DB Projects Help>>Relational algebra
Relational algebra, and kind of first-order sense (and of algebra of sets), offers with a set of finitary operations (see also regards (database)) that is shut under certain employees. These employees work on one or more operations to produce a regards. Relational algebra is a piece pc research.
Relational algebra got little interest outside of genuine numbers until the distribution of E.F. Codd's relational style of information in 1970. Codd offered such algebra as groundwork for collection problem different languages. (See area Implementations.)
Relational algebra is primarily similar in significant energy to relational calculus (and thus first-order logic); this outcome is known as Codd's theorem. You must be cautious to keep away from a mismatch, that may develop between the two different languages since negation, used to an equation of the calculus, constructs an equation that may be real on an unlimited set of possible tuples, while the distinction agent of relational algebra always dividends a specific outcome. To defeat these problems, Codd restrained the operands of relational algebra to specific operations only and also offered restrained service for negation (NOT) and disjunction (OR). Similar rules are discovered in many other logic-based pc different languages. Codd identified the phrase relational completeness to talk about a vocabulary that is total with regard to first-order predicate calculus apart from the rules he offered. In process the rules have no undesirable result on the usefulness of his relational algebra for collection requirements.
关系代数是：
the professional information of how a relational collection operatesan screen to the information saved in the collection itself the numbers which underpin SQL operations
Terminology
关系 - 一组元组。Tuple-一系列特征，总结了一些实际业务。
属性 - 由称为区域执行的实际因素。
Domain - a set of nuclear principles.
Set - a precise distinction for a selection of physical objects which contains no copies.
关系模型概念
表示DB作为关系的选择
关系 - 原理书桌。行选择与现实世界实体相对应的相关信息原则
Tuple - row
Attribute - line header
关系 - 表
Domain D - set of nuclear principles. Called. Often specified as information variety, enumeration, variety, structure, models of measurement
就关系风格而言，原子价值是不可分割的
Relation schema - R (A1, A2, An) - Regards name R; record of characteristics A1, A2, An
属性AI-某些区域D = DOM（AI）执行的因子的名称
关系程度 - 各种属性
Relation (or relation state) r of relation schema R (A1, A2, An), denoted r(R) - set of n-tuples r = {t1, t2, tm}. Be aware n vs. m
n -tuple t-请求的原理记录t = ，其中vi在dom（ai）或null中。
值与能力AI相比，表示为t [ai]或t [i]
Also relation intension for the schema R
关系条件R（R）的关系扩展
Tuples associated are not (officially) requested (attributes)
可以设置元组
正式定义：
A regards r(R) - precise regards of stage n on areas Dom (A1), dom (A2), dom (An), which is a part of Cartesian product:
R(R) part Dom (A1) x Dom (A2) x ... x Dom (An)
Amount of possible tuples - |doom (A1)| x |dom (A2)| x ... x |dom (An)|. Not all are valid
Current regards condition - appropriate tuples that signify a particular condition in the actual world
Schema is dependable in time
State changes frequently
Attribute principles are nuclear - formally no blend or multi-valued attributes
关系模型约束
Regulations on Cartesian product
固有的基于模型或ACT的要求 - 内置在信息样式中，例如，没有类似的元组
Schema-based or precise demands - immediately stated in schemas of the information model
基于申请的或语义的要求或业务规则 - 不得在信息样式的模式下立即说明 -
数据依赖关系,such as sensible dependencies and multi-valued dependencies - for examining quality of DB design
Schema-based constraints:
5.2.1区域需求-DOM（AI） - 信息种类，子兰，枚举。请参阅第8.1页，第1页。246
5.2.2关键要求 - 没有两个元素是相同的
Superkey - part of characteristics such that no two tuples have the same mixture of values
SuperKey描述了外观约束
Key - superkey with no redundancy:
Two different tuples cannot have similar principles of all characteristics in key
无法从密钥中夺走任何特征，并有1.仍然保持
钥匙的值稍微确定元组
Relation schema may have more than one key
Defining a key makes sure uniqueness
主键 - 选择选择建议之一以识别关系的元组
主要关键通常是个体（或少数）属性
null的约束 - 对于每个属性
允许，或
Not allow
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