[RESOLVED] Correct way to handle query errors

phpSimon

Hi all

What's the correct way to handle a SQL query failure, instead of die?

I read in another forum that this could be handled by a function call, but I don't know more than that.

What I'd like to do is reload the current page with a URL variable in the event of an error. The page can check for that variable and introduce some friendly error message, or handle the page differently, etc.

If I can call a function in the event of a query error I can log what data I want to, then reload the page that called the function and embed the URL variable.

Would that be considered a 'correct' way to proceed? And if so... could anyone advice the correct way to code the function call following a query error?

Thanks

bradgrafelman

$result = mysql_query($sql);

if(!$result) {
    // handle query failure here
}

phpSimon

Thank you.

If there's more than one query with the same $result variable and the first query passes but the second fails, would $result become 'unset' due to the failure, or would it be unchanged and therefore contain the results of the first query?

Ashley_Sheridan

If you're re-assigning $result to a second query, then $result will just contain any return (error or true) from that last query. If you need specific details about both, then assign them both to different variables and check them after.

phpSimon

Hi Ashley

If I use

if (!$result){
}

to check, would an error return still be picked up?

I thought checking for !$result was equivalent to checking for !isset($var) but if that was the case, wouldn't an error return mean !$result would equal false, because it IS set (and contains an error).

Thanks for your time.

bradgrafelman

phpSimon wrote:
I thought checking for !$result was equivalent to checking for !isset($var)

No, it definitely is not. It is simply evaluating $result in boolean context, and then negating the result. So if $result is a resource, for example, it will be evaluated as TRUE in boolean context and then negated to be FALSE (causing the if() block not to be executed, in other words).

phpSimon wrote:
If there's more than one query with the same $result variable and the first query passes but the second fails, would $result become 'unset' due to the failure, or would it be unchanged and therefore contain the results of the first query?

It will be whatever you last assigned to it, since all other values will simply be overwritten and cease to exist.

As such, perhaps you should be checking the results of both queries, e.g. right after you execute them (and before executing any other queries)?

Ashley_Sheridan

It depends on what goes before it. If you are doing something like this:

$result = mysql_query("query");

and your query has an error in it, then $result is still set, but it set to false. Use isset() if you aren't sure yet in your code whether or not a variable actually has a value. For example:

if(some condition)
{
    $some_var = 'message';
}

if(isset($some_var))
{
    echo $message;
}

The variable $some_var is only set if the condition is true, so you use isset() in your code to check if it's safe to use. If you changed your code to this:

$some_var = '';
if(some condition)
{
    $some_var = 'message';
}
echo $some_var;

then you don't need to use isset() because $some_var always has a value (you set it to an empty string at the start). Even if a variable is false, it's still set, if that makes sense?

phpSimon

Thank you both again, that's really helpful.