An attribute or group of attributes which uniquely identifies a relation or a table.
Types of Keys
Primary Key
- An attribute or set of attributes that uniquely identifies a record or a set of records in a relation.
- A relation can have only one primary key.
- Primary key cannot be null.
- Each value in the primary key has to be unique.
- Minimal Super Key
Super Key
-
Uniquely identifies each record in a table.
-
Max super keys in a relation -
- Where n is the number of attributes
Candidate Key
- Super key whose proper subset is not a super key.
- It is a minimal super key.
- Attributes that can be used as primary key.
Secondary Key
- Some records are required to be accessed by a field other than the primary key, in this case we use alternate key.
Composite Key
- Consists of two or more fields
- Can be used as primary key
Sort / Control Key
- Fields that dictate the sequence of the record.
| Sr # | Name |
|---|---|
| 1 | Ahmad |
| 2 | Ali |
| 3 | Asghar |
Foreign Key
- The primary key of one table is foreign key in another table.
- The relation with the foreign key is called the child table.
- The relation whose key is referred as foreign key in the child table is the parent table
- That key has to be the primary key in the parent table and refers to that table in the child table.