To get the first of the previous month:-
declare @firstofprevmonth date = datefromparts(year(dateadd(month,-1,getdate())), month(dateadd(month,-1,getdate())), 1)
And the last day:-
declare @lastofprevmonth date = eomonth(@firstofprevmonth)

Putting...

This should do the trick:-

UPDATE m
SET
    Field1 = COALESCE(Field1,(SELECT TOP 1 Field1 FROM chld c WHERE c.custid = m.custid AND Field1 IS NOT NULL...

You should use CONVERT

USE [reporting_database]
GO
INSERT INTO [dbo].[Converted_Agreement]
SELECT 
CONVERT (char(10),Dates,126)
FROM [dbo].[Agreement]
GO


You could try something like this:-

select coalesce(convert(varchar,[Days Since Account Logon]),'Unknown') as [Days Since Account Logon]

But I agree with the rest, you shouldn't be doing...

Thanks for your feedback - great question too!  I completely agree with you that triggers are a more complete solution, but this is such a handy technique I figured it...

Thanks Steve - wasn't aware it did this..