Can a primary key have attributes?
The primary key is an attribute or a set of attributes that uniquely identify a specific instance of an entity. It must have a non-null value for each instance of the entity. The value must be unique for each instance of an entity. The values must not change or become null during the life of each entity instance.
Why we say primary key to any attribute?
Primary key is the Candidate key selected by the database administrator to uniquely identify tuples in a table. Out of all the Candidate keys that can be possible for a table, there can be only one key that will be used to retrieve unique tuples from the table. This Candidate key is called the Primary Key.
How many attributes can a primary key have?
Out of these three attributes, one attribute or a set of more than one attributes can be a primary key. Attribute Stu_Name alone cannot be a primary key as more than one students can have same name….Table Name: STUDENT.
| Stu_Id | Stu_Name | Stu_Age |
|---|---|---|
| 105 | Carl | 29 |
Can a primary key be changed on a computer?
The primary key-value must exist when the record is created. The primary key must remain stable—you can’t change the primary-key field (s). The primary key must be compact and contain the fewest possible attributes. The primary-key value can’t be changed.
What does stable mean for a primary key?
Stable means that a primary key should never change after it has been assigned to an entity. Single purpose means that a primary key attribute should have no purpose other than entity identification. Typically, good choices for primary keys are integer values automatically generated by a DBMS.
What are the requirements for a primary key?
The primary key must uniquely identify each record. A record’s primary-key value can’t be null. The primary key-value must exist when the record is created. The primary key must remain stable—you can’t change the primary-key field (s). The primary key must be compact and contain the fewest possible attributes.
Why is the primary key value always unique?
Most importantly, the value will always be unique. Since the system generates the value, you’ll avoid data entry (and other human) errors. In addition, the value will always exist at the time the record is entered, so the primary-key value will never be null. A surrogate key is immune to changes in business.