First-to-Second Transaction Latency

Activation metric: how long after the first swipe does the second one come? A card that is used once and then sits for a week behaves differently from one used twice in an hour. Per account, find the first and second transactions in time order and the hours between them. Two window functions on one pass: LEAD(txn_at) to fetch the next transaction's timestamp, ROW_NUMBER() to keep only the first row per account. Order both windows by txn_at, txn_id so simultaneous timestamps cannot flip the answer.

Return account_id, first_txn_at, second_txn_at, and hours_to_second ((julianday(second) − julianday(first)) × 24, rounded to 1 decimal). Accounts with only one transaction are excluded. Order by hours_to_second ascending, then account_id ascending.