DateDiff function adding an extra year for the customer dob - sql

Question

How can I alter the SQL query to not return a 3 for customer Mike since he is not 3 years old yet. As you can see the customer turns 3 in December of 2021. But my query is giving him the age of 3. Is there anyways to alter or make a query that gives him the correct age?
SELECT
id,
name,
dob,
DATEDIFF(YYYY, dob, GETDATE())
FROM
customer
WHERE
DATEDIFF(YYYY, dob, GETDATE()) >= 2
AND DATEDIFF(YYYY, dob, GETDATE()) <= 4
Results:
id name dob datediff
-------------------------------
1 Mike 2018-12-05 3

Answer 1

There are many varied solutions to this issue on SQL Server. This answer is based on a Stack Overflow answer from a question where the accepted answer is not the best answer imo. When it comes to calculating "age from birth" there are advantages to using date format 112. You could try calculating the customer age something like this
declare
#as_of datetime=getdate(),
#bday datetime='2018-12-05';
select
(0 + Convert(Char(8),#as_of,112) - Convert(Char(8),#bday,112))/10000 age;
age
2

Answer 2

If you want an integer number of years (no fractional part)...
SELECT
*,
CASE
WHEN DATEADD(year, diff.years, customer.dob) > GETDATE()
THEN diff.years - 1
ELSE diff.years
END
AS age
FROM
customer
CROSS APPLY
(
SELECT DATEDIFF(year, customer.dob, GETDATE()) AS years
)
AS diff
WHERE
customer.dob > DATEADD(year, -5, CAST(GETDATE() AS DATE)) -- Born more recently than 5 years ago, so at most 4 years 11 month and 30 days old
AND customer.dob <= DATEADD(year, -2, CAST(GETDATE() AS DATE)) -- Born two years ago or earlier, so at least 2 years and 0 days old
The cross apply is just so that I can write the DATEDIFF() once and refer to it as many times as I like (Don't Repeat Yourself).
But then I also refactored the where clause. By moving the calculation to be on GETDATE() rather than on the dob column, I both make the maths simpler (than the case statement), but also make it so that any index on dob can be used (SARGable).
CAST(GETDATE() AS DATE) just removes the time part of today's date, on the assumption your dob (and calculations) don't account for the exact time they were born ;)