如何在查询时使用datediff函数查找年龄范围？

Question

I have a table A with date-of-birth (dd-mm-yy format) column, DOB, and suppose another table B, with data which I want to join. How can I join considering a particular age range like 18 to 25?

This is what I have written, but it's not working:

select * from B
inner join (select * from A
    where datediff(year, DOB, year(getdate())) between 18 and 25) A
on B.id = A.id

Answer 1

You should not use datediff() for this purpose. Instead:

where dob >= dateadd(year, -25, getdate()) and
      dob <= dateadd(year, -18, getdate())

There are two important reasons why. First, using a function on dob prevents the use of indexes and other optimizations. That can result in inferior query plans.

Second, datediff() measures year boundaries. So, it is actually returning people who were born in the calendar year (from Jan 1 to Dec 31) 18 to 25 years ago. Generally, you are not interested in calendars for this purpose.