Page 270 - IT 402 v2.0 class 10
P. 270

So while entering data  an error message is displayed when the user tries to save the record number 5 with
         ity as blank.  he error message is displayed because   ntry re uired  property was set to   es . If the property
        would have been set to  No  and the user would have saved the record  then in such case  the error message
        would not have been displayed.






















           RELATIONSHIP

          relationship refers to link between different tables of a database by matching the data on the basis of the
        common  rimary key field.  elational databases support different types of relationships between tables which
        are designed to enforce the concept of referential integrity.   ase  supports different types of relationships
        among tables.  s an example consider two tables:  able  and  able  where  able  has a foreign key to  able .
        In database  relationship of the tables may be between:

          One-to-one relationship: In this relationship, both the tables must have Primary Key Columns. It occurs
           when there is exactly one record of  able  that corresponds to exactly one record with  able .
          One-to-many relationships: In this relationship, one of the tables must have a Primary Key column that
           will be associated with all the columns of related tables. It occurs when each record in TableA can have
           many linked records in  able  but each record in  able  may have only one corresponding record in
           TableA.

          Many-to-many relationships: In this relationship, none of the associated tables have Primary Key column.
           It means that all the columns of  rimary  ey fields are associated with all the columns of related tables. It
           occurs when each record in  able  may have many linked records in  able  and vice-versa.
         onsider a database of shop in which there are   ustomer  table   Item  table   Order  table    ddress  table and
         OrderedItems  table with fields as:   ustomerI     OrderI     ItemI    etc.

          There is one-to-one relationship between the CustomerID in the Customers table and the CustomerID in
           the Address table. This relationship indicates that each customer may be associated with an address.
          There  is  one-to-many  relationship between  the OrderID  in the Orders  table and the OrderID  in the
           OrderedItems  table.  This relationship indicates that  each order  may contain multiple  items, but each
           OrderedItem record may only correspond to a single order.

           here  is  Many-to-many  relationship  between  the  ItemI   in  the  Items  table  and  the  ItemI   in  the
           OrderedItems table. This relationship indicates that each item of stock may be associated with many orders.

         elationships between tables help to:
          Save time as there is no need to enter the same data in separate tables that are connected



         268       Touchpad Information Technology-X
   265   266   267   268   269   270   271   272   273   274   275