MySQL SELECT Queries: DISTINCT Clause Explained

What is the DISTINCT Clause?

The DISTINCT clause is used in SELECT statements to return only unique rows. It filters out duplicate records based on the columns specified in the SELECT list.

When you use DISTINCT, MySQL compares all selected columns in each row and removes duplicates, ensuring each row in the result set is unique.

• Placed immediately after SELECT keyword.
• Applies to all columns in the SELECT statement.
• Useful for eliminating repeated data.

Syntax of DISTINCT Clause

The basic syntax to use DISTINCT in a SELECT query is straightforward.

• SELECT DISTINCT column1, column2 FROM table_name;

Examples of Using DISTINCT

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

Performance Considerations

Using DISTINCT can add overhead to your queries because MySQL must compare rows to identify duplicates.

On large datasets, this can slow down query execution, so use DISTINCT only when necessary.

• Avoid using DISTINCT if you can filter duplicates with WHERE or JOIN conditions.
• Indexes on columns used in DISTINCT can improve performance.
• Consider alternative approaches like GROUP BY when appropriate.

DISTINCT vs GROUP BY

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

DISTINCT simply removes duplicates, while GROUP BY groups rows and allows aggregate functions.

• DISTINCT returns unique rows based on selected columns.
• GROUP BY groups rows and can calculate aggregates like COUNT, SUM.
• Use GROUP BY when you need aggregation; use DISTINCT for simple uniqueness.