若有关系R（A,B,C,D,E）和S（B,C,F,G），则R与S自然联结运算后的属性列有（51）个，与表达式π1,3,6,7（σ3<6（R∞S））等价的SQL语句如下：SELECT （52） FROM （53） WHERE （54）;

75.( )

74.(  )

73.( )

72.(  )

There is nothing in this world constant but inconstancy．--SWIFT

Project after project designs a set of algorithms and then plunges into construction of customer-deliverable software on a schedule that demands delivery of the first thing built．

In most projects,the first system built is（71）usable,It may be too slow,too big,awkward to use,or all three．There is no（72）but to start again,smarting but smarter,and build a redesigned version in which these problems are solved．The discard and（73）may be done in one lump,or it may be done piece-by-piece．But all large-system experience shows that it will be done．Where a new system concept or new technology is used,one has to build a system to throw away,for even the best planning is not so omniscient（全知的）as to get it right the first time．

The management question,therefore ,is not whether to build a pilot system and throw it away．You will do that．The only question is whether to plan in advance to build a（74）,or to promise to deliver the throwaway to customers．Seen this way,the answer is much clearer．Delivering that throwaway to customers buys time,but it does so only at the（75）of agony（极大痛苦）for the user,distraction for the builders while they do the redesign,and a bad reputation for the product that best redesign will find hard to live down．

Hence plan to throw one away;you will,anyhow．

71.()

（接上一题）客户端软件与POP3服务器建立（70）连接。

(接上一题) 如果要使得纯IPv6结点可以与纯IPv4结点进行通信，则需要使用（68）。

POP3协议采用（69）模式进行通信，当客户机需要服务时，

在IPv4向IPv6过渡期间，如果要使得两个IPv6结点可以通过现有的IPv4网络进行通信，则应该使用（67）；

（接上一题）若定义问题的解空间，以深度优先的方式搜索解空间，则采用（65）算法设计策略。