In what scenarios is it advisable to use multiple SQL statements instead of a UNION query?

When dealing with complex queries or when the tables being queried have different structures, it may be advisable to use multiple SQL statements instead of a UNION query. This approach allows for more flexibility in handling the data and can be more efficient in certain situations. 

<?php

// First SQL statement
$sql1 = "SELECT column1, column2 FROM table1 WHERE condition1";

$result1 = mysqli_query($conn, $sql1);

// Second SQL statement
$sql2 = "SELECT column3, column4 FROM table2 WHERE condition2";

$result2 = mysqli_query($conn, $sql2);

// Process results from both queries
if ($result1 && $result2) {
    // Process results from $result1
    while ($row = mysqli_fetch_assoc($result1)) {
        // Process data
    }

    // Process results from $result2
    while ($row = mysqli_fetch_assoc($result2)) {
        // Process data
    }
}

?>

Keywords

PHP SQL UNION query performance optimization

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