND end >= '6')
LIMIT 1;
/* this should not insert anything, as it would cause an "overlap" of start and end */
INSERT INTO `test_table` (user_id, start, end)
SELECT '4', '11', '17' FROM DUAL
WHERE NOT EXISTS (SELECT * FROM `test_table`
WHERE user_id = '4' AND end >= '11')
LIMIT 1;

Answer 1

I'd use triggers instead:
-- This is your original table create statement
DROP TABLE IF EXISTS `test_table`;
CREATE TABLE `test_table`(
`id` INT PRIMARY KEY AUTO_INCREMENT,
`user_id` INT,
`start` INT,
`end` INT
);
DELIMITER //
CREATE TRIGGER `TRG_test_table_before_insert` BEFORE INSERT ON `test_table` FOR EACH ROW BEGIN
SELECT
COUNT(*) INTO #cnt
FROM
test_table
WHERE
`user_id` = NEW.user_id
AND `start` <= NEW.`end`
AND `end` >= NEW.`start`
;
IF(#cnt > 0) THEN
SET #msg = CONCAT('TrgErr: overlapping, user_id = ', NEW.user_id, ', start = ', NEW.`start`, ', end = ', NEW.`end`);
SIGNAL SQLSTATE '45000' SET message_text = #msg;
END IF;
END//
DELIMITER ;
Then you will be able to use normal insert statements:
INSERT INTO `test_table` (user_id, `start`, `end`) VALUES ('4', '1', '5');
INSERT INTO `test_table` (user_id, `start`, `end`) VALUES ('2', '1', '9');
INSERT INTO `test_table` (user_id, `start`, `end`) VALUES ('4', '6', '13');
INSERT INTO `test_table` (user_id, `start`, `end`) VALUES ('4', '11', '17'); -- this one will not be inserted
In addition, you can implement the similar settings for updates.
P.S. You should check overlapping logic in my code as I do not know if start = end should be allowed or not.
P.P.S. Index (user_id, begin) will also help

Answer 2

NOT EXISTS means that get results for the inner query and if it returns any rows dont include that related row in the main query. But, your subquery does not have any relation with your main query, which makes me to think that the query may not be generating correct results.
select * from tbl1
where not exists (
select 1 from tbl2 where tbl1.id = tbl2.id
)
the above query makes lot more sense. It means that for every record in tbl1 check tbl2 and if any result is found dont include it in the query result.

Answer 3

It might be easier to understand what your query is doing if it is rewritten like this:
INSERT INTO `test_table` (user_id, start, end)
SELECT user_id, start, end
FROM ( SELECT 4 AS `user_id`, 6 AS `start`, 13 AS `end`) AS candidate
WHERE NOT EXISTS (
SELECT *
FROM `test_table` AS t
WHERE t.user_id = candidate.user_id AND t.end >= candidate.`end`
)
;
Also, note I removed the single quotes around the numbers; it may or may not be a problem in this case, but in some scenarios that could have resulted in some hard to find bugs where 2 > 11 (if MySQL decided to cast the t.end to a char type to compare with candidate.end).

Answer 4

A select from DUAL will only return ever return one row, so the LIMIT 1 is not required. However if you use your table name, your query will return either as many rows as are in the table, or none, dependent on whether the EXISTS expression returns true or false. So in that case you will need the LIMIT 1.
Your interpretation of what the WHERE NOT EXISTS does is correct.
If (start,end) pairs are only ever inserted in order, your existing test on end is sufficient. If however they might go backwards e.g. (4, 1, 2), (4, 5, 6), (4, 3, 4), then you should change your WHERE clause in the subquery to also test the start value, e.g. the last query should be written as
INSERT INTO `test_table` (user_id, start, end)
SELECT '4', '11', '17' FROM DUAL
WHERE NOT EXISTS (SELECT * FROM `test_table`
WHERE user_id = '4' AND
(start <= 11 AND end >= 11 OR start <= 17 AND end >= 17))
I made a small demo on dbfiddle to show how these work.