- Primary key
-- Only one Identity column
-- Can't be null
- Identity
-- Automatic value
-- Only one Identity column
-- Can't be null
-- Can exist in addition of primary key, GUID-auto columns
- Uniqueindentifier
-- Can be multiple
-- Can be null
- Uniqueindentifier - rowguid
-- Auto generated
-- Only one Identity column
-- Can be null
-- Can exist in addition of primary key, Identity columns
- Unique Key
-- Can be null
-- Can be multiple