Question

If you had data in table 1 that you need to use to return data in table 2 for each row returned in table 1. What is more efficient to use a set of querys in PHP one inbedded in the while loop of the other or an SQL function within a query?

for example:

$qry = mysql_query(SELECT date FROM table1)

while($res = mysql_fetch_array($qry))
{
  $qry = mysql_query("SELECT name FROM table2 WHERE date=$res['date']")
}

or to do this as a function that returns the Id from table1 within the query.

Answer 1

It depends on if the data is easier to find during the while loop or in the query itself.

So if the DB has to base the sub-query on the result of each row in the main query, and there are 1000 total rows and 100 results in the main query, it has to check all of the rows 100 times, so that’s 100,000 sub-queries it runs.

So think it terms of the number of results of the main query. If your while loop has to query the DB 100 times while the DB can do it much faster and efficiently in one query, that’s better. But if you want a subset of answers that you can say ‘query only based on the last set of results’ the while loop is better.