Successful payments by payment method

A payments analyst wants to know which payment methods bring in the most money, ignoring anything that did not go through.

From payment_txns, count the successful transactions and add up their amounts for each method. Return method, txn_count and total_amount, with the highest total_amount first.

1. Step 1: Filter first with `WHERE status = 'SUCCESS'`; row-level filters belong in `WHERE`, not `HAVING`.
2. Step 2: `GROUP BY method` collapses the remaining rows into one row per payment method.
3. Step 3: `COUNT(*)` and `SUM(amount)` are computed per group, then `ORDER BY total_amount DESC` ranks the methods.

SELECT method,
       COUNT(*)    AS txn_count,
       SUM(amount) AS total_amount
FROM payment_txns
WHERE status = 'SUCCESS'
GROUP BY method
ORDER BY total_amount DESC;