Case statement to sum a value based on another value???

[ChrisB]

A financial analyst reached out to me and asked that I implement a measure to an Explore he uses frequently.

This measure "True Cost" is a simple division based upon the "breakdown type" of the transaction. In SQL, it goes like this:

SELECT
CASE
WHEN BREAKDOWN_TYPE IN (
'Scheme Fixed Fee',
'Premium Variable Fee',
'Gateway Fixed Fee',
'Interchange Fixed Fee',
'Chargeback Fixed Fee',
'Scheme Variable Fee'
) THEN SUM(
PROCESSING_CURRENCY_AMOUNT
) END 
/ 
CASE WHEN BREAKDOWN_TYPE = 'Capture' THEN SUM(PROCESSING_CURRENCY_AMOUNT) END AS TRUE_COST
FROM
"FINANCIAL_ACTIONS"
GROUP BY BREAKDOWN_TYPE;

But when I implemented it in my model, it failed to run with the following error:

SQL compilation error: Aggregate functions cannot be nested:
    COALESCE(SUM(CASE
    WHEN financial_actions."BREAKDOWN_TYPE" IN (
            'Scheme Fixed Fee',
            'Premium Variable Fee',
            'Gateway Fixed Fee',
            'Interchange Fixed Fee',
            'Chargeback Fixed Fee',
            'Scheme Variable Fee'
        ) THEN SUM(financial_actions."PROCESSING_CURRENCY_AMOUNT")
  END / CASE
    WHEN financial_actions."BREAKDOWN_TYPE" = 'Capture' THEN SUM(financial_actions."PROCESSING_CURRENCY_AMOUNT")
  END ), 0) AS "financial_actions.true_cost"

The error message does make sense but I don't see where I went wrong in my LookML:

 measure: true_cost {
type: sum
value_format_name: usd
sql: CASE
WHEN ${TABLE}."BREAKDOWN_TYPE" IN (
'Scheme Fixed Fee',
'Premium Variable Fee',
'Gateway Fixed Fee',
'Interchange Fixed Fee',
'Chargeback Fixed Fee',
'Scheme Variable Fee'
) THEN SUM(${TABLE}."PROCESSING_CURRENCY_AMOUNT")
END / CASE
WHEN ${TABLE}."BREAKDOWN_TYPE" = 'Capture' THEN SUM(${TABLE}."PROCESSING_CURRENCY_AMOUNT")
END ;;

How can I work around this issue? Should I create a dimension for the numerator and another one for the denominator?

Thank you!