SQL is NOT DRY.

gistfile1.sql

SELECT people.first_name,
       people.last_name,
       SUM(CAST(transactions.amount_in_cents AS float) / 100.00), // total transaction amount all time
       ytd.amount,
       transactions.amount_in_cents, // last transaction processed
       last_transaction.created_at // date of last transaction
FROM people 
  JOIN transactions 
  ON people.id = transactions.person_id
  
  INNER JOIN (SELECT DISTINCT(transactions.person_id) AS id, 
              SUM(CAST(transactions.amount_in_cents AS float) / 100.00) AS amount
              FROM transactions
              WHERE transactions.created_at >= CAST(extract (year FROM current_date) || '-01-01' AS date)
              GROUP BY transactions.person_id       
            ) AS ytd
  ON people.id = ytd.id

  LEFT JOIN (SELECT transactions.person_id AS id,
              MAX(transactions.created_at) AS created_at
              FROM transactions
              GROUP BY transactions.person_id
            ) AS last_transaction
  ON people.id = last_transaction.id

WHERE transactions.created_at = last_transaction.created_at
GROUP BY people.id, ytd.amount, last_transaction.created_at, transactions.created_at, transactions.amount_in_cents
ORDER BY people.last_name, people.first_name