SQL Divide Column Value By Another Column - mysql

Question

First off, please excuse my formatting. I'm new here and this is my first posting.
I would like to take the value of a column (costs) split by invoice. For example, my table currently looks like this, where worker is the worker who worked on the job, inv is the invoice number, costs is the total costs for the invoice, and amount is the amount of work each worker did on that invoice. I would like to, at the end of this, be able to sum the worker amounts and cost amounts to come up with the invoice total:
Worker | Inv | Costs | Amount
----A--- |---1 | 12 -----| 50
----B--- |---1 | 12 -----| 10
----C--- |---1 | 12 -----| 40
----A----|---2 | 1 ------| 10
----B----|---2 | 1 ------| 10
and I would like it to look like this:
Worker | Inv | Costs | Amount
----A--- |---1 | 4 -------| 50
----B--- |---1 | 4 -------| 10
----C--- |---1 | 4 -------| 40
----A----|---2 | .5 ------| 10
----B----|---2 | .5 ------| 10
The end result after I throw this into a pivot table in Excel would show that Invoice1 is for a total of $112 and the total for Invoice2 is $21

Answer 1

This one sums the cost and amount for each invoice:
SELECT inv, SUM(Cost+Amount)
FROM MyTable
GROUP BY inv
The result is $112 for Invoice 1 and $21 for Invoice 2

Answer 2

select inv,count(1) as workercount
from tbl
group by inv
This query will give the count of the number of invoices, workercount will contain the number you want to divide by. We will use it as a subquery
select worker, inv, costs/invcount.workercount as costs, amount
from tbl
inner join
(select inv,count(1) as workercount
from tbl
group by inv) invcount
on tbl.inv = invcount.inv
pivot results from there. You can calc the total invoice amount via SQL instead of the excel spreadsheet you want here.