Keys uniquely identify rows in a table from "summary" of SQL and Relational Theory by C. Date
In the relational model, a key is a set of one or more attributes that serves as a unique identifier for a row in a table. The key's purpose is to uniquely identify each row in the table, thereby ensuring that no two rows are identical in terms of the key attributes. This uniqueness property is essential for maintaining the integrity and consistency of the data stored in the table. A key can be a single attribute or a combination of multiple attributes, depending on the specific requirements of the table. For example, a table may have a single attribute key such as an EmployeeID, which uniquely identifies each employee in the organization. Alternatively, a table may have a composite key consisting of two attributes, such as a combination of EmployeeID and ProjectID, to uniquely identify the association between employees and projects. In practice, a key is defined as either a primary key or a candidate key. The primary key is the key that is chosen as the main identifier for the table and is used to establish relationships with other tables in the database. A candidate key, on the other hand, is any key that could potentially serve as the primary key but is not currently designated as such. It is important to note that a key must satisfy certain properties to be considered valid, such as being unique for each row in the table and being non-null. Additionally, a key should be minimal, meaning it should contain the smallest number of attributes necessary to uniquely identify a row. This minimality property helps to reduce redundancy and improve the efficiency of queries and data retrieval operations. In summary, keys play a crucial role in the relational model by uniquely identifying rows in a table and ensuring data integrity and consistency. By defining and enforcing keys in a database, users can establish meaningful relationships between tables, avoid duplicate records, and maintain the accuracy and reliability of the stored information.
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