MySQL将表a中查询的数据插入到表b中
insert into b select * from a;
如果表b不存在
create table b as select * from a;
扩展：
将b表中的某写字段值插入到a表中
insert into a (userID,userName) select b.userID,b.userName from tr_ajax_chat_messages;
将a表和b表userID相等的值保存到a表
update a set a.userName=(select b.userName from b where a.userID = b.userID);