Fraud Detection SQL

WITH RecentSummarizedTransactions AS (
  SELECT
    transaction_id,
    user_id,
    COUNT(*) as transactions_quantity,
    SUM(amount) as transactions_sum,
    AVG(amount) as transaction_average,
    COUNT(DISTINCT merchant_id) as unique_merchants
  FROM
    transactions
  WHERE
    transaction_date >= NOW() - INTERVAL 1 MONTH
  GROUP BY
    user_id
)

WITH AllTimeSummarizedTransactions AS (
  SELECT
    transaction_id,
    user_id,
    COUNT(*) as transactions_quantity,
    SUM (amount) as transactions_sum,
    AVG(amount) as transaction_average,
    COUNT(DISTINCT merchant_id) as unique_merchants,
    COUNT(*) / ABS(DATEDIFF(month, NOW() - INTERVAL 1 MONTH, MIN(transaction_date))) AS avg_transactions_per_month
  FROM
    transactions
  GROUP BY
    user_id
)

SELECT
    rst.transaction_id,
    rst.user_id,
    rst.transactions_quantity,
    rst.transactions_sum,
    rst.unique_merchants
FROM
    RecentSummarizedTransactions rst
JOIN
    AllTimeSummarizedTransactions ast
ON
    ast.transaction_id=rst.transaction_id
WHERE
    rst.transaction_average >= 3 * ast.transaction_average
OR
    rst.transactions_quantity > 3 * ast.avg_transactions_per_month