SQL 平均每天花费的钱

Question

I would like to get some help in writing SQL query for the table below. The table represents how much students spent at a food court on a given date. With this table, write an SQL query that calculates the number of days between the first and last date of money spent by each student and the money spent per day in this timespan.

Student_id            Date      Money Spent($)
    1              01.01.2017      15.13
    2              03.01.2017      22.94
    3              03.01.2017      37.50
    2              01.02.2017      44.26
    2              08.04.2017      52.62
    3              09.08.2017      34.67
    1              02.09.2017      43.87
    3              04.11.2017      12.67

Answer 1

Try this

SELECT Student_id,MIN([date]) mindate, MAX([date]) maxdate,SUM(MoneySpent) TotalSpent,
        SUM(MoneySpent)/DATEDIFF ( MIN([date]) , MAX([date]) ) MoneySpentPerDay 
        FROM StudentExpenses
        GROUP BY Student_id

Answer 2

用

SELECT student_id,AVG(spent) FROM `students` GROUP BY student_id;

Answer 3

Well, that's gonna need a GROUP BY per student. Oh yes Sir.
And a MIN & MAX of the date for the first & last.

The money spend per day you ask?
Ah, that could be the SUM of the spend money divided by the COUNT of the DATEDIFF between the MIN & MAX date.
Or maybe it's just an AVG of the money spend?

Not a clear answer?

Hey, don't blame me! I never went to those food courts.