根据日期时间将列更新为整数

Question

I'm trying to do a mass update on an sql table, but I don't understand the logical equivalent. What I'm wanting to do, in pseudo code, is:

UPDATE mytable
SET mycolumn = (pull down the datetime from mycolumn2. if the year of that datetime is not equal to 2013, then mycolumn = 24. if the year is 2013, then mycolumn = 24 - number of months in that datetime)

I'm just really not sure where to start on implementing this kind of logic in SQL. Does anyone have any tips or direction for me?

Thanks

Answer 1

UPDATE myTable
SET mycolumn = CASE WHEN DATEPART(year, mycolumn2) = 2013 
                        THEN 24 - DATEPART(month, mycolumn2) 
                    ELSE 24 
               END

Adjust date syntax for RDBMS as needed.

Answer 2

UPDATE mytable
SET mycolumn = CASE WHEN YEAR(mycolumn2) <> 2013 
                    THEN 24
                    WHEN YEAR(mycolumn2) = 2013 
                    THEN 24 - mycolumn
                END

Answer 3

Using a Simple Case statment:

update mytable
set mycolumn = 24 - case year(mycolumn2) 
                        when 2013 then  month(mycolumn2)
                        else 0 
                    end