Sum of two tables using SQL - sql

Question

I'm trying to get the sum of two columns, but it seems to be adding incorrectly. I have a table Tbl_Booths and another table called Tbl_Extras.
In the Tbl_Booths:
BoothId | ExhId | BoothPrice
1 | 1 | 400
2 | 1 | 500
3 | 2 | 400
4 | 3 | 600
So totalBoothPrice for ExhId = 1 is 900
Tbl_Extras:
ExtraId | ExhId | Item | ItemCost
1 | 1 | PowerSupply | 400
2 | 2 | PowerSupply | 400
3 | 1 | Lights | 600
4 | 3 | PowerSupply | 400
5 | 4 | Lights | 400
So totalItemCost for ExhId = 1 is 1000
I need to find a way to get the sum of totalBoothPrice + totalItemCost
The value should of course be 900 + 1000 = 1900
I'm a total beginner to SQL so please have patience :-)
Thank you in advance for any input you can give me, since I'm going made here !
It is used in a Caspio database system.

Answer 1

You can use union all to combine the two tables and then aggregate:
select exhid, sum(price)
from ((select exhid, boothprice as price
from tbl_booths
) union all
(select exhid, itemcost as price
from tbl_extras
)
) e
group by exhid;
This returns the sum for all exhid values. If you want to filter them, then you can use a where clause in either the outer query or both subqueries.
Here is a db<>fiddle.

Answer 2

Booth totals:
select exhid, sum(boothprice) as total_booth_price
from tbl_booths
group by exhid;
Extra totals:
select exhid, sum(itemcost) as total_item_cost
from tbl_extras
group by exhid;
Joined:
select
exhid,
b.total_booth_price,
e.total_item_cost,
b.total_booth_price + e.total_item_cost as total
from
(
select exhid, sum(boothprice) as total_booth_price
from tbl_booths
group by exhid
) b
join
(
select exhid, sum(itemcost) as total_item_cost
from tbl_extras
group by exhid
) e using (exhid)
order by exhid;
This only shows exhids that have both booth and extras, though. If one can be missing use a left outer join. If one or the other can be missing, you'd want a full outer join, which MySQL doesn't support.