How do you perform a full outer join on `txn_id` and flag rows where either side is missing?

Use a FULL OUTER JOIN on the key column and add a CASE expression to flag missing rows. For example:

SELECT

COALESCE(a.txn_id, b.txn_id) AS txn_id,

a.notional AS a_notional,

b.notional AS b_notional,

CASE

WHEN a.txn_id IS NULL THEN 'not in txn_a'
WHEN b.txn_id IS NULL THEN 'not in txn_b'
ELSE 'present in both'

END AS status

FROM txn_a a

FULL OUTER JOIN txn_b b ON a.txn_id = b.txn_id;

The COALESCE ensures you get the txn_id from whichever side is present, and the CASE expression labels rows that exist only in one table.