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    考察字符串的处理
    concat连接 以及group_concat的连接

    1. with t as(
    2. select distinct post_id,topic_id from Keywords,Posts 
    3. where locate(concat(' ',word,' '),concat(' ',content,' '))>0)
    5. select post_id, group_concat(topic_id order by topic_id,'') topic from t group by post_id
    6. union
    7. select P.post_id,'Ambiguous!' from Posts P left join t on P.post_id = t.post_id
    8. group by P.post_id having count(t.topic_id)=0