if (not exists (select name from syscolumns where id = object_id('Pub_Comment') and name = 'Position'))   
    Begin

    ALTER TABLE Pub_Comment ADD
    Position float(8)

    EXEC sys.sp_addextendedproperty @name=N'MS_Description', @value=N'评论时间点
   type:text
   default:0
   regex:decmal1
   regex_msg:经过秒数应为数字' ,@level0type=N'SCHEMA', @level0name=N'dbo', @level1type=N'TABLE', @level1name=N'Pub_Comment', @level2type=N'COLUMN', @level2name=N'Position'

    End