Unique key with NULLs

Question

This question requires some hypothetical background. Let\'s consider an employee table that has columns name, date_of_birth, tit

Answer 1

I think MySQL does it right here. Some other databases (for example Microsoft SQL Server) treat NULL as a value that can only be inserted once into a UNIQUE column, but personally I find this to be strange and unexpected behaviour.

However since this is what you want, you can use some "magic" value instead of NULL, such as a date a long time in the past

Answer 2

I had a similar problem to this, but with a twist. In your case, every employee has a birthday, although it may be unknown. In that case, it makes logical sense for the system to assign two values for employees with unknown birthdays but otherwise identical information. NealB's accepted answer is very accurate.

However, the problem I encountered was one in which the data field did not necessarily have a value. For example, if you added a 'name_of_spouse' field to your table, there wouldn't necessarily be a value for each row of the table. In that case, NealB's first bullet point (the 'wrong way') actually makes sense. In this case, a string 'None' should be inserted in the column name_of_spouse for each row in which there was no known spouse.

The situation where I ran into this problem was in writing a program with database to classify IP traffic. The goal was to create a graph of IP traffic on a private network. Each packet was put into a database table with a unique connection index based on its ip source and dest, port source and dest, transport protocol, and application protocol. However, many packets simply don't have an application protocol. For example, all TCP packets without an application protocol should be classed together, and should occupy one unique entry in the connections index. This is because I want those packets to form a single edge of my graph. In this situation, I took my own advice from above, and stored a string 'None' in the application protocol field to ensure that these packets formed a unique group.

Answer 3

I were looking for one solution and the Alexander Yancharuk suggested was good idea for me. But in my case my columns are foreign keys and employee_id can be null.

I have this structure:

+----+---------+-------------+
| id | room_id | employee_id |
+----+---------+-------------+
|  1 |       1 | NULL        |
|  2 |       2 | 1           |
+----+---------+-------------+

And the room_id with employee_id NULL can not be duplicated

I solved adding a trigger before insert, like this:

DELIMITER $$
USE `db`$$
CREATE DEFINER=`root`@`%` TRIGGER `db`.`room_employee` BEFORE INSERT ON `room_employee` FOR EACH ROW
BEGIN
    IF EXISTS (
            SELECT room_id, employee_id
            FROM room_employee
            WHERE (NEW.room_id = room_employee.room_id AND NEW.employee_id IS NULL AND room_employee.employee_id IS NULL)
        ) THEN
        CALL `The room Can not be duplicated on room employee table`;
    END IF;
END$$
DELIMITER ;

I also added a constraint unique for room_id and employee_id

Answer 4

The perfect solution would be support for function based UK's, but that becomes more complex as mySQL would also then need to support function based indexes. This would prevent the need to use "fake" values in place of NULL, while also allowing developers the ability to decide how to treat NULL values in UK's. Unfortunately, mySQL doesn't currently support such functionality that I am aware of, so we're left with workarounds.

CREATE TABLE employee( 
 name CHAR(50) NOT NULL, 
 date_of_birth DATE, 
 title CHAR(50), 
 UNIQUE KEY idx_name_dob (name, IFNULL(date_of_birth,'0000-00-00 00:00:00'))
);

(Note the use of the IFNULL() function in the unique key definition)