A partial key, also known as a non-unique identifier, is a set of attributes that can be used to uniquely identify an entity, but only in combination with other attributes. This means that a partial key cannot uniquely identify an entity on its own, but when combined with other attributes, it can form a unique identifier.
In an ER diagram, a partial key is represented by underlining the attribute(s) that form the partial key. The underline is a dotted line instead of a solid line to indicate that the attribute(s) do not form a full key, but rather a partial key.
For example, consider a database for a university. In this database, we might have an entity called 'Course' with the attributes 'CourseID,' 'CourseName,' and 'Department.' The combination of CourseID and Department forms a unique identifier for a course, but CourseName alone cannot uniquely identify a course. Therefore, CourseID and Department would be underlined in the ER diagram to indicate that they form a partial key.
Another example would be a database for a library, where we have an entity called 'Book' with attributes like 'ISBN,' 'Title,' and 'Author.' In this case, the combination of ISBN and Title could form a unique identifier for a book, but 'Author' alone cannot uniquely identify a book, so 'ISBN' and 'Title' would be underlined in the ER diagram to represent the partial key.
