Find Max Value of Multiple Columns

Nothing new, but a nice trick to find the maximum value of a number of columns within a row, due to Louis Davidson (via Simon Sabin). It uses the fact that a sub-query can reference columns from the main rowset.

select 
    WorkOrderId,
    (select max(DateValue)
     from (select StartDate as DateValue
           union all
           select EndDate
           union all
           select DueDate
           union all
           select ModifiedDate) as DateCols) as MaxDate,
    StartDate, 
    EndDate, 
    DueDate, 
    ModifiedDate
from 
    Production.WorkOrder

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