The DISTINCT keyword is used in SQL queries to eliminate duplicate rows from the result set. When you retrieve data from a database, it’s common to encounter situations where multiple rows have the same values across the selected columns. The DISTINCT keyword ensures that only unique records are returned, making it easier to analyze or display distinct values. By using DISTINCT, developers can focus on unique entries without manually filtering out duplicates after retrieving the results.
For example, consider a table named "Employees" that includes columns for employee names and their associated departments. If you run a query like SELECT department FROM Employees;, the output may show the same department multiple times if several employees are in the same department. However, if you modify the query to include the DISTINCT keyword, like this: SELECT DISTINCT department FROM Employees;, the result will provide a list of departments, showing each one only once, regardless of how many employees belong to it. This can be particularly helpful in reporting scenarios where you need to list unique categories or values without redundancy.
Using DISTINCT can also impact performance, especially on large datasets, as the database engine has to process the data to identify and filter duplicates. It’s important for developers to consider when and how often they use DISTINCT; while it can simplify queries for unique results, it may also slow down query execution if used excessively or without clear necessity. Being mindful of this trade-off helps in writing efficient and effective SQL queries that meet the data retrieval needs without adding unnecessary overhead.