mysql 数据库SQL问题，下面的SQL inner join 跟的为啥不是表名？而是个子查询？

update a inner join (SELECT * FROM b) on a.iplong >=b.ip1 and a.iplong < b.ip2
set a.country = b.country, a.city = b.city ;

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http://dev.mysql.com/doc/refman/5.6/en/update.html

这称为: Multiple-table syntax:

UPDATE [LOW_PRIORITY] [IGNORE] table_references
    SET col_name1={expr1|DEFAULT} [, col_name2={expr2|DEFAULT}] ...
    [WHERE where_condition]

table_references的定义在这里:
http://dev.mysql.com/doc/refman/5.6/en/join.html

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PS, 这个sql应该是错的. mysql里应该会报:

ERROR 1248 (42000): Every derived table must have its own alias