In SQL Server, the terms "USED DISTINCT" and "NOT LOCK" seem to refer to two distinct concepts that address different aspects of SQL

 

DISTINCT Keyword

The DISTINCT keyword is used in SQL queries to eliminate duplicate rows from the result set. It ensures that the query returns only unique records from the specified columns.

Example ====>>>

SELECT DISTINCT column_name FROM table_name;

Advantages:

Eliminates Duplicates: It ensures that only unique rows are returned from the database, which can be useful when you want to avoid redundancy.

Simplifies Queries: Instead of manually filtering out duplicates or performing complex operations, DISTINCT simplifies the process of obtaining unique results.

Disadvantages:

Performance Impact: DISTINCT requires SQL Server to scan the entire result set, which can be slow, especially for large datasets.

NOLOCK Hint (Non-locking Reads)

The NOLOCK hint in SQL Server is used to perform dirty reads on the database, meaning it allows the query to read data without placing any locks on the data. This means that other transactions can modify the data while the query is running, and the query can still read it, even if it hasn't been committed yet.

Example ===>>>>>

SELECT * FROM table_name WITH (NOLOCK);

Advantages:

Improved Performance: Since NOLOCK avoids placing locks, it can reduce contention and improve performance, especially for read-heavy queries in high-concurrency environments.

Faster Queries: Particularly in reporting and data warehousing environments where reading large amounts of data without waiting for locks is crucial, it can provide faster response times.

Disadvantages:

Dirty Reads: The query can return uncommitted data (data that might later be rolled back), leading to inaccurate or inconsistent results