Oracle SQL: How to create a constraint such that only one row in a group can be 'Checked' [duplicate]

Question

In Oracle SQL

Suppose a table with the following structure:

ID	NAME	DESCRIPTION	ACTIVE_FLAG
1	A1234567	Item Desc 1	Y
2	A1234567	Item Desc 2	N

I'd like to be able to create a constraint upon this table such that the following operations are allowed:

INSERT INTO TBL (name, description, active_flag) VALUES('A1234567', 'Item Desc 3', 'N')

UPDATE TBL SET active_flag = 'N' WHERE ID = 1

but these are is not:

INSERT INTO TBL (name, description, active_flag) VALUES('A1234567', 'Item Desc 3', 'Y')

UPDATE TBL SET active_flag = 'Y' WHERE ID = 2

Essentially, I'm interested in a constraint such that for all rows which share a value for NAME, only one of those rows can have their active flag set to 'Y'

How might one go about achieving such a constraint?