SQL SELECT Queries: Using the DISTINCT Keyword

What is the DISTINCT Keyword?

DISTINCT is a SQL keyword used in SELECT statements to return only unique rows from the result set.

It filters out duplicate rows based on the columns specified in the SELECT clause.

• Removes duplicate rows from query results.
• Works on one or multiple columns.
• Helps in data analysis and reporting by showing unique values.

Syntax and Usage

The basic syntax of DISTINCT in a SELECT query is straightforward.

You place DISTINCT immediately after SELECT followed by the columns you want to retrieve.

Examples of DISTINCT in SELECT Queries

Let's look at practical examples to understand how DISTINCT works.

Performance Considerations

Using DISTINCT can slow down queries on large datasets because the database must compare rows to eliminate duplicates.

Consider indexing columns involved or using other methods like GROUP BY if appropriate.

• DISTINCT requires sorting or hashing to identify duplicates.
• Avoid using DISTINCT unnecessarily to improve performance.

DISTINCT vs GROUP BY

Both DISTINCT and GROUP BY can be used to get unique rows, but they serve different purposes.

GROUP BY is primarily used for aggregation, while DISTINCT simply removes duplicates.

• DISTINCT returns unique rows without aggregation.
• GROUP BY groups rows and can apply aggregate functions like COUNT or SUM.