目录插入数据 select子查询(外语句先执行，内语句后执行)总结嵌套查询又称子查询，有select子查询，where子查询，from子查询，exists子查询。
I\'d like to set non-integer primary keys for a table using some kind of hash function.md5() seems to be kind of long (32-characters).
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