How can the SQL query be optimized for better performance when retrieving data from the database?

To optimize an SQL query for better performance, you can consider the following steps: 1. Use indexes on columns frequently used in the WHERE clause to speed up data retrieval. 2. Minimize the number of columns selected in the query to reduce the amount of data transferred. 3. Avoid using SELECT * and instead specify only the necessary columns. 4. Use JOINs efficiently and avoid unnecessary JOINs that can slow down the query.

// Example of optimizing an SQL query by using indexes
$query = "SELECT * FROM users WHERE username = 'john_doe'";
// Add an index on the 'username' column to improve query performance
$index_query = "CREATE INDEX idx_username ON users (username)";