How to code something like 'Update on duplicate key update to something else'?

All we need is an easy explanation of the problem, so here it is.

As part of dealing with the UK’s GDPA ‘right to be forgotten’ in a small database I want to update the (autoinc) PK of a row in table ‘member’ to an obfuscated value and let it cascade down through the FKs in related tables, such as the dates they held certain positions.

If I then set the names of the member in table ‘member’ to empty strings I can still use my related tables, for example to determine that there was someone holding a certain position at a certain time but I will no longer be able to ascertain either the name or the id of the person who held it, which is what I want.

I have written a simple a UDF obfuscate_id() that takes a positive integer ID and returns a negative ID calculated by combining the id passed with a random number.

FUNCTION obfuscate_id(id INT) RETURNS INT
COMMENT 'returns the negative version of the id passed, obfuscated by a random number'

BEGIN
  DECLARE result INT;
  SET result =  (FLOOR(RAND(id)*10000) + id ) * -1;
  IF result > 0  THEN -- ensures result is -ve even if called with a -ve parameter
     SET result = result * -1; 
  END IF;  
  RETURN  result;
END;

And I am using the sql

UPDATE member
SET  member_id = obfuscate_id(member_id)
WHERE member_id = 1234;

However, this might generate a negative id that already exists from a previous update of another member_id. If so I want to set it to something else, maybe by obfuscating the id again as in

SET  member_id = obfuscate_id(obfuscate_id(member_id));

Is there a simple way, like ON DUPLICATE KEY … to check if the value I am about to update to already exists and if so update to something else?

I did look at
how-to-use-on-duplicate-key-for-update
and
update-on-duplicate-key-update
and
mysql-update-with-on-duplicate-key-update
but am no wiser

I would add that given the size of table ‘member’ and the particular application, even the first update is going to be a very rare occurrence, so the chance of having any obfuscated IDs is extremely slim, let alone duplicate ones, but I’d like to cover the eventuality if I can.

How to solve :

I know you bored from this bug, So we are here to help you! Take a deep breath and look at the explanation of your problem. We have many solutions to this problem, But we recommend you to use the first method because it is tested & true method that will 100% work for you.

Method 1

Wouldn’t it be simpler / easier to just choose a random number who’s range is outside your current range of values in the member table? E.g. something like (FLOOR(RAND(id)) * -1) + (SELECT MIN(id) FROM member) (semi-pseudocode)?

Note: Use and implement method 1 because this method fully tested our system.
Thank you 🙂

All methods was sourced from stackoverflow.com or stackexchange.com, is licensed under cc by-sa 2.5, cc by-sa 3.0 and cc by-sa 4.0

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