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Lesson 179 / 29860%
SQL Transactions8/8

Concurrency Control

Locks · MVCC · isolation levels (READ COMMITTED, REPEATABLE READ, …).

// ACID · the four guarantees that make SQL safe at scaleAAtomicityall-or-nothing
Every operation in a transaction succeeds — or NONE of them do. No half-applied state.
CConsistencyconstraints intact
After commit, all constraints (PK, FK, CHECK) still hold. The DB never moves from a valid state to an invalid one.
IIsolationconcurrent safety
Concurrent transactions don't see each other's partial work — they behave as if they ran serially.
DDurabilitysurvives crash
Once COMMIT returns, the changes survive a power loss / crash — written to disk + log.
Visual explanation diagram · click steps to walk through it

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