Describe The Characteristics A Table Must Have To Be 254595

Describe The Characteristics A Table Must Have To Be Considered A R

Describe The Characteristics A Table Must Have To Be Considered A Relation. Define the term domain, and explain the significance of the domain integrity constraint to a relation. Using ER notation to define its entity, give an original example of a table that is not a proper relation. Explain what makes it not a relation. Under what circumstances can a relation have duplicate rows? Define the term non-unique key and give an original example with non-unique keys, using ER notation to define its entity. Give an original example of a relation with a unique composite key, using ER notation to define its entity. What is a surrogate key, and under what circumstances would you use one? Explain the term foreign key and give an original example, using ER notation to define its entity.

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The concept of a relation in relational database theory is fundamental for understanding how data is organized and manipulated within a database management system. A table qualifies as a relation if it adheres to specific characteristics that define its structure and integrity. These include having a set of attributes (columns), each attribute having a specific domain, and the data being organized into rows (tuples) where each row is a unique record. Additionally, in a relation, each tuple must be unique within the table, meaning no duplicate rows are allowed unless specific circumstances permit them. The relation must also conform to the principles of atomicity, where each cell should hold only scalar data values, avoiding composite or multi-valued attributes.

The term "domain" refers to the set of all possible values that an attribute can have. It underpins the concept of data integrity and consistency across the database. Domain integrity constraints ensure that attribute values are valid and adhere to predefined rules, which preserve data quality. For example, a "date" domain might restrict values to valid calendar dates, preventing impossible entries like February 30th. Enforcing domain constraints maintains the correctness of the data and prevents anomalies, supporting reliable data retrieval and analysis.

Using ER notation, consider an entity Employee with attributes EmpID and Name. If we define a table without a proper key, such as one with multiple duplicate rows of EmpID and Name, it is not a proper relation. For instance, if EmpID is not unique and we have multiple rows with the same EmpID and Name, the table violates the entity integrity rule, making it not a proper relation. This lack of unique identification for each participant disqualifies it as a true relation in the relational model.

A relation can have duplicate rows under specific circumstances, such as when the primary key is not defined or when the relation intentionally allows duplicate data for particular analytical purposes. However, in standard relational database design, duplicate tuples are generally disallowed because they violate the principle of entity integrity.

A non-unique key is an attribute or set of attributes that do not uniquely identify tuples within a relation. For example, in an entity Order with attributes OrderID and CustomerID, the attribute CustomerID may serve as a non-unique key if multiple orders are placed by the same customer. Using ER notation, Order can be represented with attributes where CustomerID is a non-unique key because it does not guarantee uniqueness of all tuples.

An example of a relation with a unique composite key is an entity ClassSchedule with attributes CourseID, TimeSlot, and RoomID. The composite key could be (CourseID, TimeSlot)>, ensuring that each class scheduled at a specific time is uniquely identified regardless of the room. This combination prevents duplicate entries and enforces entity integrity.

A surrogate key is an artificially generated identifier, often a sequential number or GUID, used as a primary key when natural keys are insufficient or unsuitable. Surrogate keys are particularly useful when natural keys are composite, too lengthy, or subject to change, which could complicate referential integrity. They provide a simple, stable, and efficient means for uniquely identifying records in a relation. For instance, in a table Customer, a surrogate key CustomerID might be used instead of a natural key like email or username, especially if that data can change or is not guaranteed to be unique.

The foreign key is an attribute or set of attributes in one relation that references the primary key in another relation, establishing a link between the two. It enforces referential integrity, ensuring that a record in one table corresponds to a valid record in the related table. For example, consider entities Order and Customer. In the Order entity, the attribute CustomerID acts as a foreign key referencing CustomerID in the Customer entity. This ensures that each order is associated with a valid customer, maintaining consistency across the database.

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