MySQL unique index by multiple fields

We have a special kind of table in our DB that stores the history of its changes in itself. So called "self-archived" table:

CREAT TABLE coverages (
   id INT, # primary key, auto-increment
   subscriber_id INT,
   current CHAR,  # - could be "C" or "H".
   record_version INT,
   # etc.
);

It stores "coverages" of our subscribers. Field "current" indicates if this is a current/original record ("C") or history record ("H").

We could only have one current "C" coverage for the given subscriber, but we can't create a unique index with 2 fields (*subscriber_id and current*) because for any given "C" record there could be any number of "H" records - history of changes.

So the index should only be unique for current == 'C' and any subscriber_id.

That coul开发者_高级运维d be done in Oracle DB using something like "materialized views": where we could create a materialized view that would only include records with current = 'C' and create a unique index with these 2 fields: *subscriber_id, current*.

The question is: how can this be done in MySQL?

You can do this using NULL values. If you use NULL instead of "H", MySQL will ignore the row when evaluating the UNIQUE constraint:

A UNIQUE index creates a constraint such that all values in the index must be
distinct. An error occurs if you try to add a new row with a key value that
matches an existing row. This constraint does not apply to NULL values except
for the BDB storage engine. For other engines, a UNIQUE index permits multiple
NULL values for columns that can contain NULL.

Now, this is cheating a bit, and it means that you can't have your data exactly as you want it. So this solution may not fit your needs. But if you can rework your data in this way, it should work.