Placement preparation.
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The extended ER model adds inheritance to ER. Specialisation splits an entity into subclasses, generalisation combines subclasses into a superclass, a...
A complete guide to ER notation and a repeatable eight step method for turning a written requirement into a correct, readable ER diagram.
A weak entity has no key of its own and can only be identified through an owner entity. It is drawn with a double rectangle, joined by an identifying...
Cardinality says how many entities on one side may relate to entities on the other; participation says whether every entity must take part. Together t...
A key attribute distinguishes one entity from another. Understanding super keys, candidate keys, primary keys and partial keys at design time prevents...
Attributes are simple or composite, single valued or multivalued, stored or derived, and may be complex, key or null valued. Each type converts into t...
The relational model stores everything in tables and connects them by matching values rather than pointers. That one decision delivers declarative que...
The hierarchical model arranges records in a tree with one parent each; the network model allows many parents through a graph of sets. Both used physi...
Tier architecture describes how many machines or layers a database request passes through. One tier is local, two tier puts the application on the cli...
A schema is the design of a database and rarely changes. An instance is the data in it at one moment and changes constantly. Metadata is the stored de...
Data independence is the ability to change a schema at one level without changing the level above. Physical independence is easy to achieve; logical i...
The ANSI-SPARC architecture separates a database into external, conceptual and internal levels. That separation is what makes data abstraction and dat...