How can the use of DISTINCT affect the performance of a PHP application when querying data from multiple tables?

Using DISTINCT in a query can impact the performance of a PHP application when querying data from multiple tables because it requires the database to scan the entire result set and remove any duplicate rows. This can be resource-intensive, especially when dealing with large datasets. To improve performance, it's recommended to optimize the query by using appropriate indexes, restructuring the query, or reducing the number of tables involved.

// Example of optimizing a query by using appropriate indexes
$query = "SELECT DISTINCT column_name FROM table_name WHERE condition";
// Add indexes to the columns involved in the WHERE clause to improve query performance
// For example: CREATE INDEX index_name ON table_name (column_name);