DBMS
Foreign Keys and Referential Actions
What happens to child rows when a referenced parent row is deleted or updated. Cascade, restrict, set null, set default and no action, with the rule f...
#Interview
Interview preparation material.
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DBMS
Entity, Referential and Domain Integrity
The three integrity rules of the relational model: a primary key may not be null, a foreign key must match an existing primary key or be null, and eve...
DBMS
Keys in the Relational Model
Super key, candidate key, primary key, alternate key, composite key, foreign key and surrogate key, defined formally with a worked method for finding...
DBMS
Relation Schema and Relation Instance
A relation schema is the fixed definition of a relation; a relation instance is the set of tuples in it right now. A relational database schema is the...
DBMS
Relations, Tuples, Attributes and Domains
The relational model has exactly four structural terms. A relation is a set of tuples, a tuple is one fact, an attribute is a named column and a domai...
DBMS
Converting an ER Diagram into Relational Tables
Seven rules turn any ER diagram into a relational schema: entities become tables, one to many puts a foreign key on the many side, many to many become...
DBMS
Extended ER: Generalisation, Specialisation and Aggregation
The extended ER model adds inheritance to ER. Specialisation splits an entity into subclasses, generalisation combines subclasses into a superclass, a...
DBMS
Drawing and Reading ER Diagrams
A complete guide to ER notation and a repeatable eight step method for turning a written requirement into a correct, readable ER diagram.
DBMS
Strong and Weak Entities
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...
DBMS
Cardinality and Participation Constraints
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...
DBMS
Relationships, Relationship Sets and Degree
A relationship is an association among entities. Relationship types have a degree, may carry their own attributes, and may connect an entity type to i...
DBMS
Keys in the ER Model
A key attribute distinguishes one entity from another. Understanding super keys, candidate keys, primary keys and partial keys at design time prevents...