http://dev.mysql.com/doc/refman/5.5/en/innodb-consistent-read.html
"Consistent read(即普通select) is the default mode in which InnoDB processes SELECT statements in READ COMMITTED and REPEATABLE READ isolation levels. "
"
A consistent read does not set any locks on the tables it accesses"
因此另一个事务可以随便改：
" and therefore other sessions are free to modify those tables at the same time a consistent read is being performed on the table."
不用读锁了，那怎么保证repeatable read? 这就要归功于mysql的MVCC机制了。在MVCC机制中，我们可以理解每个事务对同一份数据有自己的数据空间，你的改动跟我无关（类似于java中的threadlocal)