Unique Data Columns in SQL Server 2005: A Quick Look

**- 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

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