I'm struggling a little bit with how to word this question. Edits are welcomed:

I have a SQL Server table (PlanID, PlanDesc, PlanType) where the PlanID is the PK. For the column PlanType - I want this to have the property where Only 1 PlanID can be assigned the PlanType value "AP". BUT Many PlanIDs are allowed to be assigned any other PlanType values. 

So something like this would be valid:

```
INSERT INTO MyTable VALUES (1, 'Plan A', OP)
INSERT INTO MyTable VALUES (2, 'Plan B', OP)
INSERT INTO MyTable VALUES (3, 'Plan C', AP)
INSERT INTO MyTable VALUES (4, 'Plan D', LP)
INSERT INTO MyTable VALUES (5, 'Plan E', LP)

```

But as soon as I try to insert (or update) any other plan with PlanType AP, it would fail if an existing plan already has the PlanType AP.

Is there any way to accomplish this type of constraint in the DB structure. Or is my only choice to carefully code this logic into the application?