SQL date eval, choose the lowest out of 3 date columns

Hi Experts,

I am trying to figure out a way to evaluate 3 different date columns and select the lowest time value.  I also have NULL's in these fields

For example

       A                                      b                                  c
1/1/2010 1:51               1/1/2010 13:51          1/1/2010 14:00
NULL                             1/4/2010 11:01          1/3/2010 11:00

so for the first row the date value from A should be returned, and in the 2nd row the date value from C should be returned.

Thanks in advance!
Aaron GoodwinAsked:
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Shaun KlineLead Software EngineerCommented:
Does this example help?
declare @test table
  date1 datetime,
  date2 datetime,
  date3 datetime

insert into @test values ('1/1/2010 1:51', '1/1/2010 13:51', '1/1/2010 14:00')
insert into @test values (NULL, '1/4/2010 11:01', '1/3/2010 11:00')

select * from @test

select case when date1 < date2 then
            case when date1 < date3 then date1 else date3 end 
         else case when date2 < date3 then date2 else date3 end end
from @test

Open in new window

Create the reusable function first, then use it twice
create function min_dt(@a datetime, @b datetime) returns datetime
return case when (@b is null) or (@b < 0) or @a < @b then @a else @b end

declare @t table (a datetime, b datetime, c datetime)
insert @t values ('1/1/2010 1:51', '1/1/2010 13:51', '1/1/2010 14:00')
insert @t values (NULL, '1/4/2010 11:01', '1/3/2010 11:00')
insert @t values ('1/4/2010 11:01', NULL, '1/3/2010 11:00')
insert @t values ('1/1/2011 11:01', NULL, '1/3/2011 11:00')

select *, MIN_=dbo.min_dt(dbo.min_dt(a,b),c) from @t

Open in new window


Your query fails on the last one

insert @t values ('1/1/2011 11:01', NULL, '1/3/2011 11:00')
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Shaun KlineLead Software EngineerCommented:
True, it does fail. This correction does work:

select case when date2 is null or date1 < date2 then
        case when date3 is null or date1 < date3 then date1 else date3 end
      else case when date3 is null or date2 < date3 then date2 else date3 end end
from @test

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awking00Information Technology SpecialistCommented:
select case when first_compare <= second_compare then first_compare
            else second_compare
       end as lowest_time_value from
(select case when a < b then a else b end as first_compare,
        case when b < c then b else c end as second_compare
 from tablex);
Aaron GoodwinAuthor Commented:
thank you very much!
I would have thought I at least assisted in the solution?
Also, are you opposed to functions which would be a reusable way of doing things...
Aaron GoodwinAuthor Commented:
Hi CyberKiwi,

Sorry, I did not mean to offend you.
The answer to you question is Yes, I am not comfortable with calling functions at my level of understanding.  Just straight forward SQL queries are what I like to work with.  I appreciate that you caught the error in the first round of code from Shaun.....but ultimately Shaun's principle idea, and provided the working solution, hence my assignment of the points.
I could have let that error go undetected, but I assisted didn't I?
Anyway, I'm off.
Life is too short.
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Microsoft SQL Server 2008

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