"SELECT id,subjectId,title,content,addTime,author,(SELECT top 1 smallImage FROM pmo_images WHERE pmo_images.topicId = pmo_topic.id) AS smallImage FROM pmo_topic WHERE subjectId = ".$subjectId." ORDER BY addTime DESC LIMIT ".($pageIndex*$pageSize).",".$pageSize
/*
SELECT top 1 smallImage FROM pmo_images WHERE pmo_images.topicId = pmo_topic.id
这个语句的查询结果有可能为空
*/