oracle - How to constrain a database table so only one row can have a particular value in a column?

Question

Using Oracle, if a column value can be 'YES' or 'NO' is it possible to constrain a table so that only one row can have a 'YES' value?

I would rather redesign the table structure but this is not possible.

[UDPATE] Sadly, null values are not allowed in this table.

Answer 1

Use a function-based index:

create unique index only_one_yes on mytable
(case when col='YES' then 'YES' end);

Oracle only indexes keys that are not completely null, and the CASE expression here ensures that all the 'NO' values are changed to nulls and so not indexed.