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 ...
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