Unique indexes should allow multiple NULL values to adhere to ANSI standard

[bummzack]

It looks like unique indexes on MsSQL don't allow multiple NULL values. Since SilverStripe enforces ANSI compatibility, multiple NULL values on unique indexes should be allowed.

According to this answer on Stackoverflow, indexes should be defined as follows:

CREATE UNIQUE NONCLUSTERED INDEX idx_yourcolumn_notnull
ON YourTable(yourcolumn)
WHERE yourcolumn IS NOT NULL;