6238 Databaser Agenda/ER til Relationel.pdf
Contents
6238 Databaser Agenda ER til Relationel
Fra ER-Diagram til Relationel model i 7 step
Denne slide viser et eksempel på hvordan man kommer fra ER-Diagram til den Relationeller model i 7 step. Eksemplet er en simpliceret udgave og kan i meget specielle tilfælde have fejl og mangler © Mercantec 2014
Den oprindelige udgave
STEP 1: For regular entity type E in ER schema, create a relation R that includes all the simple attributes, and component attributes of composite attributes. Select the primary key.
STEP 2: For weak entity type W in ER schema, with owner entity type E, create a relation R, include all simple attributes (or component attributes of composite attributes) of W as attributes of R. In addition, include the primary key attributes of the relation Q for the owner entity type E. Primary key is the combination of primary key of Q and partial key of R.
STEP 3: For 1:1 relationship X, suppose S and T are the relations for the entity types participating in it. Include primary key of T as foreign key of S. Include other attributes of the relationship X as attributes of S.
STEP 4: For 1:N relationship Y, suppose S relation corresponds to the entity type at the N-side, and T relation corresponds to the entity type at the other side. Include primary key of T as foreign key of S.
STEP 5: For M:N relationship Z, create a new relation R to represent Z. Include simple attributes of Z in R. Include the primary keys of S and T as foreign keys of R, their combination forms the primary key of R.
STEP 6: For multivalued attribute A, create a new relation R that includes an attribute corresponding to A. Include primary key of the relation of the entity type having A as an attribute. Primary key is their combination.
STEP 7: For n-ary relationship type X, and n>2, create a new relation R. Include primary key of each participating entity type's relation as foreign key of R. Include attributes of X as simple attributes of R.
Oplæg
Step 1
Step 2
STEP 2: For weak entity type W in ER schema, with owner entity type E, create a relation R include all simple attributes (or component attributes of composite attributes) of W as attributes of R. In addition, include the primary key attributes of the relation Q for the owner entity type E. Primary key is the combination of primary key of Q and partial key of R. STEP2 : Lav en tabel for alle svage entiteter. Indsæt alle attributter I tabellen. Indsæt også en attribut for alle nøgler I den entitet den er svag overfor Primærnøglen er alle nøgle attributter fra den entitet den er svag overfor plus evt. ekstra nøgler.Vi har ikke nogen svage entiteter I viste eksempel.
Step 3
STEP 3: For 1:1 relationship X, suppose S and T are the relations for the entity types participating in it. Include primary key of T as foreign key of S. Include other attributes of the relationship X as attributes of S. STEP 3: Find alle 1:1 reletioner.�Hvis der er tvungen deltagelse på en af siderne sættes FK nøglen på denne side.�Hvis der er tvungen deltagelse på begge sider vælges en af siderne som nøgle bærer Her indsættes Sælger som foreign key I Bil
Step 4
STEP 4: For 1:N relationship Y, suppose S relation corresponds to the entity type at the N-side, and T relation corresponds to the entity type at the other side. Include primary key of T as foreign key of S.
STEP 4: For alle 1:Mange relationer indsættes en FK nøgle på mange siden. Her indsættes Sælger som FK på Kunde
Step 5
STEP 5: For M:N relationship Z, create a new relation R to represent Z. Include simple attributes of Z in R. Include the primary keys of S and T as foreign keys of R, their combination forms the primary key of R. STEP 5: For alle mange til mange relationer oprettes en ny tabel. Tabellen kopierer nøgle fra begge tabeller som en ny sammensat nøgle. Indsætter tabellen købes af
Step 6
STEP 6: For multivalued attribute A, create a new relation R that includes an attribute corresponding to A. Include primary key of the relation of the entity type having A as an attribute. Primary key is their combination. STEP6: For alle multivalue attributter oprettes en ny tabel.�Lav en attribut i tabellen der peger på primærnøglen i den entitet den hører til. Opretter tabellen TelefonNr og sætter Sælger som (FK)
Step 7
STEP 7: For n-ary relationship type X, and n>2, create a new relation R. Include primary key of each participating entity type's relation as foreign key of R. Include attributes of X as simple attributes of R.
Vi har ikke nogen n-ary relationer