MySQL query in a loop vs using a SQL join

Question

I'm having an inner debate at my company about looping queries in this matter:

$sql = "
  SELECT foreign_key
  FROM t1";

foreach(fetchAll($sql) as $row)
{
  $sub_sql = "
    SELECT *
    FROM t2
    WHERE t2.id = " . $row['foreign_key'];

  foreach(fetchAll($sub_sql) as $sub_row)
  {
    // ...
  }
}

Instead of using an sql join like this:

$sql = "
  SELECT t2.*
  FROM t2
  JOIN t1
  ON t1.foreign_key = t2.id";

foreach(fetchAll($sql) as $row)
{
  // ...
}

---

Additional information about this, the database is huge, millions of rows.

I have of course searched an answer to this question, but nobody can answer this in a a good way and with a lot of up votes that makes me certain that one way is better then the other.

Question

Can somebody explain to me why one of thees methods is better then the other one?

Answer 1

The join method is generally considered better, if only because it reduces the overhead of sending queries back and forth to the database.

If you have appropriate indexes on the tables, then the underlying performance of the two methods will be similar. That is, both methods will use appropriate indexes to fetch the results.

From a database perspective, the join method is far superior. It consolidates the data logic in one place, making the code more transparent. It also allows the database to make optimizations that might not be apparent in application code.