Describe the Composite Key

Composite Key: Sometimes one key is not enough to uniquely recognize a row. E.g. in a single class Roll is enough to search a student, but in the whole school, merely searching by the Roll is not enough, because there could be 10 classes in the school and every one of them may have a certain roll no 5. To uniquely recognize the student we have to say something like "class VII, roll no 5". So, a combination of two or more attributes is combined to make a unique combination of values, such as Class + Roll.