2019独角兽企业重金招聘Python工程师标准>>> 
关于这条报错信息，意思是指每个派生出来的表都必须有一个自己的别名。
_mysql_exceptions.OperationalError: (1248, 'Every derived table must have its own alias')
如下两条select语句可以说明这个问题：
select group_concat(message)
from ( select if(root=0,message,root) as message from table where message in (...) ); ERROR 1248 (42000): Every derived table must have its own aliasselect group_concat(message)
from (select if(root=0,message,root) as message from table where message in (...) ) as messages;+---------------------------------+
| group_concat(msg_id) |
+---------------------------------+
| ............................... |
+---------------------------------+
1 row in set (0.00 sec)
