The DISTINCT keyword in SQL is used to eliminate duplicate rows from the results of a SELECT query, ensuring that only unique values are returned. When applied to one or more columns, it filters out rows where all selected columns have identical values, leaving only one instance of each unique combination. For example, if a table contains multiple entries for the same city in a city column, using SELECT DISTINCT city would return each city name only once, regardless of how many times it appears. This is particularly useful for generating clean, non-repetitive datasets for analysis or reporting.
Behind the scenes, databases process DISTINCT by sorting or grouping the result set to identify and remove duplicates. This can involve temporary data structures or algorithms to compare rows, which may impact performance, especially with large datasets or complex queries. For instance, a query like SELECT DISTINCT name, age FROM employees forces the database to check every combination of name and age to ensure uniqueness. While this is straightforward for small tables, it can become resource-intensive with millions of rows. Developers should also note that DISTINCT operates on the entire set of selected columns, not individual ones. A common mistake is assuming DISTINCT applies to the first column listed, but uniqueness is determined by all columns in the SELECT clause.
DISTINCT is best used when you explicitly need unique results, such as counting distinct values (e.g., COUNT(DISTINCT department) to find unique departments in a company) or avoiding redundant data in reports. However, it should not be a default choice for every query. Overusing it can mask underlying issues like unintended duplicates from incorrect joins or poor schema design. For example, if a query joining orders and customers tables returns duplicate customer records due to multiple orders, adding DISTINCT might hide the problem instead of addressing the root cause (e.g., using proper joins or adjusting the query logic). Always validate whether duplicates are legitimate before relying on DISTINCT as a quick fix.