What is the difference between a UNION and a JOIN in PHP SQL queries?

The main difference between a UNION and a JOIN in PHP SQL queries is that a UNION combines the result sets of two or more SELECT statements into a single result set, while a JOIN combines rows from two or more tables based on a related column between them. A UNION does not require a relationship between the tables being combined, while a JOIN requires a common column to match rows from different tables.

// Example of using UNION in a PHP SQL query
$sql = "SELECT column1 FROM table1
        UNION
        SELECT column2 FROM table2";

// Example of using JOIN in a PHP SQL query
$sql = "SELECT table1.column1, table2.column2
        FROM table1
        JOIN table2 ON table1.id = table2.id";