mysql-thinkphp中union使用问题

代码
$result4=$activity->where("topic='%s' and city='%s'","top",I('get.city'))->field('createTime,title,readCount,id,belong')->table('gq_activity')->union('SELECT createTime,title,readCount,id,belong FROM gq_preferential WHERE topic="top"')->union('SELECT createTime,title,readCount,id,belong FROM gq_multimedia WHERE topic="top"')->union('SELECT createTime,title,readCount,id,belong FROM gq_travel WHERE topic="top"')->select();
主要问题
怎么样在union('SELECT createTime,title,readCount,id,belong FROM gq_preferential WHERE topic="top"')里在where里使用变量用于筛选出数据。因为如果用union('SELECT createTime,title,readCount,id,belong FROM gq_preferential WHERE topic="top" and id="$id")，会报错。