从表topic中随机查询topictype为1和2的记录各10条，
select * from
(
select * from
(
select top 10 topicid,topic,topictype,content,answer,author,addtime,itemid from cean_topic where topictype=1 order by newid()
) as a1
union all
select * from
(
select top 10 topicid,topic,topictype,content,answer,author,addtime,itemid from cean_topic where topictype=2 order by newid()
) as a2
) as a3
使用union all进行查询结果的合并，
SQL数据库用上面语句需要对子查询语句列出详细的列名，而Access则不需要，
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