计算特定课程的平均时间

Question

I have this tiny issue:

I need to find average time each student spend to study math, and I'm not sure how exactly to do it. for example:

+-------+------------+----------------+
| Name  |   Course   |      Time      |
+-------+------------+----------------+
| Sara  | Chemistry  | 1.1.2020 10:00 |
| Sara  | Math       | 1.1.2020 10:15 |
| Sara  | Biology    | 1.1.2020 10:17 |
| Sara  | Math       | 1.1.2020 10:45 |
| Sara  | Chemistry  | 1.1.2020 10:55 |
| John  | Math       | 1.1.2020 10:00 |
| John  | Biology    | 1.1.2020 10:35 |
| John  | Math       | 1.1.2020 11:00 |
| John  | Literature | 1.1.2020 11:30 |
| Kolin | Chemistry  | 1.1.2020 10:00 |
| Kolin | Math       | 1.1.2020 10:30 |
| Kolin | Biology    | 1.1.2020 11:00 |
+-------+------------+----------------+

I tried with Lead(), but I don't know how to specify that I need average time Only for Math. For Sara, for example average function shoul return average for 2minutes and 10 minutes.

Answer 1

Ok, so it looks like Sara doesn't spend very long on a particular subject; she did 15 mins Chem then 2 mins math, 28 mins Bio, 10 mins math and an unknown amount of time on Chemistry again..

We can use LEAD to get the time she switched to the next activity from the current:

SELECT *, LEAD(time) OVER(PARTITION BY name) nxttime FROM table

Now let's find out how long (from tine to nxttime) the students spent on each subject (And this is where we need to know your db, cos date math is different in all of them. I'll use nxttime - time Because that works in oracle and sql server and returns the number of days between the dates:

SELECT name, course, 1440.0 * (LEAD(time) OVER(PARTITION BY name) - time) 
FROM table

Now we just want to average it for maths, per student:

SELECT x.name, AVG(x.spent)
FROM 
  (
    SELECT name, course, 1440.0 * (LEAD(time) OVER(PARTITION BY name) - time) as spent
    FROM table
  ) x
WHERE course = 'Math'
GROUP BY name

Sara's 2 mins and 10 mins should average to 6