Why do you need to include a field in GROUP BY when using OVER (PARTITION BY x)? - sql

Question

I have a table for which I want to do a simple sum of a field, grouped by two columns. I then want the total for all values for each year_num.
See example: http://rextester.com/QSLRS68794
This query is throwing: "42803: column "foo.num_cust" must appear in the GROUP BY clause or be used in an aggregate function", and I cannot figure out why. Why would an aggregate function using the OVER (PARTITION BY x) require the summed field to be in GROUP BY??
select
year_num
,age_bucket
,sum(num_cust)
--,sum(num_cust) over (partition by year_num) --THROWS ERROR!!
from
foo
group by
year_num
,age_bucket
order by 1,2
TABLE:
| loc_id | year_num | gen | cust_category | cust_age | num_cust | age_bucket |
|--------|-----------|------|----------------|-----------|-----------|-------------|
| 1 | 2016 | M | cash | 41 | 2 | 04_<45 |
| 1 | 2016 | F | Prepaid | 41 | 1 | 03_<35 |
| 1 | 2016 | F | cc | 61 | 1 | 05_45+ |
| 1 | 2016 | F | cc | 19 | 2 | 02_<25 |
| 1 | 2016 | M | cc | 64 | 1 | 05_45+ |
| 1 | 2016 | F | cash | 46 | 1 | 05_45+ |
| 1 | 2016 | F | cash | 27 | 3 | 03_<35 |
| 1 | 2016 | M | cash | 42 | 1 | 04_<45 |
| 1 | 2017 | F | cc | 35 | 1 | 04_<45 |
| 1 | 2017 | F | cc | 37 | 1 | 04_<45 |
| 1 | 2017 | F | cash | 46 | 1 | 05_45+ |
| 1 | 2016 | F | cash | 19 | 4 | 02_<25 |
| 1 | 2017 | M | cash | 43 | 1 | 04_<45 |
| 1 | 2017 | M | cash | 29 | 1 | 03_<35 |
| 1 | 2016 | F | cc | 13 | 1 | 01_<18 |
| 1 | 2017 | F | cash | 16 | 2 | 01_<18 |
| 1 | 2016 | F | cc | 17 | 2 | 01_<18 |
| 1 | 2016 | M | cc | 17 | 2 | 01_<18 |
| 1 | 2017 | F | cash | 18 | 9 | 02_<25 |
DESIRED OUTPUT:
| year_num | age_bucket | sum | sum over (year_num) |
|----------|------------|-----|---------------------|
| 2016 | 01_<18 | 5 | 21 |
| 2016 | 02_<25 | 6 | 21 |
| 2016 | 03_<35 | 4 | 21 |
| 2016 | 04_<45 | 3 | 21 |
| 2016 | 05_45+ | 3 | 21 |
| 2017 | 01_<18 | 2 | 16 |
| 2017 | 02_<25 | 9 | 16 |
| 2017 | 03_<35 | 1 | 16 |
| 2017 | 04_<45 | 3 | 16 |
| 2017 | 05_45+ | 1 | 16 |

Answer 1

You need to nest the sum()s:
select year_num, age_bucket, sum(num_cust),
sum(sum(num_cust)) over (partition by year_num) --WORKS!!
from foo
group by year_num, age_bucket
order by 1, 2;
Why? Well, the window function is not doing aggregation. The argument needs to be an expression that can be evaluated after the group by (because this is an aggregation query). Because num_cust is not a group by key, it needs an aggregation function.
Perhaps this is clearer if you used a subquery:
select year_num, age_bucket, sum_num_cust,
sum(sum_num_cust) over (partition by year_num)
from (select year_num, age_bucket, sum(num_cust) as sum_num_cust
from foo
group by year_num, age_bucket
) ya
order by 1, 2;
These two queries do exactly the same thing. But with the subquery it should be more obvious why you need the extra aggregation.