Functional dependency is a property that describes the relationship between two attributes in a table. It refers to the fact that the value of one attribute (the dependent attribute) is determined by the value of another attribute (the determinant).
Formally, a functional dependency is denoted by X -> Y, which means that for any two tuples in the table that have the same value for the determinant attribute(s) X, they must have the same value for the dependent attribute(s) Y.
In a well-designed database, every table should have a primary key, which is a unique identifier for each record in the table. The primary key is used to enforce data integrity and ensure that each record in the table can be uniquely identified.
The primary key is also closely related to functional dependencies, because it is often used as the determinant in functional dependencies. For example, if we have a table of employees, the employee ID might be the primary key, and other attributes like the employee's name and department might be dependent on the employee ID. In this case, we could write the functional dependency as Employee_ID -> {Employee_Name, Department}.
In general, it is good practice to use a table's primary key as the determinant in functional dependencies, as this ensures that each record can be uniquely identified and the relationships between attributes are well-defined. By using functional dependencies and primary keys together, we can ensure that the database is well-structured and maintains data integrity over time.
