php combining multiple sql queries

Question

I have a section of my page that refreshes every 10 seconds to check if the user has any mails or alerts.

I have 2 queries straight after one another.

$sql_result7 = mysql_query("SELECT date FROM alerts WHERE to_id='$id' AND date > '$lastcheck'", $db);
$alerts = mysql_num_rows($sql_result7);
$rs7 = mysql_fetch_array($sql_result7); 
$last_alert = $rs7[date];

$sql_result7 = mysql_query("SELECT date FROM mobmail WHERE to_id='$id' AND read = '0'", $db);
$mails = mysql_num_rows($sql_result7);
$rs7 = mysql_fetch_array($sql_result7);
$last_mail_alert = $rs7[date];

Is it possible to save trips to the database by combining this into one query? Is there any need to?

Answer 1

Why just use a union?

SELECT date
FROM alerts where...
UNION
SELECT date
FROM mobmail where...

If you do want duplicates... utilize UNION ALL

Answer 2

You can do it using a UNION .. basically put the second query after the first with UNION in between.

It doesn't seem like you can JOIN on anything here, except for to_id , but you don't really get any benefit out of that. This would almost be a cross join, which would be unnecessarily inefficient.

Answer 3

Why not just use a union, and the MAX aggregate command...

SELECT max(date) lastdate,'alert' dtype FROM alerts WHERE .. GROUP BY to_id
UNION
SELECT max(date) lastdate,'mail' dtype FROM mobmail WHERE ... GROUP BY to_id

You'd now only need a single SQL query and decreased PHP side processing:

$sql_result7=mysql_query(/* as above with your params*/);
$res=mysql_fetch_array($sql_result7);
$last_alert=$last_mail=null;
foreach ($res as $row) {
   if ($row['dtype']=="alert") {
      $last_alert=$row['lastdate'];
   } elseif ($row['dtype']=="mail") {
      $last_mail=$row['lastdate'];
   }
}

... Or something to that effect (I can't easily test ;)).