Difference Between Primary Key and Unique Key (in Terms of DBMS)

Edited by Diffzy | Updated on: September 08, 2022


Difference Between Primary Key and Unique Key (in Terms of DBMS) Difference Between Primary Key and Unique Key (in Terms of DBMS)

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In MySQL, a key is a column or combination of columns used to build a link between one or more two tables. Keys may also be used to identify individual rows inside a database. They are also used in the process of accessing records contained inside the table. Both keys ensure that a column or collection of columns inside a table or relation is entirely distinct from one another. The primary key is used to identify each record in the database, while the unique key is used to prevent duplicate entries in a column, except a NULL value. This is the fundamental distinction between the two types of keys. In this piece, we will contrast the primary differences between primary and unique keys based on various criteria. Primary keys and special keys are both types of primary keys.

It is possible to get records from tables by using either the Primary Key or the Unique Key. A linkage between the tables may also be established using these keys. Both a table's Primary Key and its Unique Key may be used to identify individual entries inside that table. Only one primary key may be assigned to a table simultaneously, but several unique keys can be associated with the same relation or table.

Primary Key vs Unique Key

In a relational database, both the primary and unique keys are considered special keys since they both ensure that the values of a column or collection of columns are entirely distinct from one another. Within the confines of a primary key constraint, a unique key constraint has already been established. On the other hand, a unique key is used to avoid duplicate entries in a column, except for an invalid entry. In contrast, a primary key is specifically used to identify each record in the database. A null entry is the only exception to this rule.

Nevertheless, the keys may each include more than one column from the associated table, and they both play an essential part in saving and retrieving data. The data consists of a series of tables with columns, and the information of various sorts is stored in the columns of these tables. This information may then be accessed or retrieved by following specific instructions. This is the point at which the answers become apparent. The Primary Key and the Unique Key are examples of unique keys used to specify how the data should be kept inside the system.

Because null values are not permitted with primary keys, this kind of key is ideal for use in record identification because it guarantees that each record has an identifier that is both unique and not null. To prevent there from being more than one identifier associated with each record, a table may only have one primary key set for it. In contrast, a unique key permits null values and instead focuses on ensuring the one-of-a-kindness of non-null values. You may establish a unique key on a column that needs its values to be unique but not necessarily non-null. You are allowed to specify numerous special keys for a single table due to the fact that a single table may have several columns that each only holds unique values.

It is possible for a table to have both a primary key and a unique key, as shown by some of the examples; however, it is not logical to declare both a unique key and a primary key on the same column. This is because the two types of keys serve different purposes (s). In circumstances such as these, you should remain with the primary key since it not only guarantees that no null values are kept but also guarantees that the primary key itself is unique, just as the unique key does.

Difference Between Primary Key and Unique Key in Tabular Form

Table: Primary Key vs Unique Key
Parameters of Comparison
 Primary Key
Unique Key
They are used so that each row in a table may be assigned its own distinct identity.
Determines uniquely a row that is not the primary key.
NULL value acceptance
It cannot accept NULL values.
It can take NULL values.
The number of keys that a table may specify
Only one main key
More than one unique key
A clustered index is created.
Produces an index that is not clustered.
Auto Increment
A primary key supports the value of an auto-increment.
unique key does not support auto-increment value.
We cannot modify or remove values stored in primary keys.
We may alter unique vital values.

What is a Primary Key?

A primary key is a column in a database that identifies each tuple (row) in that table in an unambiguous manner. The primary key enforces the integrity constraints of the table. Each table may have exactly one primary key at any time. The primary key will not let any duplicate or NULL values be used in its place because of how seldom the value of the primary key in a table changes; careful consideration must be given to its selection in situations where such shifts are possible. It is possible to refer to one table's primary key using another database's foreign key.

The term "primary key" refers to a key used in relational databases and is used interchangeably with the word "main keyword." A primary key is a unique key that may be used to identify each record in a database table. A social security number, a phone number, the number on a driver's license, or the number on a vehicle's license plate are all examples of this kind of unique identifier. Only one primary key may exist in a database at any one time. Each row in a table in a database has a column or collection of columns that contain values that identify it in a way that is distinct from the other rows in the table. This column or group of columns is called the table's primary key, and it must include values that are not repeated anywhere else in the database. Null values are not allowed in the primary key. A relational database cannot function properly without a primary key.

When a table is created or modified, the PRIMARY KEY constraint must be specified to provide a primary key. In the SQL Standard, a primary key may include one or more columns, and each column inside that key is automatically specified as NOT NULL. Primary keys can also contain foreign keys. Because it is possible to end up with duplicate values inside a column if you select a PRIMARY KEY constraint on more than one column, each combination of values must be utterly unique across all of the columns.

What is a Unique Key?

A record in a database table may be uniquely identified by using a set of one or more than one column or field of the table. This is referred to as a "unique key." The UNIQUE KEY constraint ensures that all values stored in a column are distinct throughout the database. A unique key, like a primary key, could have more than one column in the data it refers to. On the other hand, a unique key can only take on a single null value. When you look at a database table, no two rows will have the same values. A special key is relatively similar to a primary key, and both types of keys can be specified while the table is being created. When a column or collection of columns in a relational database system is tagged as unique, the system checks the integrity of the values in those columns before imposing the constraint. This ensures that no two records have the same value in a given column.

In DBMS, the primary key is used to identify an individual tuple in a relationship; the unique key constraint, on the other hand, is used for the same purpose but in a different context. The term "unique key constraints" refers to a grouping of either one or several attributes and columns of any table that is used to uniquely identify a record (in a given database table). One null value is all that may be associated with a key that is considered to be unique. It is impossible for it to have any duplicate values, much like the primary key. In other words, in the same way that the primary key provides a guarantee for the uniqueness of a single column, a unique key offers a contract for the identity of many columns. In every given table, there may be several special keys, but there will only be one primary key constraint for that table.

Including unique key, constraints help to guarantee that the information stored in any column of the database does not appear more than once in any of the rows that make up the database. The provision of data validation in this manner is the appropriate approach. It is allowed for the relational database to contain a single row with a null value for the unique, essential requirement, but just that one row. In the example that was just shown, the column Aadhar ID may be given the role of the unique key. In this configuration, there will be no duplicate entries; instead, each one will have its one-of-a-kind identification number that has been designated by the government. In the event that a student has moved to India from another country, the value of the Aadhar ID field in his or her case shall be set to NULL. This is allowed since the unique constraint may only include a single instance of the NULL value. The primary key and the unique key have this significant distinction between them.

Any foreign key may be made to reference the unique limitations because of how they were created. They are available for usage in the event that it is necessary to implement restrictions on one or more columns that do not make up the main key. In contrast to the situation with primary keys, unique constraints are known to produce the non-clustered index, which has a structure that is different from data rows. The usage of pointers is necessitated by the fact that non-clustered indexes include individual key-value entries that refer to the data row that contains the key value.

Main Differences Between Primary Key and Unique Key

  • The one-of-a-kind value character is ensured by using the UNIQUE KEY constraint.
  • Multiple unique keys may be specified on a table.
  • A NULL value may be stored in a column. However, there may be no more than one such value in any column.
  • When used alone, a unique key may automatically provide a non-clustered index.
  • A primary key is a unique key identifier used to identify a row inside a database table uniquely. On the other hand, a unique key identifies all of the potential rows that may exist in a table rather than just the rows that are presently present in the table.
  • While there may be one and only one primary key on a table, there can be several unique keys for a table in a database system. However, there can only be one primary key on a table.
  • A Primary key allows auto-increment value, but A unique key does not support the auto-increment weight.
  • A table can have only one primary key, although it may have more than one unique key. The primary key is the only key that can access the data in the database.


In this piece, I will contrast primary and unique key constraints. Direct critical controls are more stringent. When we want to ensure that the columns do not include any duplicate data, we have concluded that it is helpful to have a unique key to the table. Additionally, the primary key is valid when we do not want to maintain a NULL value in the database. When YOU have a foreign key in another table, which is suitable for creating a link across plains, it may also be an ideal solution.

Even though both primary and unique keys have a few sets of commonalities, it is clear that direct and special keys have substantial distinctions. For example, although each table can have only one primary key, we are permitted to have many unique keys for that table. In the same manner, a primary key column will not take the value NULL. However, special key columns will allow for the presence of up to one null value inside each column. And last, the primary key column has a unique clustered index, while a particular key column has a unique non-clustered index. This is the single most critical distinction between the two types of key columns.

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"Difference Between Primary Key and Unique Key (in Terms of DBMS)." Diffzy.com, 2023. Sun. 02 Apr. 2023. <https://www.diffzy.com/article/difference-between-primary-key-and-unique-key-993>.

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