difficult constraint for a mysql-table - mysql

Question

I need a constraint for a mysql-table. The table has the fields 'id', 'ref', 'from' and 'to'. The constraint schould guarantee that there are no datasets with the same 'ref' and a time overlapping (fields 'from' and 'to').
In sql: The following statement should always return '0'.
select count(*)
from `TABLE` d1 inner join `TABLE` d2 on
d1.`ref` = d2.`ref` and d1.`id` <> d2.`id` and
d1.`to` >= d2.`from` and d1.`from`<=d2.`to`
Is there a way to handle this with constrains?

Answer 1

Now I have the following triggers. Thanks for your help!
DELIMITER $$
USE `devtestplandb`$$
CREATE
TRIGGER `db`.`trig1`
BEFORE INSERT ON `db`.`TABLE`
FOR EACH ROW
BEGIN
SET #CNT = (
select count(*)
from `TABLE` d
where d.`ref` = NEW.`ref` and
d.`to` >= NEW.`from` and
d.`from` <= NEW.`to`);
IF #CNT != 0 THEN
CALL error_001();
END IF;
END$$
CREATE
TRIGGER `db`.`trig2`
BEFORE UPDATE ON `db`.`TABLE`
FOR EACH ROW
BEGIN
SET #CNT = (
select count(*)
from `TABLE` d
where d.`ref` = NEW.`ref` and
d.`ID` <> NEW.`ID` and
d.`to` >= NEW.`from` and
d.`from` <= NEW.`to`);
IF #CNT != 0 THEN
CALL error_002();
END IF;
END$$

Answer 2

"Is there a way to handle this with constrains?"
Yes, SQL Standard 2011 supports this kind of scenarios in readable declarative manner:
unique constraint definition
<without overlap specification> ::=
<application time period name> WITHOUT OVERLAPS
In your example:
CREATE TABLE tab (
id INT AUTO_INCREMENT PRIMARY KEY,
ref VARCHAR(100),
from_date DATE,
end_date DATE,
PERIOD FOR ref_period(from_date, end_date),
UNIQUE (ref, ref_period WITHOUT OVERLAPS)
);
And sample inserts:
INSERT INTO tab(ref, from_date, end_date) VALUES ('a', '2020-01-01','2020-03-01');
-- OK
INSERT INTO tab(ref, from_date, end_date) VALUES ('a', '2020-03-01','2020-05-01');
-- OK
INSERT INTO tab(ref, from_date, end_date) VALUES ('a', '2020-04-01','2020-07-01')
-- Duplicate entry 'a-2020-07-01-2020-04-01' for key 'ref
SELECT * FROM tab;
db<>fiddle demo - Maria DB 10.5