Why do we NOLOCK AND LOCK in SQL server with an example

In SQL Server, NOLOCK and LOCK are hints used in queries to control how the database engine handles locking and concurrency. These hints affect how data is read and modified in a multi-user environment.

1. NOLOCK: It's also known as the READUNCOMMITTED isolation level. When you use NOLOCK, it allows a query to read data without waiting for other transactions to release locks on that data. This can improve query performance in certain scenarios because it doesn't wait for locks held by other transactions. However, it comes with the risk of reading uncommitted or "dirty" data that might be rolled back by another transaction.

Example:

SELECT * FROM TableName WITH (NOLOCK);

1. LOCK: This hint is used to acquire locks explicitly on the data being accessed, ensuring consistency and preventing other transactions from modifying it until the lock is released. There are different types of locks (ROWLOCK, TABLOCK, XLOCK, etc.) that can be specified depending on the granularity of the lock required.

Example:

SELECT * FROM TableName WITH (TABLOCK);

However, it's essential to use these hints cautiously because:

• NOLOCK can lead to reading inconsistent or incorrect data.
• LOCK hints can cause blocking and affect concurrency, potentially leading to performance issues if not used appropriately.

In many cases, it's better to rely on the default isolation levels provided by the database system to manage locking and ensure data consistency. Proper indexing, query optimization, and understanding the transaction isolation levels usually provide better long-term solutions for performance and consistency concerns.