SQL Server CASE WHEN ELSE STATEMENT not working as should

Question

I am struggling with why the following case statement is not working.

What I want to do is convert the month from a date ("date of death") into two characters so that January becomes 01, February becomes 02 and October to December remain as their two character lengths

However, the code seems to be stumping me

SELECT  
    LEN(MONT(RAW.dbo.ONS_Death_DOD201516FYQ1.DATE_OF_DEATH)) AS length_of_month,
    (CASE
        WHEN LEN(MONTH(CAST(RAW.dbo.ONS_Death_DOD201516FYQ1.DATE_OF_DEATH as VARCHAR(20)))) = 1
           THEN CONCAT('0', MONTH(RAW.dbo.ONS_Death_DOD201516FYQ1.DATE_OF_DEATH))
     END) AS DERIVED_MOD
FROM    
    RAW.dbo.ONS_Death_DOD201516FYQ1

The above works and converts those with 1 digit to 2 but when I try to implement the second option below it does not seem to work.

SELECT  
    LEN(MONTH(ONS.DATE_OF_DEATH)) as length_of_month,
    (CASE
        WHEN LEN(MONTH(CAST(RAW.dbo.ONS_Death_DOD201516FYQ1.DATE_OF_DEATH as VARCHAR(20)))) = 2
            THEN MONTH(RAW.dbo.ONS_Death_DOD201516FYQ1.DATE_OF_DEATH)
        WHEN LEN(MONTH(CAST(RAW.dbo.ONS_Death_DOD201516FYQ1.DATE_OF_DEATH as VARCHAR(20)))) = 1
            THEN CONCAT('0', MONTH(RAW.dbo.ONS_Death_DOD201516FYQ1.DATE_OF_DEATH))
    END) AS DERIVED_MOD
FROM    
    RAW.dbo.ONS_Death_DOD201516FYQ1

Any help is much appreciated.

Answer 1

To get the two digit month, just pad with zero, like this:

RIGHT('0' + RTRIM(MONTH(RAW.dbo.ONS_Death_DOD201516FYQ1.DATE_OF_DEATH)), 2)

No need of CASE WHEN .

The RTRIM is just there to convert the month number to a string.

If your version is SQL Server 2012 or higher, there is even a shorter way with format :

FORMAT(RAW.dbo.ONS_Death_DOD201516FYQ1.DATE_OF_DEATH, 'MM') 

Answer 2

What I want to do is convert the month from a date ("date of death") into two characters so that January becomes 01, February becomes 02 and October to December remain as their two character lengths

Replace @dt with your datetime field..works from SQLserver 2005

Declare @dt datetime
set @dt='20160901'
select
case when month(@dt)>=10 then cast( month(@dt) as varchar(2))
     else cast(0 as varchar(2))+cast(month(@dt) as varchar(2))
     end
     ,month(@dt)