SQL DISTINCT Tutorial

The DISTINCT keyword in SQL is used to return only unique (different) values. It is particularly useful when you want to eliminate duplicate records from your query results.

Syntax

SELECT DISTINCT column1, column2, ...
FROM table_name;

Example

Consider a table named Customers:

CustomerID	CustomerName	City
1	John Doe	New York
2	Jane Smith	Los Angeles
3	John Doe	New York
4	Alice Johnson	Chicago

To select distinct customer names:

SELECT DISTINCT CustomerName FROM Customers;

Result:

CustomerName
John Doe
Jane Smith
Alice Johnson

Key Points

• DISTINCT can be used with one or more columns.
• It removes duplicate rows from the result set.
• It is often used with the SELECT statement.

Best Practices

• Use DISTINCT only when necessary, as it can impact performance on large datasets.
• Combine DISTINCT with other SQL clauses like WHERE, ORDER BY, and GROUP BY for more complex queries.

Conclusion

The DISTINCT keyword is a powerful tool in SQL for ensuring that your query results contain only unique values. By understanding its usage and best practices, you can write more efficient and effective SQL queries.