SQL: without an explicit transaction, every statement is its own

-- autocommit: two independent transactions
UPDATE account SET bal = bal - 100 WHERE id = 1;
UPDATE account SET bal = bal + 100 WHERE id = 2;   -- if this fails, money is gone

-- one unit:
BEGIN;
  UPDATE account SET bal = bal - 100 WHERE id = 1;
  UPDATE account SET bal = bal + 100 WHERE id = 2;
COMMIT;                       -- or ROLLBACK, and neither happened

-- After an error inside a PostgreSQL transaction, every later statement fails:
--   ERROR: current transaction is aborted, commands ignored until end of
--          transaction block
-- SAVEPOINT lets you recover part of the way rather than losing all of it.