Difference Between Primary Key vs. Unique Key

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Main Difference

The difference between primary key and the unique key is that primary key cannot be null whereas unique key can be null.

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Primary Key vs. Unique Key

DBMS stands for a database management system that is a group of data that is related to each other. DBMS is a combination of programs that are used to access, maintain and utilize a database. The database is an orderly collection of data that is associated in a way. Retrieval of data is very important in database management. Retrieval of data depends on the way you store the data in your database. There are many files in the database management system, and all the files are inter related. There are many application specific software, but DBMS is not an application software it is general purpose software. The main purpose of the database management system is storing and accessing data. There are many features of a database management system such as its input data, it edits data, and it can share data, display data and can manipulate data. DBMS is also a file based system that is a set of an application program that has a purpose of carrying out the services for the user. For every database, there is a separate application program. Primary key and foreign key are important concepts in database management system primary key is the chosen candidate keys by database designer whereas a foreign key is a key that refers to the primary key of other relation. This article is about the difference between primary key and unique key that is very simple that the primary key cannot be null whereas unique key can be null.

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In database tuples in relation are uniquely defined using the primary key. The primary key can be a set of attributes in a relation. The primary key is an attribute that can never be changed. This key can be used to identify any record in a database. In database management system one of the candidate keys is primary key by taking some points into consideration. Unique key in DMBS is used to identify an individual tuple that is unique in a relation. Unique key is very similar to primary, but the difference is that there are no null values in primary key, but there can be null values in unique keys. More than one unique key can be present in a relation. The unique thing is that primary key can never be NULL. In primary key two tuples in a table can contain the same value of the primary key attribute. There can be many relations in the database, and there can be only one primary key for every relation in the database. In database primary key are cluster indexed. In the database, all tuples in a table are sorted. Constraints in the database are a temporary table. If any intermediary tables are created, then it is called temporary tables. Only one NULL value for a column can be accepted by the constraints of the unique key. If we have a student table with roll numbers, name, batch, phone number and citizen id of the very student. We put roll number as a primary key because every student has a unique primary key. The unique key can be citizen Id as it is also unique cannot two students can have same citizen number. A foreign key is used as a referenced key for unique key. Unique key generates a non-clustered index that has a distinct structure. The primary key can be deleted one must take care of the foreign key.

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Comparison Chart

BasisPrimary KeyUnique Key
NullThe primary key cannot be nullThe unique key can be null.
MeaningThe primary key is a unique identifierThe unique key uniquely define a row
NumberThere can be only one primary keyThere can be more than one unique key
IndexPrimary key creates clustered indexUnique key does not create a clustered index

What is Primary Key?

In database tuples in relation are uniquely defined using the primary key. The primary key can be a set of attributes in a relation. The primary key is an attribute that can never be changed. This key can be used to identify any record in a database. In database management system one of the candidate keys is primary key by taking some points into consideration. The unique thing is that primary key can never be NULL. In primary key two tuples in a table can contain the same value of the primary key attribute. There can be many relations in the database, and there can be only one primary key for every relation in the database. In database primary key are cluster indexed. In the database, all tuples in a table are sorted. Constraints in the database are a temporary table. If any intermediary tables are created, then it is called temporary tables. The primary key can be deleted one must take care of the foreign key.

Foreign Key

If there is a relation R1 that has a primary key of the other relation R2, then the attribute is called foreign key. RI in the foreign key is called referencing relation and R2 is referenced relation. The basis difference between primary key and foreign key is that primary key cannot have NULL value whereas foreign key can have NULL values. A foreign key can also accept duplicate values. In the database, there can be one primary key, and there can be multiple foreign keys. Inserting a value into a foreign key column of referencing relation is what makes sure that inserting value must be present in the primary key.

What is a Unique Key?

Unique key in DMBS is used to identify an individual tuple that is unique in a relation. The unique key is very similar to primary, but the difference is that there are no null values in primary key, but there can be null values in unique keys. More than one unique key can be present in a relation. Only one NULL value for a column can be accepted by the constraints of the unique key. If we have a student table with roll numbers, name, batch, phone number and citizen id of the very student. We put roll number as a primary key because every student has a unique primary key. The unique key can be citizen Id as it is also unique cannot two students can have same citizen number. A foreign key is used as a referenced key for unique key. Unique key generates a non-clustered index that has a distinct structure.

Key Differences

  1. The primary key cannot be null whereas unique key can be null.
  2. The primary key is a unique identifier whereas unique key uniquely defines a row.
  3. There can be only one primary key whereas there can be more than one unique key.
  4. Primary key creates a clustered index whereas unique key does not create a clustered

Comparison Video