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ReDtModel, DDL, DDL, App program - Coggle Diagram

- - - - relation algebra expression: new relation
      - query
      - relational calculus
    - - IC: key, entity, domain, referential
        
        reject; reason
        
        reject; correct(rare)
    - - IC: referential
        
        restrict
        
        cascade: delete all referencing tuples together
        
        set null, set default of the referencing attri values
        
        combo
    - - not PK or FK
      - PK: 等同del +insert
      - FK: referential IC
    - - OLTP systems 处理transactions 高速
  - - - key constraints
        
        all tuples in a relation(a set of tuples: set consists of dintinct elements) must be distinct
        
        superkey(SK):
        
        定义：subsets of attributes of R and no 2 tuples in any r of R should have the same combo of values for these attr
        
        at least one for every relation: all atrributes
        
        can have redundant attr
        
        key(k)
        
        性质1. It is a SK: two distinct tuples in any state of the relation 没有相同的属性组合值
        
        性质2. It is a result of any SK removing a set of attributes that is not a SK of R： a minimal SK, we can no longer remove any attris without violating uniqueness constraints
        
        determined from the meaning of the attribute(s); time-invariant
        
        can >1, called candidate keys
        
        primary key
        
        identify tuples in r; underlined in the table;
        
        unique key
      - domain constraints
        
        atomic attr value within each tuple
        
        examples: integerts, real numbers; Characters, Booleans, fixed-length strings, variable-length strings, date, time, timestamp; subrange of values from above time, an enumerated data type(all possible vals are explicitly listed).
      - constraints on NULLs
        
        定义: specify whether NULL values are or ar not permited
      - entity integrity constr
        
        定义：no primary key value can be NULL
      - referential integrity const
        
        informal定义：a tuple in one relation that refers to another relation must refer to an existing tuple in that relation
        
        formal定义：FK in R1 is said to reference R2 if:1. FK have the same domains as PK in R2, say FK references R2; 2. t1(FK) = t2(PK)/NULL, if not NULL, we say, tuple t1 references tuple t2.
        
        FK概念： FK 需要和“引用”同时出现，需要满足2个条件
        
        图形表示：FK到PK的带箭头折线
    - - meaning and behavior of attributes
        
        work hours limit; salary limit...
    - - no duplicate tuples
    - - functional denpendencies
      - multivalued dependencies
  - - - atomic
      - data type w/o data format
      - domain names: more readable, similar in meaning to attribute
        
        关系中定义域不同于数学中的定义域（数域），比如，usa_phone_numer（所有十位有效数字集）, 是常用定义域（已经得到共识的）。很多属性可以使用这个定义域，比如，家庭电话，办公室电话，移动电话。
      - interpretation of name
      - units
    - - attributes(A1,A2..An): name of roles played by some domain D
      - realtion/relation state(r/r(R): r = (t1,t2,..tm)。也叫relation extension。
        
        n-tuples(t):order list of n values, t=<v1,v2,..vn>。tuple 可以说是从R到D上的mapping，那么tuple = a set of (attribute, value) pairs, value 顺序就无所谓了。这种字典描述法：叫self-describing data
        
        value(vi, i=1~n): an ele of dom(Ai) or NULL. 记为t[Ai], t.Ai, t[i]
        
        atomic, not divisible, composite/multivalued attributes not allowed
        
        This model is called flat rModel(first normal form assumption)
        
        Null meaning
        
        attributes does not apply
        
        value exists but not avaliable
        
        value unknow: know nothing about it.
        
        决定了算术或者比较的时候如何处理
        
        better avoid NULL
        
        interprete: a fact or a instance of the assertion
        
        facts of a entity
        
        facts of a relationship
        
        用集合论概念定义: 在dom(A1),dom(A2)..dom(An)的n度数学关系，r(R)“属于（dom(A1)dom(A2)..*dom(An)）
        
        特征
        
        no ordering of tuples, simply a sett of tuples; but file and table have orders;
        
        has ordering of values, or attributes within a n-tuple; not really if you can maintain correspondence btw attrites and vales
        
        degree/arity(n): #of attributes
      - interpretation: a declaration or a type of assertion
      - interpretion: a predicate，断言，是一个表达式，返回真假/unknown值
      - 性质
        
        a set of attributes 组成key
  - - - 定义：: a set of relation states {r1, r2,...rn} such that ri 是Ri的一个state and ri满足IC.
      - valid state: 满足IC中所有contrains
      - invalid state