—个链式队列的队头和队尾指针分别为f和r,则判断队空的条件为（）。

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SQL Error: select id,body,options from ***_enewstiku where sourceid='30251' and id < 2447197 and id<>'2447197' order by id desc limit 5 SQL Error: select id,body,options from ***_enewstiku where sourceid='30251' and id > 2447197 and id<>'2447197' order by id asc limit 5