Relasjonsdatamodell i DBMS | Database Concepts & Eksempel

The table name and column names are helpful to interpret the meaning of values in each row. The data are represented as a set of relations. In the relational model, data are stored as tables. However, the physical storage of the data is independent of the way the data are logically organized.

Some popular Relational Database management systems are:

• DB2 og Informix Dynamic Server – IBM
• Oracle og RDB – Oracle
• SQL Server og tilgang – Microsoft

Relasjonsmodell Concepts i DBMS

1. Egenskap: Each column in a Table. Attributes are the properties which define a relation. e.g., Student_Rollno, NAME,etc.
2. tabeller – I relasjonsmodellen lagres relasjoner i tabellformatet. Den lagres sammen med dens enheter. En tabell har to egenskaper rader og kolonner. Rader representerer poster og kolonner representerer attributter.
3. tuppel – It is nothing but a single row of a table, which contains a single record.
4. Relasjonsskjema: A relation schema represents the name of the relation with its attributes.
5. Grad: The total number of attributes which in the relation is called the degree of the relation.
6. Cardinality: Totalt antall rader i tabellen.
7. Column: The column represents the set of values for a specific attribute.
8. Relation instance – Relation instance is a finite set of tuples in the RDBMS system. Relation instances never have duplicate tuples.
9. Relasjonsnøkkel – Every row has one, two or multiple attributes, which is called relation key.
10. Attribute domain – Every attribute has some pre-defined value and scope which is known as attribute domain

Relasjonelt Integrity begrensninger

Relasjonelt Integrity begrensninger i DBMS henvises til betingelser som må være tilstede for en gyldig relasjon. Disse relasjonelle begrensningene i DBMS er avledet fra reglene i miniverdenen som databasen representerer.

There are many types of Integrity Begrensninger i DBMS. Begrensninger på det relasjonelle databasestyringssystemet er for det meste delt inn i tre hovedkategorier:

1. Domain Constraints
2. Nøkkelbegrensninger
3. Referensielt Integrity begrensninger

Domain Constraints

Domenebegrensninger kan brytes hvis en attributtverdi ikke vises i det tilsvarende domenet, eller den ikke er av riktig datatype.

Domain constraints specify that within each tuple, and the value of each attribute must be unique. This is specified as data types which include standard data types integers, real numbers, characters, Booleans, variable length strings, etc.

Eksempel:

Create DOMAIN CustomerName
CHECK (value not NULL)

The example shown demonstrates creating a domain constraint such that CustomerName is not NULL

Nøkkelbegrensninger

Et attributt som unikt kan identifisere en tuppel i en relasjon kalles nøkkelen til tabellen. Verdien av attributtet for forskjellige tupler i relasjonen må være unik.

Eksempel:

I den gitte tabellen er CustomerID et nøkkelattributt i CustomerTable. Det er mest sannsynlig at det bare er én nøkkel for én kunde, CustomerID =1 er bare for CustomerName = Google".

Kunde ID	Kundenavn	status
1	Google	Aktiv
2	Amazon	Aktiv
3	Apple	inaktiv

Referensielt Integrity begrensninger

Referensielt Integrity constraints in DBMS are based on the concept of Foreign Keys. A foreign key is an important attribute of a relation which should be referred to in other relationships. Referential integrity constraint state happens where relation refers to a key attribute of a different or same relation. However, that key element must exist in the table.

Eksempel:

I eksemplet ovenfor har vi 2 relasjoner, Kunde og Billing.

Tuple for CustomerID =1 is referenced twice in the relation Billing. Så vi vet Kundenavn=Google har faktureringsbeløpet $300

Operasjoner i relasjonsmodellen

Fire grunnleggende oppdateringsoperasjoner utført på relasjonsdatabasemodellen er

Insert, update, delete and select.

• Insert is used to insert data into the relation
• Delete is used to delete tuples from the table.
• Modify allows you to change the values of some attributes in existing tuples.
• Select allows you to choose a specific range of data.

Når en av disse operasjonene brukes, må integritetsbegrensninger spesifisert på relasjonsdatabaseskjemaet aldri brytes.

innfelt Operasjon

The insert operation gives values of the attribute for a new tuple which should be inserted into a relation.

Oppdater Operasjon

You can see that in the below-given relation table CustomerName= ‘Apple’ is updated from Inactive to Active.

Delete Operasjon

For å spesifisere sletting velger en betingelse på attributtene til relasjonen tuppelen som skal slettes.

In the above-given example, CustomerName= “Apple” is deleted from the table.

The Delete operation could violate referential integrity if the tuple which is deleted is referenced by foreign keys from other tuples in the same database.

Velg Operasjon

I eksemplet ovenfor, CustomerName=”Amazon” is selected

Beste praksis for å lage en relasjonsmodell

• Data må representeres som en samling av relasjoner
• Each relation should be depicted clearly in the table
• Rows should contain data about instances of an entity
• Columns must contain data about attributes of the entity
• Cells of the table should hold a single value
• Each column should be given a unique name
• Ingen to rader kan være identiske
• The values of an attribute should be from the same domain

Fordeler med relasjonell databasemodell

• Enkelhet: A Relational data model in DBMS is simpler than the hierarchical and network model.
• Strukturell uavhengighet: The relational database is only concerned with data and not with a structure. This can improve the performance of the model.
• Lett å bruke: Relasjonsmodellen i DBMS er enkel da tabeller som består av rader og kolonner er ganske naturlige og enkle å forstå
• Query capability: It makes possible for a high-level query language like SQL for å unngå kompleks databasenavigasjon.
• Data independence: The Structure of Relational database can be changed without having to change any application.
• Skalerbar: Regarding a number of records, or rows, and the number of fields, a database should be enlarged to enhance its usability.

Ulemper med relasjonsmodell

• Few relational databases have limits on field lengths which can’t be exceeded.
• Relational databases can sometimes become complex as the amount of data grows, and the relations between pieces of data become more complicated.
• Komplekse relasjonsdatabasesystemer kan føre til isolerte databaser der informasjonen ikke kan deles fra ett system til et annet.

Sammendrag

• The Relational database modelling represents the database as a collection of relations (tables)
• Attribute, Tables, Tuple, Relation Schema, Degree, Cardinality, Column, Relation instance, are some important components of Relational Model
• Relasjonelt Integrity constraints are referred to conditions which must be present for a valid Relation approach in DBMS
• Domenebegrensninger kan brytes hvis en attributtverdi ikke vises i det tilsvarende domenet eller den ikke er av riktig datatype
• Insert, Select, Modify and Delete are the operations performed in Relational Model constraints
• The relational database is only concerned with data and not with a structure which can improve the performance of the model
• fordeler med Relational model in DBMS are simplicity, structural independence, ease of use, query capability, data independence, scalability, etc.
• Few relational databases have limits on field lengths which can’t be exceeded.