Question

I am using the datediff function

SELECT stName
       ,stId
       ,stDob  --(varchar(15))
       ,stJoinDt --(datetime)
FROM student stu
WHERE
DATEDIFF(yy,stu.stDob,stu.stJoinDt) between 18 and 75

Since the between operator is not effective I have also changed the code to

SELECT stName
           ,stId
           ,stDob
           ,stJoinDt
    FROM student stu
    WHERE
    DATEDIFF(yy,stu.stDob,stu.stJoinDt) >= 18 
    AND  DATEDIFF(yy,stu.stDob,stu.stJoinDt) < 75

Is there any other effective way to write datediff to capture all the missing records?

The missing records are
stDob             stJoinDt
10/08/1925        2011-01-03
04/18/1935        2011-01-19
12/11/1928        2011-06-06
1/24/1927         2011-04-18
04/18/1918        2011-04-20

Answer 1

Those records should be missing because the number of years between stDob and stJoinDt is not between 18 and 75, as you are filtering them out with your condition that stDob and stJoinDt differ by between 18 and 75 years:

with student as (
    select 'Bob' as stName, 1 as stId, '10/08/1925' as stDob,        '2011-01-03' as stJoinDt
    union select 'Bob' as stName, 2 as stId, '04/18/1935',        '2011-01-19'
    union select 'Bob' as stName, 3 as stId, '12/11/1928',        '2011-06-06'
    union select 'Bob' as stName, 4 as stId, '1/24/1927 ',        '2011-04-18'
    union select 'Bob' as stName, 5 as stId, '04/18/1918',        '2011-04-20'
)
    SELECT stName
           ,stId
           ,stDob  --(varchar(15))
           ,stJoinDt --(datetime)
           ,datediff(yy, stu.stDob, stu.stJoinDt) as DiffYears
    FROM student stu

Output:

stName  stId    stDob           stJoinDt    DiffYears
Bob     1       10/08/1925      2011-01-03  *86* (>75)
Bob     2       04/18/1935      2011-01-19  *76* (>75)
Bob     3       12/11/1928      2011-06-06  *83* (>75)
Bob     4       1/24/1927       2011-04-18  *84* (>75)
Bob     5       04/18/1918      2011-04-20  *93* (>75)

My guess would be you were wanting to capture all records where the person is at least 18 years old. In that case, remove the 75 part from the filter:

WHERE
DATEDIFF(yy,stu.stDob,stu.stJoinDt) >= 18 
-- STOP HERE

Although technically this does not perform the correct calculation, because it is only finding the difference in the year values and not taking into account day and month. For instance, a date-of-birth of 12/31/1990 and a join date of 1/1/2008 would register as 18 years even though the person is only 17 years, 1 day old. I would recommend instead using the solution provided in this question:

where
    (DATEDIFF(YY, stu.stDob, stu.stJoinDt) -
        CASE WHEN( 
            (MONTH(stDob)*100 + DAY(stDob)) > (MONTH(stJoinDt)*100 + DAY(stJoinDt))
        ) THEN 1 ELSE 0 END
    ) >= 18