此汇总查询如何找到正确的值

Question

I am having trouble understanding this code. It actually seems to work, but I don't understand how the correct value for activity year and month is "found" get the proper min and max? Or is it running all permutations getting the highest? This is very strange to me.

I do understand how the dateadd works, just not how the query is actually working on the whole. This may be a bad question since I don't actually need help solving a problem, just insight into why this works.

select 
    EmployeeNumber,
    sum(BaseCalculation) as BaseCalculation,
    min(dateadd(mm, (ActivityYear - 1900) * 12 + ActivityMonth - 1 , 0)) as StartDate,
    max(dateadd(mm, (ActivityYear - 1900) * 12 + ActivityMonth - 1 , 0)) as EndDate
from 
    Compensation 
where 
    1=1
    -- and
group by 
    EmployeeNumber

Answer 1

It is converting the columns ActivityYear and ActivityMmonth to a date. It is doing so by counting the number of months since 1900 and adding them to time zero. So, Jan 2000 would become something like Jan, 100. This seems like a very arcane calculation, because dates that are about 2,000 years old are not really useful.

Of course, this assumes that ActivityYear is a recognizable recent year.

I would convert the year and month to the first day of the beginning of the month, with something like this:

min(cast(cast(ActivityYear * 10000 + ActivityMonth + 1 as varchar(255)) as date)

Answer 2

Sql Server will calculate every value of that statement, and then only return the min and max.

Although I cannot say for sure that sql server executes this way internally, the way I think about it I imagine that the engine strips off the group by and all the aggregate functions, runs that query. And then just sums/finds the min etc off of that.

Answer 3

for both the min and max function call, the algorithm is

dateadd(mm, (ActivityYear - 1900) * 12 + ActivityMonth - 1 , 0)

Your query compute all possible date from the Compensation table using this algorithm. Then, you select the minimum date as StartDate and the maximum as EndDate .

This is how the proper max and min are returned.

Note that the dateadd signature is DATEADD (datepart , number , date )

Since the last parameter is 0, you are addind to month(mm) the number calculated in the algorithm, and return the corresponding date starting from 0.

Check this out for more information : https://msdn.microsoft.com/en-us/library/ms186819.aspx