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 recommend to create additional table column checksum which will contain md5 hash of name and date_of_birth. Drop unique key (name, date_of_birth) because it doesn't solve the problem. Create one unique key on checksum.

ALTER TABLE employee 
    ADD COLUMN checksum CHAR(32) NOT NULL;

UPDATE employee 
SET checksum = MD5(CONCAT(name, IFNULL(date_of_birth, '')));

ALTER TABLE employee 
    ADD UNIQUE (checksum);

This solution creates small technical overhead, cause for every inserted pairs you need to generate hash (same thing for every search query). For further improvements you can add trigger that will generate hash for you in every insert:

CREATE TRIGGER before_insert_employee 
BEFORE INSERT ON employee
FOR EACH ROW
    IF new.checksum IS NULL THEN
      SET new.checksum = MD5(CONCAT(new.name, IFNULL(new.date_of_birth, '')));
    END IF;