IF NOT EXISTS (SELECT 1 FROM [sys].[extended_properties] WHERE [major_id] = OBJECT_ID(N'[dbo].[Repository]')
  AND [minor_id] = (SELECT [column_id] FROM [sys].[columns] WHERE [object_id] = OBJECT_ID(N'[dbo].[Repository]') AND [name] = N'RepositoryType') AND [name] = N'MS_Description')
    EXEC [sys].[sp_addextendedproperty] N'MS_Description', N'RDM / DVLS = 0, Asset = 1',
        N'SCHEMA', N'dbo', N'TABLE', N'Repository', N'COLUMN', N'RepositoryType';