It is very common way of usage that  when ever you need some new attribute for your model objects/table you add a new column to the related database table.  After several requirements you will notice:

• there will be lots of column in the same table
• most of them are not always used
• there is key columns you have for which columns will be used (eg: if type==individual then read name,surname,mobile else company name,tel,vs… )

even after you optimize the database for this type of usage and optimize your queries it will be very expensive way of storage.

solution

group related attributes/columns and store them in separate tables with same ID

benefits

• self documentation: related attributes are organized among themselves
• query performance: since data is separted between multiple tables you index will be more efficient and there will be less empty data cells
• more live object oriented thinking in schema design

ORM practices

hibernate:

http://www.hibernate.org/hib_docs/reference/en/html/inheritance.html#inheritance-mixing-tableperclass-tablepersubclass

<br>
<class name="Payment" table="PAYMENT"><br>
<id name="id" type="long" column="PAYMENT_ID"><br>
<generator class="native"/><br>
</id><br>
<discriminator column="PAYMENT_TYPE" type="string"/><br>
<property name="amount" column="AMOUNT"/><br>
...<br>
<subclass name="CreditCardPayment" discriminator-value="CREDIT"><br>
<join table="CREDIT_PAYMENT"><br>
<property name="creditCardType" column="CCTYPE"/><br>
...<br>
</join><br>
</subclass><br>
<subclass name="CashPayment" discriminator-value="CASH"><br>
...<br>
</subclass><br>
<subclass name="ChequePayment" discriminator-value="CHEQUE"><br>
...<br>
</subclass><br>
</class><br>

JPA:

sample from

@Entity
@Inheritance(strategy=InheritanceType.JOINED)
public class Boat implements Serializable { ... }

@Entity
public class Ferry extends Boat { ... }

@Entity
@PrimaryKeyJoinColumn(name="BOAT_ID")
public class AmericaCupClass  extends Boat { ... }

reference:

• http://www.hibernate.org/hib_docs/annotations/reference/en/html/entity.html
• http://www.hibernate.org/hib_docs/reference/en/html/inheritance.html#inheritance-mixing-tableperclass-tablepersubclass
• http://www.jpox.org/docs/1_2/jpa_orm/inheritance.html

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