[meekaaku]

Its not a problem in practice. There might be a way to enforce that through database constraints, but in practice, checking transaction lines sums to 0 in business logic is not that hard. Having said that, you can run accounting entry sanity checks on the entire database. As previously said

SELECT SUM(amount) /* this should sum to zero */

FROM transaction_line

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Also to identify any non-balancing transaction is easy:

SELECT tx.id, tx.date, SUM(amount) tx_sum

FROM transaction tx

INNER JOIN transaction_line txl ON txl.transaction_id = tx.id

GROUP BY tx.id

HAVING tx_sum != 0

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This will identify them even if caused by your business logic bug, database bug, disk corruption etc.

This can also be done on the single accounting transaction just after insert too and can be done within the same database transaction.