Key plays an important role in relational database; it is used for identifying unique rows from table. It also establishes relationship among tables.
Types of keys in DBMS
Primary Key – A primary is a column or set of columns in a table that uniquely identifies tuples (rows) in that table.
Super Key – A super key is a set of one of more columns (attributes) to uniquely identify rows in a table.
Candidate Key – A super key with no redundant attribute is known as candidate key
Alternate Key – Out of all candidate keys, only one gets selected as primary key, remaining keys are known as alternate or secondary keys.
Composite Key – A key that consists of more than one attribute to uniquely identify rows (also known as records & tuples) in a table is called composite key.
Foreign Key – Foreign keys are the columns of a table that points to the primary key of another table. They act as a cross-reference between tables.