Keys are defined to easily identify any row of data in a table.
- Key plays a vital role in the relational database (is a collection of data items with pre-defined relationships between them)
- It is used for identifying unique rows from the table.
- It also establishes the relationship among tables.
- A Key can be a single attribute or a group of attributes
Types of Keys
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 or more columns (attributes) to uniquely identify rows in a table.
Candidate Key – A super key with no redundant attribute is known as the 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 a 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.
Non-key Attributes
Non-key attributes are the attributes or fields of a table, other than candidate key attributes/fields in a table.
Non-prime Attributes
Non-prime Attributes are attributes other than the Primary Key attribute(s).
student_id, name, phone and age.student_id, (student_id, name), phone
Candidate Key:
student_id and phone
Primary Key:
student_id
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