For Identifiable record types, the new selectID method is easier to use than selectPrimaryKey(as:):

struct Player: Identifiable, TableRecord {
    var id: Int64
}

// Before
let ids = try Player.selectPrimaryKey(as: Int64.self).fetchSet(db)

// After
let ids = try Player.selectID().fetchSet(db)

This pull request reverts 7ffc332.

At the time, selectID was replaced with selectPrimaryKey(as:) because:

Since Optional is a database value like others, selectID would be a request of optionals for records with an optional ID type:

struct Player: Identifiable, MutablePersistableRecord {
    var id: Int64?
}

Player.selectID() // QueryInterface<Int64?>

This is not good, because primary keys are not null. And those requests would fetch impractical types such as [Int64?], Set<Int64?>, Int64??.

This problem is reintroduced, but we now recommend against Identifiable types with optional ids in Recommended Practices for Designing Record Types.

Records with optional ids can use selectPrimaryKey(as:) in order to remove the undesired optional:

// NOT RECOMMENDED: Identifiable type with optional ID 
struct Player: Identifiable, MutablePersistableRecord {
    var id: Int64? 
}

// NOT RECOMMENDED: QueryInterface<Int64?>
Player.selectID() 

// OK: QueryInterface<Int64>
Player.selectPrimaryKey(as: Int64.self)