Nested transaction
Transaction within transaction. Standardized in SQL but sparse support in RDBMS. PostgreSQL/MySQL emulate via savepoints. Begin-within-begin = often no-op or error.
PostgreSQL: BEGIN; ... BEGIN; ... gives WARNING "there is already a transaction in progress" — inner BEGIN ignored. SQL Server supports nested transactions but commit semantics is confusing (inner commit does nothing, only outer commit applies). True nested transaction semantics: inner abort only rolls back inner state. PostgreSQL via PL/pgSQL: BEGIN-EXCEPTION blocks are effectively nested transactions via savepoints. ORM frameworks abstract: Django atomic() can be nested, internally savepoints.