unique constraint when column value is equal to false? - mysql

Question

In mysql is it possible to have a unique constraint only when a value is false?
For example, I want a unique constraint on Name but only if Archived = false.
In other words I have a field archived which I'm using as a soft delete and deleting two things with the same name will result in a violation if I put the unique constraitn around Name and archived. hence wondering if the database can handle this type of unique constraint

Answer 1

Relevant Example
I think the only way to do this would be with Triggers, and Signals.
Just put your trigger for update and insert.
Relevant Example of Similar Approach From here: http://cvuorinen.net/2013/05/validating-data-with-triggers-in-mysql/
DELIMITER $$
CREATE TRIGGER example_before_insert_allow_only_one_active
BEFORE INSERT ON example_tbl FOR EACH ROW
BEGIN
IF NEW.active = 1 AND (SELECT COUNT(id) FROM example_tbl
WHERE active=1 AND foreign_key_id=NEW.foreign_key_id) > 0
THEN
SIGNAL SQLSTATE '45000'
SET MESSAGE_TEXT = 'Cannot add or update row: only one active row allowed per type';
END IF;
END;
$$
CREATE TRIGGER example_before_update_allow_only_one_active
BEFORE UPDATE ON example_tbl FOR EACH ROW
BEGIN
IF NEW.active = 1 AND (SELECT COUNT(id) FROM example_tbl
WHERE id<>NEW.id AND active=1 AND foreign_key_id=NEW.foreign_key_id) > 0
THEN
SIGNAL SQLSTATE '45000'
SET MESSAGE_TEXT = 'Cannot add or update row: only one active row allowed per type';
END IF;
END;
$$
Applied to your case
DELIMITER $$
CREATE TRIGGER example_before_insert_allow_only_one_not_archived
BEFORE INSERT ON example_tbl FOR EACH ROW
BEGIN
IF NEW.archived = 0 AND (SELECT COUNT(id) FROM example_tbl
WHERE name=NEW.name) > 0
THEN
SIGNAL SQLSTATE '45000'
SET MESSAGE_TEXT = 'Cannot add or update row: existing name exists and you are entering with archived = 0 (false)';
END IF;
END;
$$
CREATE TRIGGER example_before_update_allow_only_one_not_archived
BEFORE UPDATE ON example_tbl FOR EACH ROW
BEGIN
IF NEW.archived = 0 AND (SELECT COUNT(id) FROM example_tbl
WHERE name=NEW.name) > 0
THEN
SIGNAL SQLSTATE '45000'
S SET MESSAGE_TEXT = 'Cannot add or update row: existing name exists and you are entering with archived = 0 (false)';
END IF;
END;
$$
Finally, you may actually be interested in using the following statement in your SQL if you only want one NAME for archived = false, but unlimited for archived = true.
SELECT COUNT(id) FROM example_tbl WHERE name=NEW.name AND archived = 0;