SQL Server Isolation Levels Quiz
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Questions and Answers

What is the primary characteristic of the Read Uncommitted isolation level?

  • It allows transactions to read data that has not yet been committed. (correct)
  • It prevents other transactions from modifying data until the current transaction is complete.
  • It uses shared locks to block any reads until the transaction is finalized.
  • It guarantees that all data reads will return the last committed state.

    • Which feature differentiates the Read Committed isolation level from Read Uncommitted?

  • It commits all changes immediately after execution.
  • It allows dirty reads to occur between transactions.
  • It enables transactions to run faster by ignoring locks.
  • It uses shared locks to restrict access to uncommitted data. (correct)

    • In the context of isolation levels, what does a dirty read imply?

  • Reading a value in a transaction that has already been closed.
  • Reading a value that is currently being modified and not yet committed. (correct)
  • Reading a value that has been committed and is stable.
  • Reading a value that has been rolled back after modification.

    • How can the isolation level be changed in SQL Server Management Studio (SSMS)?

    <p>By selecting the appropriate isolation level from the Options menu under Query Execution.</p> Signup and view all the answers

    What scenario exemplifies the behavior of transactions when using Read Uncommitted isolation level?

    <p>Transaction A reads a value that has been updated by Transaction B but not yet committed.</p> Signup and view all the answers

    What happens to Transaction 2 when it tries to modify the examDesc value for examId = 201 while Transaction 1 is still active?

    <p>Transaction 2 is forced to wait until Transaction 1 commits or rolls back.</p> Signup and view all the answers

    In the serializable isolation level, what restriction is placed on other transactions?

    <p>Other transactions can modify data read by the current transaction only after it completes.</p> Signup and view all the answers

    Which of the following statements is true regarding snapshot isolation?

    <p>Data read by statements in a transaction is consistent with the data at the start of that transaction.</p> Signup and view all the answers

    What occurs in a situation where the READ_COMMITTED_SNAPSHOT option is set to ON?

    <p>Transactions can access the last committed records without waiting.</p> Signup and view all the answers

    What kind of rows might Transaction 1 encounter upon re-executing the SELECT statement after Transaction 2 inserts a new record?

    <p>Phantom rows.</p> Signup and view all the answers

    Which isolation level allows the highest level of concurrency while increasing the chance of concurrency effects like dirty reads?

    <p>Read Uncommitted.</p> Signup and view all the answers

    During Transaction 2's attempt to add a new record while Transaction 1 is still active, what does Transaction 2 experience in the serializable isolation case?

    <p>It is blocked from committing its changes.</p> Signup and view all the answers

    What occurs when the READ_COMMITTED_SNAPSHOT option is set to OFF?

    <p>Locks prevent modifications while reads are in progress.</p> Signup and view all the answers

    In the Read Committed isolation level, which issue is prevented?

    <p>Dirty Reads</p> Signup and view all the answers

    What is a consequence of READ_COMMITTED_SNAPSHOT being set to ON?

    <p>Row versioning allows a consistent snapshot at the statement start.</p> Signup and view all the answers

    What does the Repeatable Read isolation level achieve?

    <p>Prevents other transactions from modifying rows that have been read.</p> Signup and view all the answers

    Which scenario would likely result in a Non-Repeatable Read?

    <p>A transaction modifies data and commits before another reads it.</p> Signup and view all the answers

    In the provided examples, the output of Query2.sql when executed after Query1.sql in a READ_COMMITTED_SNAPSHOT environment is determined by which value?

    <p>80</p> Signup and view all the answers

    Under which condition can transactions cause a Phantom Read in the Repeatable Read isolation level?

    <p>When they insert new rows matching the current transaction's search criteria.</p> Signup and view all the answers

    What is the primary function of exclusive locks during the execution of a transaction in Read Committed isolation level?

    <p>They block concurrent modifications until the transaction is complete.</p> Signup and view all the answers

    Study Notes

    SQL Server Isolation Levels

    • SQL Server isolation levels control how concurrent transactions access data.
    • Different levels offer varying degrees of isolation.

    Isolation Levels Explained

    • Read Uncommitted:
      • Allows transactions to read data that hasn't been committed yet (dirty reads).
      • No locks while reading, minimizing read overhead, but increasing risk of dirty reads.
    • Read Committed:
      • Prevents dirty reads.
      • An exclusive lock is issued during data modification, preventing other transactions from reading the modified data until it's committed.
      • READ_COMMITTED_SNAPSHOT option affects behavior:
        • OFF: Uses shared locks to prevent modifications while reading. Transactions wait until other transactions finish.
        • ON (default in SQL Azure): Uses row versioning for a consistent snapshot at the start of each statement, no locks.
    • Repeatable Read:
      • Prevents non-repeatable reads.
      • Holds a shared lock on all data during read operations until the transaction completes, preventing other transactions from modifying data.
      • Allows other transactions to insert new data that matches the read conditions (phantom reads).
    • Serializable:
      • Highest level of isolation.
      • Prevents both non-repeatable reads and phantom reads.
      • Holds locks until the transaction completes, severely limiting concurrency.
    • Snapshot Isolation:
      • Guarantees a consistent view of the data at the start of the transaction, regardless of concurrent changes.
      • No locks are issued; reads do not block writes, and writes do not block reads.
      • The ALLOW_SNAPSHOT_ISOLATION database option must be enabled.
      • The behavior of READ_COMMITTED is determined by the READ_COMMITTED_SNAPSHOT database option.

    Impact of Isolation Levels

    • Lower isolation levels let multiple users access data concurrently, but increased concurrency risks (like dirty reads).
    • Higher isolation levels reduce risks but consume more system resources and might cause blocking.
    • The choice of isolation level depends on specific application requirements and acceptable concurrency levels.

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    Description

    Test your knowledge on SQL Server isolation levels and understand how transactions access data concurrently. Explore the differences between read uncommitted, read committed, and repeatable read, and their implications for data integrity and performance.

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