摘要:
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 in 阅读全文
摘要:
1单值(Scalar operand) 只有当外层(Parent)不为空时,才返回相应值;否则返回NULL。 note:For the subquery just shown, if t1 were empty, the result would be NULL even though s2 is 阅读全文