Building a database

Implementation of desirable features• Integrity

– A field’s validation can be declared when the field is declared. If this validation is used, then the integrity of the field remains intact.

• E.g. date, haulage container, payroll/student number.

– Entity integrity - No attribute participating in the primary key of a base relation is allowed to accept null values.

• You cannot make a phone call without a phone number.

– Domain constraints - what are the possible valid values that can be used?

Referential integrity

– Through the propagation and use of foreign keys, no detail can be created where a master is needed, nor can a master be deleted without consent to the deletion of the details

– E.g. I cannot order stock unless I know who the supplier is and how to contact the company. If I add a stock item to my database without a supplier, I cannot order stock.

Implementation of desirable features

• Data independence – The implementation of relational databases

causes the external and conceptual schema to be data independent. The internal schema and the physical level are data dependent.

• Controlled redundancy– The relational model reduces redundancy at the

conceptual level, because it is in third normal form.

Building tables

• Use primary keys• Put the tables with ONLY primary keys in first.

– This is the first layer.• Put the tables that reference those tables in next.

– This is the second layer.– This layer uses the keys of the first layer as FOREIGN

keys.– The second layer cannot be placed until the first layer is

complete.• See the Joe’s Yard example following:

Required new data structure

Stock

Stock CodeStock DescriptionUnit Price

* Supplier IdUnitCostPriceStock levelReorder level

COrder

COrderNoOrder Date

* Customer IdStaffPaid

* StaffIssued* Staff no

COrderLine

* COrderNoQuantityRequired

Customer

Customer IdCustomer NameCustomer Address

Supplier

Supplier IdSupplier NameSupplier AddressAmount Owed

Staff

Staff noStaff nameStaff role

SupplierOrder

SupplierOrderNo* Supplier IdSupplierOrderDateDeliveredDate

SupplierOrderLine

* SupplierOrderNo* Stock CodeStockRequired

Stock

Stock CodeStock DescriptionUnit Price

* Supplier IdUnitCostPriceStock levelReorder level

COrder

COrderNoOrder Date

* Customer IdStaffPaid

* StaffIssued* Staff no

COrderLine

* COrderNoQuantityRequired

Customer

Customer IdCustomer NameCustomer Address

Supplier

Supplier IdSupplier NameSupplier AddressAmount Owed

Staff

Staff noStaff nameStaff role

SupplierOrder

SupplierOrderNo* Supplier IdSupplierOrderDateDeliveredDate

SupplierOrderLine

* SupplierOrderNo* Stock CodeStockRequired

Hierarchy of data structure

Layers of tables

• The tables Customer, Staff and Supplier have only primary keys. They are the foundation layer. Layer 1.

• The tables COrder, Stock and SOrder have foreign keys that only reference the foundation layer. They are Layer 2.

• COrderline and Sorderline depend on the tables in Layer 2. They are layer 3.

Analogous to building bricks

Layer 1

Custom

erC

ustomer

Staff

Staff

SupplierSupplier

The customer table is added, with key CustomerId, the Staff with key StaffNo and the Supplier with key SupplierId.

Layer 2

COrder

Stock

SOrder

CustomerStaffSupplier

The stock and the sOrder depend on the Supplier, both having foreign key SupplierId.

The COrder depends on BOTH Staff and Customer, having foreign keys StaffPaid, StaffIssued and CustomerId.

Layer 3

Supplier Staff Customer

COrderStock

SOrder

SOrder SOrder lineline

COrder COrder lineline

•Both the sorder line and the corder line depend on the stock, having stockcode as a foreign key and part of their key.

•cOrder line depends on Corder.

•Sorder line depends on SOrder

The built database

Recap• Look back at the blocks.

– The table creates are the structure or the framework - i.e. the architect’s drawing

– The inserts are like the bricks. You cannot insert into a table unless there is supporting data in the table on which it depends.

• Do– Creates starting with the one(s) with no dependents– Inserts starting with the one(s) with no dependents– Deletes starting with the one(s) on which no other

depends– Drops starting with the one(s) on which no other depends