A weird mysql_fetch_(assoc,array,object) error [duplicate]

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Possible Duplicate:

php warning mysql_fetch_assoc

I have a weird problem 开发者_JAVA百科about my script. It returns always error for mysql_fetch_array or mysql_fetch_assoc. I have used mysql_fetch many times in my project and I checked for this error many times but I am blind about what is happening. Is there something wrong about my script?

My functions aim is learning the biggest value of specified mysql field.

Here is the function:

function nextIncrement($table,$field) {
    $sql = mysql_query("SELECT '$field' FROM '$table' ORDER BY '$field' DESC LIMIT 0,1");
    while($row = mysql_fetch_assoc($sql)) {
        $next = $row[$field];
    }
    $next = (int)$next;
    return $next;
}

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

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Most likely, your mysql_query() returned false for some reason.

See the manual for a list of possible values that mysql_query() can return.

Do a echo mysql_error(); to see what's wrong.

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Check to see that the query actually succeeds before proceeding to fetch results.

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There may be an error in your SQL statement, or maybe you don't have an open database connection?