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Database - Coggle Diagram

- - - - entity
        
        strong entity can be defined solely by its own attributes weak entity cannot be defined solely by its own attributes, An entity set
        that do not have sufficient attributes to form a primary key (customer - loan name and loan date)
        
        entity keys, key types
        
        Candidate Key
        
        when multiple possible identifiers exist, each is a candidate key
        
        allow null
        
        but candidate keys only have the bare minimum needed.
        
        Primary Key
        
        Identifier used to uniquely identify one particular instance of an entity. - must be unique - cannot be duplicate- Value should not change over time... Can be one or more attributes.
        
        Foreign Key
        
        Foreign keys reference a related table through the primary key of that
        related table
        
        Referential Integrity Constraint
        
        Composite Key
        
        consists of two or more columns in a table. These columns, when combined, uniquely identify each row in the table
        
        Partial Key
        
        The set of attributes that are used to uniquely identify a weak entity set is called the Partial key.
        
        Alternate key
        
        the alternative key is the key in the database table which is not selected as a primary key. because there can be multiple candidate keys all cannot be selected. So, the remaining keys which are not selected as primary keys are called alternative keys. The main reason for having an alternate key is to provide more additional means with the help of. Then we can access data in the table in case the primary key is not applicable. It makes sure that there are more ways than the primary key
        
        Super Key
        
        allow null
        
        . Super keys can have extra information that’s not needed to identify records, but candidate keys only have the bare minimum needed.
        
        All super keys can’t be candidate keys but the reverse is true
        
        1 more item...
        
        Super keys provide the foundation for selecting the candidate keys while candidate keys in turn serve as the basis for choosing the primary key of the relation.
        
        Why do we need both super keys and candidate keys?
        Super keys give us all the ways to identify records. From these, we pick candidate keys, which help us choose the best key (primary key) for the table.
      - Attribute
        
         Composite Attribute
        
        Sub-attributes spring from an attribute
        
        Derived Attribute
        
        Attributed is calculated or otherwise derived from another attribute,
        such as age from a birthdate.
        
        Simple (Scalars) Attribute
        
        value is atomic and can’t be further divided, such
        as a phone number.
        
         Multi-valued Attribute
        
        More than one attribute value is denoted, such as multiple phone
        numbers for a person
        
         Complex Attribute
        
        multi-valued + Composite
      - relationship
        
        Degree of Relationships
        
        number of entity types that participate in a relationship: three casses:
        
         Binary: between the instances of two entity types
        
         Ternary: among the instances of three entity types
        
        Unary: between two instances of one entity type
        
        Recursive Relationships: the same entity participates more than once
        
         Cardinality Constraint
        
        How many instances of one entity will or must be connected to a single
        instance from the other entities
        
        one - one (one employee own one car) , one - many (one department has many employees), many - many (many students take many courses)
        
         Participation Constraint
        
        the minimum and maximum participation of entities or relationships in a given relationship set. While partial participation permits discretionary involvement, total participation requires every entity in one set to take part in a relationship in another set.
        
        partial participation would permit courses with no enrolled students, while entire participation might require all students to be enrolled in at least one course
        
        can have attributes
        
        customerand loan and the relationship set borrower. We could associate the attribute date-issued to that relationship to specify the date when the loan was issued.
- - - - mapping strong entity : create table for each entity, choose the primary key for each entity
        
        Multivalued attribute becomes a separate relation with foreign key : بناخد المالتيفاليو لتيبل تانية ومعاها البرايمري كي كفورين كي والاتنين مع بعض كومبوزيت كي
        
        التيبل التاني بيكون فيه الفورين كي متكرر والسكيل ورا بعض
        
        mapping derived & complex
        
        derived does not be stored in the db
        
        complex : بنماب عادي المالتي وبعدين نفصلهم زي الكومبوزيت في الجدول السيباريت
        
        mapping composite: in separate columns
        
        mapping weak entity: 1- partial key + primary key of a strong entity as a foreign key (primary key here is partial key + foreign key)
        
        البارشال بيكون ايدينتيفير للتيبل بس مش كفاية انه يبقى برايمري لوحده فبناخد معاه البرايمري بتاع السترونج
        
        mapping 1 to 1 binary total form two sides: one table with any one to be primary key
        
        one to one total + partial : تو تيبلز الناحية الاوبشنال بيكون فيها برايمري كي والناحية التوتال برايمري كي وفورين كي من الاوبشنال
        
        employee may have a computer.. a computer must be owned by employee توتال يعني لازم يكون مملوك لحد يعني هو اللي معاه الفورين كي
        
        one to one optional from both sides: تلاتة تيبلز وفيهم التو كيز فورين واي واحد فيهم برايمري
        
        one to many many is total : البرايمري كي بتاع الوان اجطه في الميني
        
        one to many and many is partial: ثري تيبلز التيبل الجديد البرايمري كي هو اللي ناحية الميني
        
        many to many : مش ببص على البارتيسيبيشن وبناخد لبرايمري كيز بتو التو تيبلز والاتنين بيبقوا composite key
        
        كام حالة ظهر فيها composite key?
        
        1-weak entity , 2-multivalued attribute, 3-complex attribute, 4-many to many