Question

Im having trouble with a sql join involving a union. I’m trying to pull a COUNT and a field from 2 tables but getting an error.

The query:

$sql_result7 = mysql_query("(SELECT COUNT (*) as alertcount, date as alertdate FROM alerts WHERE to_id='$id' AND date > '$lastcheck') UNION (SELECT COUNT (*) as mailcount, date maildate FROM mobmail WHERE to_id='$id' AND to_del=0 AND seen = '0')", $db); 

$rs7 = mysql_fetch_array($sql_result7);
$alerts = $rs7[alertcount]; 
$mails = $rs7[mailcount]; 
$last_alert = $rs7[alertdate]; 
$last_mail = $rs7[maildate];

Is it something to do with the date as alertdate part?

The error im getting is:

Warning: mysql_fetch_array(): supplied argument is not a valid MySQL result resource

Answer 1

Besides the space between COUNT and (*), there is another issue. You can’t use $rs7[mailcount] nor $rs7[maildate] in your PHP code because your query is equivalent to:

SELECT 
      COUNT(*) as alertcount
    , date as alertdate 
FROM alerts 
WHERE to_id = '$id' 
  AND date > '$lastcheck' 
UNION 
SELECT 
      COUNT(*)                      --- No "as mailcount" here
    , date                          --- No "as maildate" either 
FROM mobmail 
WHERE to_id = '$id' 
  AND to_del = 0 
  AND seen = '0'

and will return two rows and only 2 columns:

alertcount | alertdate
-----------|------------
 24        | 2012-01-04
 73        | 2011-11-11

---

Two ways to solve this problem:

Either keep the query (changing the UNION to UNION ALL to ensure that you always get 2 rows) and chnage the PHP to use the 2 rows.

Or change the query to:

SELECT alertcount, alertdate, mailcount, maildate 
FROM
      ( SELECT 
              COUNT(*) AS alertcount
            , date     AS alertdate 
        FROM alerts 
        WHERE to_id = '$id' 
          AND date > '$lastcheck' 
      ) AS a 
  CROSS JOIN
      ( SELECT 
              COUNT(*) AS mailcount
            , date     AS maildate 
        FROM mobmail 
        WHERE to_id = '$id' 
          AND to_del = 0 
          AND seen = '0'
      ) AS b