Transaction isolation levels, locking, and the ACID properties, each explained with a short example.
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- What does ACID stand for in database transactions?
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Atomicity, Consistency, Isolation, Durability- Explain Atomicity in ACID.
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A transaction either completes entirely or not at all; partial changes never persist.- Explain Consistency in ACID.
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The database moves from one valid state to another; all constraints and rules are maintained.- Explain Isolation in ACID.
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Concurrent transactions do not interfere with each other; each operates as if alone.- Explain Durability in ACID.
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Once committed, a transaction's changes persist permanently, even after failure.- What is a transaction in a relational database?
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A sequence of SQL operations treated as a single unit, either all executed or all rolled back.- What is a dirty read?
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Reading uncommitted data from another transaction that may later be rolled back.- What is a non-repeatable read?
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Reading the same data twice in one transaction yields different values because another transaction modified and committed it.- What is a phantom read?
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A query returns different sets of rows on repeated execution because another transaction inserted or deleted rows.- Which isolation level allows dirty reads?
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Read Uncommitted- What anomalies does Read Committed prevent?
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Dirty reads. Does not prevent non-repeatable reads or phantom reads.- What anomalies does Repeatable Read prevent?
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Dirty reads and non-repeatable reads. Does not prevent phantom reads.- What anomalies does Serializable prevent?
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All three: dirty reads, non-repeatable reads, and phantom reads.- What is a shared lock?
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A lock that allows multiple transactions to read the same data simultaneously, but prevents writing.- What is an exclusive lock?
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A lock that allows only one transaction to access data; prevents both reading and writing by other transactions.
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