有关锁的简介，可参考 ：
http://www.cnblogs.com/renolei/p/4673842.html
row-level lock:shared(s)locks and exclusive(x)locks.
intention locks,table locks,used to indicate whick type of lock will be used.
Intention shared (IS),intends to set S locks on individual rows
Intention exclusive(IX),intends to set X locks on those rows.
