Using COALESCE in Postgres and grouping by the resulting value - sql

Question

I have two tables in a Postgres database:
table a
transaction_id | city | store_name | amount
-------------------------------
123 | London | McDonalds | 6.20
999 | NULL | KFC | 8.40
etc...
table b
transaction_id | location | store_name | amount
-----------------------------------
123 | NULL | McDonalds | 6.20
999 | Sydney | KFC | 7.60
etc...
As you can see, the location might be missing in one table but present in another table. For example with transaction 123, the location is present in table a but missing in table b. Apart from that, the rest of the data (amount, store_name etc.) is the same, row by row, assumed that we join on the transaction_id.
For a given merchant, I need to retrieve a list of locations and the total amount for that location.
An example of the desired result:
KFC sales Report:
suburb | suburb_total
---------------
London | 2500
Sydney | 3500
What I tried:
select
coalesce(a.city, b.location) as suburb,
sum(a.amount) as suburbTotal
from tablea a
join tableb b on a.transaction_id = b.transaction_id
where a.store_name ilike 'KFC'
group by(suburb);
But I get the error column "a.city" must appear in the GROUP BY clause or be used in an aggregate function
So I tried:
select
coalesce(a.city, b.location) as suburb,
sum(a.amount) as suburbTotal,
max(a.city) as city_max,
max(b.location) as location_max
from tablea a
join tableb b on a.transaction_id = b.transaction_id
where a.store_name ilike 'McDonalds'
group by(suburb);
But, surprisingly, I'm getting the same error, even thought I'm now using that column in an aggregate function.
How could I achieve the desired result?
NB there are reasons why we have de-normalised data across two tables, that are currently outside of my control. I have to deal with it.
EDIT: added FROM and JOIN, sorry I forgot to type those...

Answer 1

I can only imagine getting that error with your query if suburb were a column in one of the tables. One way around this is to define the value in the from clause:
select v.suburb,
sum(a.amount) as suburbTotal,
max(a.city) as city_max,
max(b.location) as location_max
from tablea a join
tableb b
on a.transaction_id = b.transaction_id cross join lateral
(values (coalesce(a.city, b.location))) as v(suburb)
where a.store_name ilike 'McDonalds'
group by v.suburb;
This is one of the downsides of allowing column aliases in the group by. Sometimes, you might have conflicts with table columns.

Answer 2

Your querires are missing a from clause, which makes it unclear which logic you are trying to implement.
Based on your sample data and expected results, I think that's a full join on the transaction_id, and then aggregation. Using a positional parameter in the group by clause avoids repeating the expression:
select
store_name,
coalesce(a.city, b.location) as suburb,
sum(amount) suburb_total
from tablea a
full join tableb b using(transaction_id)
group by 1, 2