Datediff - establish age between two columns

I have the following SQL statement.

DATEDIFF(YEAR, BirthDate, DOR)  

Bascally, I want to take the Age between the BirthDate column and DOR column and input the value into a third column called AgeReg.

How can I do this in SQL
al4629740Asked:
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Dale BurrellDirectorCommented:
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KwoofCommented:
dale has it right
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al4629740Author Commented:
I just need to do this once for a specific set of records.  I don't want to make it a computed column
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Dale BurrellDirectorCommented:
Well just add the SQL you've shown to the query to wish to use it on?

You seem to already know what to do, or else I'm misunderstanding the question.
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PortletPaulfreelancerCommented:
Assumptions
   DOR = date of registration
  [BirthDate] and [DOR] are of date/smalldatetime/datetime "data types"

Using a simple datediff(year,BIRTHDATE ,DOR) will not produce a reliable age at registration I'm afraid. Look at these results:
    | USERID |  BIRTHDATE |        DOR | AGEREG | UNRELIABLE_AGEREG |
    |--------|------------|------------|--------|-------------------|
    |    abc | 1975-01-17 | 2013-10-02 |     38 |                38 |
    |    def | 1975-12-31 | 2013-10-02 |     37 |                38 |
    |    ghd | 1996-02-29 | 2013-10-02 |     17 |                17 |
    |    ijk | 1996-02-29 | 2013-01-02 |     16 |                17 |

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The issue is that if the DOR day/month is earlier than the BIRTHDATE's day/month then the age will be incorrect by one year.

The above results were produced by this query:
    SELECT
            UserID
          , convert(varchar(10) ,BirthDate, 121) AS BirthDate
          , convert(varchar(10) ,DOR, 121)       AS DOR
          , YEAR(DOR) - YEAR(BirthDate)
            - ( CASE
                   WHEN MONTH(BirthDate) > MONTH(DOR)
                     OR ( MONTH(BirthDate) = MONTH(DOR)
                        AND DAY(BirthDate) > DAY(DOR)
                        )
                   THEN 1
                   ELSE 0
                 END ) AS AgeReg
    
          , DATEDIFF(YEAR, BirthDate, DOR)  AS unreliable_AgeReg
    FROM Users
;

-- sample data used:
    CREATE TABLE Users
    	( id int identity primary key, [UserID] varchar(12), [BirthDate] datetime, [DOR] datetime)
    ;
    	
    INSERT INTO Users
    	([UserID], [BirthDate], [DOR])
    VALUES
    	('abc', '1975-01-17 00:00:00', '2013-10-02 00:00:00'),
    	('def', '1975-12-31 00:00:00', '2013-10-02 00:00:00'),
    	('ghd', '1996-02-29 00:00:00', '2013-10-02 00:00:00'),
    	('ijk', '1996-02-29 00:00:00', '2013-01-02 00:00:00')
    ;
-- http://sqlfiddle.com/#!3/41e4b/1

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Regarding the computed column, it would make sense if you can implement it as that calculation could be done once (as both dates don't change except for a mistake). However as you can see above it can be simply included into a query like any other calculation would be.

nb: IF (and I hope it's not true) either of the fields used in the calculation are a string type then the calculation would need amendment.
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Anthony PerkinsCommented:
For a little history behind this question, see the question the author posed a couple of days ago:
add computed column from birthdate and time of registration column

What is strange is that Scott provided a valid answer, it was accepted and yet now the author wants to use the following which we all know is not the way to calculate age:
DATEDIFF(YEAR, BirthDate, DOR)  

In case they still think that this is valid I would encourage them to use the following much repeated example:
SELECT DATEDIFF(YEAR, '20121231', '20130101')

Clearly the output here cannot be used as an age.
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al4629740Author Commented:
The question is not the same as the difference I am looking for this time is using an update statement rather than a computed column.

Thanks for your effort
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Anthony PerkinsCommented:
The question is not the same
I did not say it was.  I merely pointed out that your solution to calculate the age is flawed and it is surprising as Scott had already given you the correct solution, regardless of whether you use a computed column or not.
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PortletPaulfreelancerCommented:
If you wanted an update statement I'd suggest asking for exactly that :)

But in addition, if you have been provided a correct algorithm, that should also have been part of the question too. We went down a rabbit hole to correct the algorithm I'm afraid.

Scott's accepted answer on Q_28254154 is neat, and applying that to an update statement would appear like the following (assuming a table called [Users]):
--optional, may already be done, note tinyint caters for 0 to 255
ALTER TABLE Users
add [AgeReg] tinyint
;

-- sample update statement
UPDATE Users
SET AgeReg = DATEDIFF(YEAR, BirthDate, DOR) -
              CASE WHEN CONVERT(char(5), BirthDate, 101) > CONVERT(char(5), DOR, 101) 
              THEN 1 ELSE 0 END
;

-- optional
SELECT
       BirthDate, DOR, AgeReg
FROM Users
;

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al4629740Author Commented:
Gentlemen....Thank you
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