Question

Let’s suppose I have this MySQL table:

id place_id area_mq n
-- -------- ------- --
1  1             50 1

2  2             90 1
3  2             20 1

4  3             10 1
5  3             10 2
6  3             10 3

7  4             20 1
8  4             20 2

9  5             15 1
10 5             25 1

id is the primary key, place_id stands for an area id, area_mq is the surface area of the place in mq.

What I have to do is find a compact query to calculate the sum of area_mq with these rules:

• if n is equal for the same place_id, then count every area_mq (f.ex for place_id=2, I count 90 and 20 in the sum)
• if n is different for the same place_id, then count area_mq only once (possible to do because for these kind of “places” the value will be the same; f.ex. place_id=4, there are two 20’s, i sum only 20 and not 40).

In this example, my correct total would be: 50 + (90 + 20) + 10 + 20 + (15 + 25).

Can I accomplish this with a query (no code or procedures)? If requirements for n were reversed, it would be simple with a GROUP BY and a subquery… but with these conditions, I’m stuck.

Answer 1

select place_id, sum(area_sq)/count(distinct n)
from your_table
group by place_id;

Tested here