Calculate exact date difference in years using SQL

Question

I receive reports in which the data is ETL to the DB automatically. I extract and transform some of that data to load it somewhere else. One thing I need to d

Answer 1

I have found a better solution. This makes the assumption that the first date is less than or equal to the second date.

declare @dateTable table (date1 datetime, date2 datetime)
insert into @dateTable 
    select '2017-12-31', '2018-01-02' union
    select '2017-01-03', '2018-01-02' union 
    select '2017-01-02', '2018-01-02' union
    select '2017-01-01', '2018-01-02' union
    select '2016-12-01', '2018-01-02' union
    select '2016-01-03', '2018-01-02' union
    select '2016-01-02', '2018-01-02' union
    select '2016-01-01', '2018-01-02' 
select date1, date2, 
        case when ((DATEPART(year, date1) < DATEPART(year, date2)) and 
                    ((DATEPART(month, date1) <= DATEPART(month, date2)) and 
(DATEPART(day, date1) <= DATEPART(day, date2)) ))
                    then DATEDIFF(year, date1, date2)
            when (DATEPART(year, date1) < DATEPART(year, date2))
                    then DATEDIFF(year, date1, date2) - 1
            when (DATEPART(year, date1) = DATEPART(year, date2))
                    then 0
        end [YearsOfService]
from @dateTable

date1                   date2                   YearsOfService
----------------------- ----------------------- --------------
2016-01-01 00:00:00.000 2018-01-02 00:00:00.000 2
2016-01-02 00:00:00.000 2018-01-02 00:00:00.000 2
2016-01-03 00:00:00.000 2018-01-02 00:00:00.000 1
2016-12-01 00:00:00.000 2018-01-02 00:00:00.000 1
2017-01-01 00:00:00.000 2018-01-02 00:00:00.000 1
2017-01-02 00:00:00.000 2018-01-02 00:00:00.000 1
2017-01-03 00:00:00.000 2018-01-02 00:00:00.000 0
2017-12-31 00:00:00.000 2018-01-02 00:00:00.000 0