What are the advantages and disadvantages of using UNION ALL versus UNION when merging data from two tables in PHP?

When merging data from two tables in PHP, using UNION ALL will include all rows from both tables, even if there are duplicates. This can be advantageous if you want to retain all the data. However, using UNION will remove duplicates, which can be useful if you only want unique rows. The choice between UNION ALL and UNION depends on the specific requirements of your project.

// Using UNION ALL to merge data from two tables
$query = "SELECT * FROM table1
          UNION ALL
          SELECT * FROM table2";
$result = mysqli_query($connection, $query);