Best way to prevent a value from going negative in mysql

Question

We have a table that maintains account balances by recording transactions in that table. i.e. the most recent row is the account balance.

When recording a withdrawal, we

Answer 1

Why don't you just do:

INSERT INTO `txns`
  (`account_id`, `prev_balance`, `txn_type`, `new_balance`,  `amount`, `description`)
SELECT *
FROM (
 SELECT
  t.account_id, t.new_balance, $txn_type, t.new_balance - $amount, $amount, $description
 FROM `txns` t
 WHERE t.account_id = '$account'
 ORDER BY txn_id desc
 LIMIT 1
)
WHERE new_balance - $amount > 0