mysql-thinkphp中union使用問題

mysqlthinkphpphpunion

代碼
$result4=$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();
主要問題
怎麼樣在union('SELECT createTime,title,readCount,id,belong FROM gq_preferential WHERE topic="top"')裡在where裡使用變數用於篩選出資料。因為如果用union('SELECT createTime,title,readCount,id,belong FROM gq_preferential WHERE topic="top" and id="$id")，會報錯。

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