自动生成一个唯一的varchar字段 - mySQL，Java，Hibernate

Question

Background : I have a database table called Contact. All users of my system have details of their contacts in this table including a firstname, a lastname, and also a varchar field called 'UniqueId'. Users of the system may put anything in the UniqueId field, as long as it is unique from that user's other contact's unique ids.

Aim : I now need to change my code so a unique id is automatically generated if the user does not provide one. This should be short and visually pleasing. Ideally it could just be an auto-incrementing number. However, AUTO_INCREMENT works for an integer field, not a varchar field.

Also note that each contact UniqueId needs to be unique from the other contacts of that user, but not neccesarily unique to the entire system. Therefore, the following UniqueIds are valid :

Contact
    UserId  Firstname Lastname UniqueId
    1       Bob       Jones    1
    1       Harold    Smith    2
    2       Joe       Bloggs   1

Question : So, how can I achieve this? Is there a reliable and clean way to get the database to generate a unique id for each contact in the existing UniqueId varchar field (Which is my preference if possible)? Or am I forced to make Java go and get the next available unique id, and if so, what is the most reliable way of doing this? Or any alternative solution?

Edit - 11th April AM: We use hibernate to map our fields. I'm just beginning to research if that may provide an alternative solution? Any opinions?

Edit - 11th April PM: 2 options are currently standing out, but neither seem as ideal as I would like.

1. As @eis suggests, I could have an auto-incrementing field in addition to my current varchar field. Then, either when a contact is saved the int can also be saved in the varchar field, or when a contact is retrieved the int can be used if the varchar is empty. But it feels messy and wrong to use two fields rather than one

2. I am looking into using a hibernate generator, as discussed here . But this involves holding a count elsewhere of the next id, and Java code, and seems to massively overcomplicate the process.

If my existing uniqueId field had been an int field, AUTO_INCREMENT would simply work, and work nicely. Is there no way to make the database generate this but save it as a String?

Answer 1

I think what you really should do is ditch your current 'uniqueid' and generate new ones that are really unique across the system, being always autogenerated and never provided by the user. You would need to do separate work to migrate to the new system. That's the only way I see to keep it sane. User could provide something like an alias to be more visually pleasing, if needs be.

On the upside, you could use autoincrement then.

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Ok, one additional option, if you really really want what you're asking. You could have a prefix like §§§§ that is never allowed for a user, and always autogenerate ids based on that, like §§§§1 , §§§§2 etc. If you disallow anything starting with that prefix from the end user, you would know that there would be no collisions, and you could just generate them one-by-one whenever needed.

Sequences would be ideal to generate numbers to it. You don't have sequences in MySQL, but you could emulate them for example like this .

Answer 2

I apologize, I really don't know MySQL syntax, but here's how I'd do it in SQL Server. Hopefully that will still have some value to you. Basically, I'm just counting the number of existing contacts and returning it as a varchar.

CREATE FUNCTION GetNewUniqueId
  (@UserId int)
RETURNS varchar(3)
AS
BEGIN
  DECLARE @Count int;

  SELECT @Count = COUNT(*) 
  FROM Contacts 
  WHERE UserId = @UserId;

  SET @Count = @Count + 1;

  RETURN CAST(@Count AS varchar(3));
END

But if you really want something "visually pleasing," why not try returning something more like Firstname + Lastname ?

CREATE FUNCTION GetNewUniqueId
  (@UserId int, @FirstName varchar(255), @LastName varchar(255))
RETURNS varchar(515)
AS
BEGIN
  DECLARE @UniqueId varchar(515), @Count int;

  SET @UniqueId = @FirstName + @LastName;

  SELECT @Count = COUNT(*) 
  FROM Contacts 
  WHERE UserId = @UserId AND LEFT(UniqueId, LEN(@UniqueId)) = @UniqueId;

  IF @Count > 0
    SET @UniqueId = @UniqueId + '_' + CAST(@Count + 1 AS varchar(3));

  RETURN @UniqueId;
END