What are the advantages and disadvantages of using UNION versus separate SELECT statements in PHP MySQL queries?

When deciding between using UNION versus separate SELECT statements in PHP MySQL queries, the advantage of using UNION is that it allows you to combine the results of multiple SELECT queries into a single result set. This can be useful when you want to retrieve data from multiple tables with similar structures. However, the disadvantage of using UNION is that it can be less efficient than separate SELECT statements, especially if the tables being queried are large or if there are complex conditions involved. 

// Using UNION to combine results from two SELECT queries
$query = "SELECT column1 FROM table1 WHERE condition1
          UNION
          SELECT column1 FROM table2 WHERE condition2";

$result = mysqli_query($connection, $query);

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

Keywords

UNION SELECT statements PHP MySQL advantages disadvantages

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