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Difference Between Primary Key and Unique Key

Difference Between Primary Key and Unique Key

The primary key and the unique key both are unique keys in a relational database that guarantee the uniqueness of the values on a column or a set of columns. There is already a pre-defined unique key constraint within a primary key constraint. While a primary key is particularly used to identify each record in the table, a unique key, on the other hand, is used to prevent duplicate entries in a column with the exception of a null entry. However, both the keys can contain more than just one column from a given table and they both play a crucial role in storing and retrieving data. The data is a series of tables with columns and these columns store information of all types which further can be accessed or retrieved using instructions. This is where the keys come to the picture. Primary Key and Unique Key are two unique keys that determine how the data should be stored in the system.

Primary Key

A primary key (also goes by primary keyword), is a unique key in a relational database which identifies each record in a database table. It’s sort of a unique identifier, such as a person social security number, phone number, driver license number, or vehicle license plate number. A database must have only one primary key.

A table in a database contains a column or a set of columns which contain values that uniquely identify each row in the table. This column or a set of columns is called the primary key of the table which must contain unique values, and cannot contain null values. Without a primary key, the relational database won’t work.

A primary key is created by defining a PRIMARY KEY constraint when creating or modifying a table. In the SQL Standard, a primary key may contain one or multiple columns, whereas each column is implicitly defined as NOT NULL. If you define a PRIMARY KEY constraint on more than one column, it may result in duplication of values within one column, that’s why each combination of values must be unique for all the columns.

A primary key has the following functions:

  • Each table must have one and only one primary key, not more than one.
  • A primary key cannot contain NULL values.
  • It may consist of one or more columns.
  • All columns must be defined as NOT NULL.
  • A primary key is clustered unique index by default.

Unique Key

A unique key is a set of one or more than one column/field of a table that uniquely identify a record in a database table. The UNIQUE KEY constraint makes sure all the values in a column are unique within the database. Just like a primary key, a unique key can also comprise of more than one column. However, a unique key can accept only one null value. No two rows have the same values in a database table.

A unique key is quite similar to a primary key and can be defined during the creation of the table. When a column or a set of columns is marked as unique in the relational database system, it checks for integrity of values before assigning the constraint so that to prevent two records from having identical values in a particular column.

UNIQUE is a constraint on a non-PRIMARY KEY column which characterizes the following:

  • A UNIQUE KEY constraint guarantees the uniqueness of the values.
  • Multiple unique keys can be defined on a table.
  • A column may contain a NULL value, but only one NULL value per column is allowed.
  • A unique key may create a non-clustered index by default.

Difference Between Primary Key and Unique Key

1. Function

A primary key is sort of a unique key identifier that uniquely identifies a row within a database table, while a unique key identifies all possible rows that exist in a table and not just the currently existing rows.

2. Behavior

A primary key is used to identify a record in a database table, whereas a unique key is used to prevent duplicate values in a column with the exception of a null entry.

3. Indexing

A primary key creates a clustered unique index by default while a unique key is a unique non-clustered index in a database table by default.

4. Null Values

A primary key cannot accept NULL values in a database table whereas a unique key can accept only one NULL value in the table.

5. Limit

There can only be one and only one primary key on a table, however, there can be multiple unique keys for a table in a database system.

Primary Key vs. Unique Key

Primary Key

Unique Key

A primary key is used to uniquely identify a record/row in a database table. A unique key is used to uniquely identify all possible rows in a table and not only the currently existing rows.
It does not accept NULL values. It can accept only one NULL value in a table.
It is clustered index by default which means data is organized in the clustered index sequence. It is a unique non-clustered index by default.
There can be only one primary key in a table. A table can have multiple unique keys.
Primary key is defined by using PRIMARY KEY constraint. Unique key is represented using a UNIQUE constraint.
Used to identify a row in a table. Used to prevent duplicate values in a column.
Primary key values cannot be changed or deleted. Unique key values can be modified.

Summary

  • Both primary key and unique key are entity integrity constraints that are similar in many aspects. However, they have their fair share of differences when it comes to programming. Both are essential concepts that are primarily used in database management systems.
  • Primary key is a set of one or more columns/fields of a database table that uniquely identify a record in a table. Unique key, on the other hand, prevents two records from having identical values in a column.
  • Conceptually, there can only be one PRIMARY KEY for a given table, while there can be more than one UNIQUE KEY for a table.
  • A primary key must be unique but a unique key does not necessarily have to be the primary key.
  • A primary key cannot accept NULL values in a table, whereas a unique key can allow NULL values with an exception of only one NULL in a table.

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References :


[0]Schwartz, Baron, Peter Zaitsev, and Vadim Tkachenko. High Performance MySQL. Sebastopol: O’Reilly Media, 2008. Print

[1]DuBois, Paul. MySQL. San Francisco: New Riders Publishing, 1999. Print

[2]Silberschatz, Abraham and Hank Korth. Database System Concepts. NYC: McGraw Hill Education, 1986. Print

[3]https://stackoverflow.com/questions/9565996/difference-between-primary-key-and-unique-key

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