get_maincat官方定义，供大家参考

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function get_maincat($catid, $moduleid, $level = -1) {
        global $db;
        $condition = $catid ? "parentid=$catid" : "moduleid=$moduleid AND parentid=0";
        if($level >= 0) $condition .= " AND level=$level";
        $cat = array();
        $result = $db->query("SELECT catid,catname,child,style,linkurl,item FROM {$db->pre}category WHERE $condition ORDER BY listorder,catid ASC", 'CACHE');
        while($r = $db->fetch_array($result)) {
                $cat[] = $r;
        }
        return $cat;
}