What are the potential pitfalls of using 'UNION' in SQL queries with multiple tables in PHP?

Using 'UNION' in SQL queries with multiple tables can lead to performance issues, as it combines the results of two or more SELECT statements into a single result set. To avoid these pitfalls, consider using 'UNION ALL' instead of 'UNION' if you do not need to remove duplicate rows from the result set. Additionally, ensure that the columns selected in each SELECT statement have the same data types and are in the same order. 

<?php
// Example SQL query with UNION ALL
$sql = "SELECT column1, column2 FROM table1
        UNION ALL
        SELECT column1, column2 FROM table2";
$result = mysqli_query($conn, $sql);

// Process the result set
if (mysqli_num_rows($result) > 0) {
    while ($row = mysqli_fetch_assoc($result)) {
        // Process each row
    }
} else {
    echo "No results found.";
}

// Free result set
mysqli_free_result($result);

// Close connection
mysqli_close($conn);
?>

Keywords

UNION SQL queries multiple tables PHP pitfalls

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