Exam 1 Database

Strong entity

An entity that can be uniquely identified by its own attributes (has its own primary key) and does not depend on another entity.

Weak entity

An entity that cannot be uniquely identified by its own attributes alone and depends on a strong (owner) entity, using a partial key plus the owner's key.

Multivalued attribute

An attribute that can hold more than one value for a single entity (e.g., a person's phone numbers). Drawn as a double oval in an ER diagram.

Composite attribute

An attribute made up of smaller sub-attributes (e.g., Address = street, city, state, zip).

Derived attribute

An attribute whose value is calculated from other attributes rather than stored (e.g., Age derived from Date of Birth). Drawn as a dashed oval.

Super key

Any set of one or more attributes that uniquely identifies each row in a table. It may include extra, unneeded attributes.

Candidate key

A minimal super key: a set of attributes that uniquely identifies each row, with no unnecessary attributes.

Primary key

The candidate key chosen to uniquely identify rows in a table. It cannot be null.

Alternate key

A candidate key that was not chosen as the primary key.

Foreign key

An attribute in one table that refers to the primary key of another table, used to link the tables together.

1:1 relationship

One instance of entity A is related to at most one instance of entity B, and vice versa.

1:M relationship

One instance of entity A can be related to many instances of entity B, but each B relates to only one A.

M:M relationship

Many instances of entity A can be related to many instances of entity B.

Cardinality

The maximum number of times an instance of one entity can be related to instances of another entity (e.g., one vs. many).

Ordinality

The minimum number of times an instance of one entity must be related to another (whether participation is optional (0) or mandatory (1)).

Entities in an ER diagram

Drawn as squares (rectangles).

Attributes in an ER diagram

Drawn as ovals, including multivalued, composite, and derived attributes.

Relationships in an ER diagram

Shown using crow's foot notation to indicate cardinality and ordinality.

Mapping entities to relations

Each entity becomes its own table (relation), with its attributes as columns and its key as the primary key.

Mapping a 1:1 relationship

Place the primary key of one table as a foreign key in the other table.

Mapping a 1:M relationship

Place the primary key of the "one" side as a foreign key in the table on the "many" side.

Mapping an M:M relationship

Create a join (junction) table containing the primary keys of both entities as foreign keys, which together often form its composite primary key.

Domain violation

A value that has the wrong data type or breaks the rules for that attribute's allowed values.

Entity violation

A problem with the primary key, such as a null or duplicate primary key value.

Referential violation

A foreign key problem, such as a foreign key value that does not match any primary key in the referenced table.

Semantic violation

Data that is technically valid but doesn't make sense in context (e.g., a negative age or an end date before a start date).