“Keys is a set of one or more columns whose combined values are unique among all occurrences in a given table. A key is the relational means of specifying uniqueness. In patient database a patient number could be used as a key field to uniquely identify each patient’s record in the doctor’s patient file. There must be at least one key field in each table. Sometimes a record may contain more than one key field for e.g. the doctor’s patient file may contain both a patient number an a national insurance number for each patient. Both of these are key fields.
TYPES OF KEYS:-