May 2019
Intermediate to advanced
484 pages
11h 23m
English
Content preview from Learn T-SQL Querying
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Using NOLOCK
SQL Server uses isolation levels to preserve the logical order of all transactions, and to protect transactions from the effects of updates performed by other concurrent transactions. The goal is to uphold the atomicity, consistency, isolation, and durability (ACID) properties of relational databases.
Read more about ACID in http://en.wikipedia.org/wiki/ACID.
Different isolation levels have trade-offs between concurrency and isolation requirements: using a more restrictive isolation means fewer concurrent transactions. In a nutshell, SQL Server complies with ANSI-99 standard isolation levels:
- Read uncommitted: In read uncommitted (lowest isolation level, maximum concurrency), statements can read rows that have been modified ...
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