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I am just starting SQL and I have been stuck on this for few hours.
I have two tables SERVICE and RUNNING, SERVICE table stores (car_id, service date) of multiple cars, one car can have multiple entries, RUNNING table stores (date, car_id, kms of running). I have to calculate the average running of cars after the last servicing.

And also how can I cap the maximum number of days, using max() function doesn't work?

What I have tried:

I am trying this, it's not working.

select r.car_id, sum( as avg_run from RUNNING r where >= (select date from SERVICE s where s.car_id = r.car_id order by date desc limit 1) group by car_id;
Updated 15-Jul-21 4:03am

1 solution

Start by doing it in segments: Write a SELECT with a GROUP BY that returns the car ID and the last service date.

When that works, use the car_id and last service date to get the mileage With a JOIN back to the Service tabel to get you the ID, date, and mileage.

When that works, write a JOIN that combines that with your Running table and return the date, car_id, KMs, last service date and last service mileage.

When that works, it's trivial to get the average!
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on_my_way 15-Jul-21 10:44am    
Thank you very much :)
OriginalGriff 15-Jul-21 11:13am    
You're welcome!

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