Keys are one of the basic requirements of a relational database model. It is widely used to identify the tuples(rows) uniquely in the table. We also use keys to set up relations amongst various columns and tables of a relational database.
Different Types of Database Keys
- Candidate Key
- Primary Key
- Super Key
- Alternate Key
- Foreign Key
- Composite Key
Candidate Key
The minimal set of attributes that can uniquely identify a tuple is known as a candidate key. For Example, STUD_NO in STUDENT relation.
- It is a minimal super key.
- It is a super key with no repeated data is called a candidate key.
- The minimal set of attributes that can uniquely identify a record.
- It must contain unique values.
- It can contain NULL values.
- Every table must have at least a single candidate key.
- A table can have multiple candidate keys but only one primary key.
- The value of the Candidate Key is unique and may be null for a tuple.
- There can be more than one candidate key in a relationship.
Example:
STUD_NO is the candidate key for relation STUDENT.
Table STUDENT
| STUD_NO | SNAME | ADDRESS | PHONE |
|---|---|---|---|
| 1 | Shyam | Delhi | 123456789 |
| 2 | Rakesh | Kolkata | 223365796 |
| 3 | Suraj | Delhi | 175468965 |
- The candidate key can be simple (having only one attribute) or composite as well.
Example:
{STUD_NO, COURSE_NO} is a composite
candidate key for relation STUDENT_COURSE.
Table STUDENT_COURSE
| STUD_NO | TEACHER_NO | COURSE_NO |
|---|---|---|
| 1 | 001 | C001 |
| 2 | 056 | C005 |
Note: In SQL Server a unique constraint that has a nullable column, allows the value ‘null‘ in that column only once. That’s why the STUD_PHONE attribute is a candidate here, but can not be a ‘null’ value in the primary key attribute.
Primary Key
There can be more than one candidate key in relation out of which one can be chosen as the primary key. For Example, STUD_NO, as well as STUD_PHONE, are candidate keys for relation STUDENT but STUD_NO can be chosen as the primary key (only one out of many candidate keys).
- It is a unique key.
- It can identify only one tuple (a record) at a time.
- It has no duplicate values, it has unique values.
- It cannot be NULL.
- Primary keys are not necessarily to be a single column; more than one column can also be a primary key for a table.
Example:
STUDENT table -> Student(STUD_NO, SNAME,
ADDRESS, PHONE) , STUD_NO is a primary key
Table STUDENT
| STUD_NO | SNAME | ADDRESS | PHONE |
|---|---|---|---|
| 1 | Shyam | Delhi | 123456789 |
| 2 | Rakesh | Kolkata | 223365796 |
| 3 | Suraj | Delhi | 175468965 |
Super Key
The set of attributes that can uniquely identify a tuple is known as Super Key. For Example, STUD_NO, (STUD_NO, STUD_NAME), etc. A super key is a group of single or multiple keys that identifies rows in a table. It supports NULL values.
- Adding zero or more attributes to the candidate key generates the super key.
- A candidate key is a super key but vice versa is not true.
- Super Key values may also be NULL.
Example:
Consider the table shown above.
STUD_NO+PHONE is a super key.
Relation between Primary Key, Candidate Key, and Super Key
Alternate Key
The candidate key other than the primary key is called an alternate key.
- All the keys which are not primary keys are called alternate keys.
- It is a secondary key.
- It contains two or more fields to identify two or more records.
- These values are repeated.
- Eg:- SNAME, and ADDRESS is Alternate keys
Example:
Consider the table shown above.
STUD_NO, as well as PHONE both,
are candidate keys for relation STUDENT but
PHONE will be an alternate key
(only one out of many candidate keys).
Primary Key, Candidate Key, and Alternate Key
Foreign Key
If an attribute can only take the values which are present as values of some other attribute, it will be a foreign key to the attribute to which it refers. The relation which is being referenced is called referenced relation and the corresponding attribute is called referenced attribute the relation which refers to the referenced relation is called referencing relation and the corresponding attribute is called referencing attribute. The referenced attribute of the referenced relation should be the primary key to it.
-
It is a key it acts as a primary key in one table and it acts as
secondary key in another table. - It combines two or more relations (tables) at a time.
- They act as a cross-reference between the tables.
- For example, DNO is a primary key in the DEPT table and a non-key in EMP
Example:
Refer Table STUDENT shown above.
STUD_NO in STUDENT_COURSE is a
foreign key to STUD_NO in STUDENT relation.
Table STUDENT_COURSE
| STUD_NO | TEACHER_NO | COURSE_NO |
|---|---|---|
| 1 | 005 | C001 |
| 2 | 056 | C005 |
It may be worth noting that, unlike the Primary Key of any given relation, Foreign Key can be NULL as well as may contain duplicate tuples i.e. it need not follow uniqueness constraint. For Example, STUD_NO in the STUDENT_COURSE relation is not unique. It has been repeated for the first and third tuples. However, the STUD_NO in STUDENT relation is a primary key and it needs to be always unique, and it cannot be null.
Relation between Primary Key and Foreign Key
Composite Key
Sometimes, a table might not have a single column/attribute that uniquely identifies all the records of a table. To uniquely identify rows of a table, a combination of two or more columns/attributes can be used. It still can give duplicate values in rare cases. So, we need to find the optimal set of attributes that can uniquely identify rows in a table.
- It acts as a primary key if there is no primary key in a table
- Two or more attributes are used together to make a composite key.
- Different combinations of attributes may give different accuracy in terms of identifying the rows uniquely.
Example:
FULLNAME + DOB can be combined
together to access the details of a student.
Different Types of Keys
Conclusion
In conclusion, the relational model makes use of a number of keys: Candidate keys allow for distinct identification, the Primary key serves as the chosen identifier, Alternate keys offer other choices, and Foreign keys create vital linkages that guarantee data integrity between tables. The creation of strong and effective relational databases requires the thoughtful application of these keys.
FAQs on Types of Keys in Relational Model
Q.1: Why keys are necessary for DBMS?
Answer:
Keys are one of the important aspects of DBMS. Keys help us to find the tuples(rows) uniquely in the table. It is also used in developing various relations amongst columns or tables of the database.
Q.2: What is a Unique Key?
Answer:
Unique Keys are the keys that define the record uniquely in the table. It is different from Primary Keys, as Unique Key can contain one NULL value but Primary Key does not contain any NULL values.
Q.3: What is Artificial Key?
Answer:
Artificial Keys are the keys that are used when no attributes contain all the properties of the Primary Key or if the Primary key is very large and complex.