description of er version
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HAVING FAMILIAR WITH TERMS
- ONE-TO-ONE: Every organization in each one of the two organization set have reached most linked to one entity each.
- ONE-TO-MANY: A great entity in customer can be associated with many entities to be able. But Any entity as a way can have at most 1 entity in customer.
- MANY-TO-ONE: A great entity in set A is associated with at most 1 entity in B. Although Any entity in W can be connected with a number of choices in A.
- MANY-TO-MANY: An entity in product can be associated with numerous entities to be able and vice versa is also the case.
- RECTANGLES Enterprise sets ( CUSTOMER, MERCHANDISE, ORDER)
- ELLIPSES -Attributes (C-ID, FIRSTNAME, LASTNA Organizations and Qualities Entity: an object that is active in the enterprise and that be recognized from other objects. (not shown in the EMERGENY ROOM diagramis a great instance) May be person, place, event, subject, concept inside the real world Can be physical thing or hysteria Ex: Steve, CSE305 Enterprise Type: pair of similar items or a category of entities, they are really well described A rectangle represents an entity established Ex: college students, courses We regularly just say entity and mean organization type Attribute: describes one aspect of an entity type, generally [and best when] sole valued and indivisible (atomic) Represented simply by oval on E-R picture Ex: identity, maximum registration May be multi-valued ” employ double oblong on E-R diagram Can be composite ” attribute has further composition, also use oblong for amalgamated attribute, with ovals pertaining to components connected to it simply by lines Could possibly be derived ” a virtual attribute, the one which is computable from existing data inside the database, make use of dashed oval. This helps lessen redundancy
Entity Types An business type is known as and is explained by pair of attributes College student: Id, Brand, Address, Interests Domain: likely values of your attribute. Remember that the value pertaining to an feature can be a arranged or set of values, at times called multi-valued attributes This is certainly in contrast to the pure relational model which requires atomic values Elizabeth. g., (111111, John, 123 Main St, (stamps, coins)) Key: part of attributes that distinctively identifies an entity (candidate key) Entity Schema: The meta-information of entity type name, characteristics (and affiliated domain), essential constraints Enterprise Types tend to correspond to subjective, attributes are usually nouns even though descriptions from the parts of choices May have got null principles for some business attribute instances ” simply no mapping to domain for all those instances
Tips Superkey: a great attribute or set of features that distinctively identifies a great entitythere may be many of these Composite key: the requiring more than one attribute Prospect key: a superkey in a way that no appropriate subset of its characteristics is also a superkey (minimal superkey ” has no unnecessary attributes) PrimaryTkey: the candidate key decided to be used pertaining to identifying choices and being able to access records. Unless of course otherwise known key means primary essential Alternate key: a candidate crucial not intended for primary important Secondary important: attribute or perhaps set of features commonly used to get accessing records, but not necessarily unique Foreign essential: term used in relational databases (but not really in the E-R model) intended for an feature that is the main key of another stand and is used to establish a romance with that desk where it appears as an attribute also. So a foreign key benefit occurs inside the table and again in the other desk. This conflicts with the idea that a value is definitely stored only one time, the idea that an undeniable fact is stored once is usually not undermined.