2 different small query vs 1 query with subquery

Question

I have table like this

name       | personal_number 
-----------------------------------------
Jon        | 222
Alex       | 555
Jon        | 222
Jimmy               


        

Answer 1

First query has heavy subquery. You must avoid this. The best solution for your problem is only one query:

SELECT name FROM mytable GROUP BY personal_number HAVING COUNT(*) > 1;

This query will return you each repeated name only once. If you want to display the name of the duplicate as many times as they met you must use next query:

SELECT name, COUNT(*) AS count FROM mytable GROUP BY personal_number HAVING COUNT(*) > 1;

And then in PHP do something like this:

foreach ($rows as $row) {
  for ($i = 0; $i++; $i < $row['count']) {
    echo $row['name'] . "\n";
  }
}