某公司的数据库信息: ① 部门具有部门编号、部门名称、办公地点等属性; ② 部门员工具有员工编号、姓
空白(2)处应选择()
A. NOTNULL
B. UNIQUE
C. KEYUNIQUE
D. PRIMARYKEY
A. 实体完整性、参照完整性、用户定义完整性
B. 参照完整性、实体完整性、用户定义完整性
C. 用户定义完整性、实体完整性、参照完整性
D. 实体完整性、用户定义完整性、参照完整性
图书表(图书编号,图书名称,出版社编号,出版社名称,出版时间,出版数量,版次)
图书销售表(图书编号,销售日期,销售数量,书店编号,读者编号,读者姓名,读者电话)
书店表(书店编号,联系电话,所在城市编号,城市名称)
该系统所涉及的数据存在如下约束:
I. 一个出版社可以出版多本图书,一本图书只能在一个出版社出版,在该系统中记录的图书出版信息包括出版时间、版次及出版数量信息;
II. 一个书店可以出售多本图书给多个读者 , 每位读者可以从多个书店购买多本图书 , 一本图书可以通过多个书店出售给读者,书店把图书出售给读者后会在系统中记录售书日期和售书数量信息;
III. 每个书店只能位于一个城市,一个城市可以有多个书店。
① 请根据以上信息画出合理的图书销售数据库的概念模型(用 ER 图表示)。( 8 分)
② 以图书销售表为例说明原数据库设计的不合理之处。( 4 分)
③ 给出该数据库符合 3NF 要求的全部关系模式,并指出关系模式中的全部主码和外码。( 8 分)
A、公司(公司编号,公司名,地址)主键 公司编号
B、仓库(仓库编号,仓库名,地址,公司编号)主键 仓库编号 外键 公司编号
C、职工(职工编号,姓名,性别,仓库编号,聘期,工资) 主键 职工编号 外键 仓库编号
D、聘用(职工编号,仓库编号,聘期,工资)主键 职工编号 外键 仓库编号
某公司的部门(部门号,部门名,负责人,电话)、商品(商品号,商品名称,单价,库存量)和职工(职工号,姓名,住址)3个实体之间的关系如表2-18、表2-19和表2-20所示。假设每个部门有一位负责人和一部电话,但有若干名员工;每种商品只能由一个部门负责销售。部门关系不属于第三范式的原因是(1)。如果用户要求得到表2-21所示的结果,需要(2),并增加关系模式(3)。
空白(2)处应选择()
A. 修改表1的结构,在表1中增加一个职工号
B. 修改表2的结构,在表2中增加一个职工号
C. 修改表2的结构,在表2中增加一个部门号
D. 修改表3的结构,在表3中增加一个部门号
In 2020, COVID-19 is raging around the world. Should the government give priority to health care? This has been an intensely discussed question for years. Read carefully the opinions from both sides and write your response in NO LESS THAN 200 words, in which you should first summarize briefly the opinions from both sides and give your view on the issue. Marks will be awarded for content relevance, content sufficiency, organization and language quality. Failure to follow the above instructions may result in a loss of marks. Yes It is quite obvious that people will be less economically burdened if governments prioritize health care. This is because more favorable policies will be issued and more funds will be granted to the public, which make it more convenient and cost-effective for the public to see a doctor. Admittedly, physical fitness is also one of the essential basic needs of each individual. Governments have an obligation to increase the welfare benefits of all citizens. The development in health service has a long-term impact on our future, but it needs a huge amount of money and efforts to be improved. And few countries could enhance those services without the government’s support. No The governments’ revenues come from the tax payers’ money, so the legitimate goals of any government should all be to serve the people, the most basic of which is to build adequate and quality infrastructure. No government can afford to ignore the construction of infrastructure. Education, which is closely linked to the future developments of all countries, should also be set a priority when governments allocate large portions of taxpayers’ money. When it comes to environmental problems, individual actions are indispensable in the problem-solving process. But the most pressing and complicated problems go far beyond an individual’s capability, which require the concerted effort of the governments.
A、根据频率高低不同,噪声可以分为高频、中频和低频噪声,高频噪声频率一般大于1000Hz,低频噪声频率一般小于400Hz
B、一般情况下,人耳可听到的声波频率为20~20,000Hz,称为可听声
C、频率是描述声音的主要物理量,是媒介特质函数
D、阻性消声器的高频失效是指消声器对高到某一频率的噪声消声效果显著降低
A、将可听声的频率范围( 20Hz~20kHz)按倍数变化,划分为若干较小的频段,称其为频程或频带
B、频谱包括线状谱、连续谱和复合谱
C、频谱分析指研究声音强度(声压级、声强级、声功率级)随声音的频率分布的规律
D、n倍频程是指频率f2是频率f1的n倍
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