Notable Difference Between Primary Key and Unique Key

Notable Difference Between Primary Key and Unique Key: These are the unique keys in a related database that warrant the uniqueness of the values on a column or a set of columns. There is also a pre-defined unique key restriction within a primary key restriction. In this article, we will show you the notable difference between primary key and unique key.

While a primary key is specially used to identify each record in the table, a distinctive key, on the contrary, it is used to prevent duplicate records in a column with the exception of a null entry. Although, both the keys can contain over one column from a given table and they both play a major role in storing and retrieving data.

The data is a sequence of tables with columns and these columns record information of all types which further can be accessed or retrieved using instructions. This is where the differences come to the picture. Primary Key and Unique Key are two different keys that determine how the data should be recorded in the system.


Primary Key

A primary key is a related database that identifies each record in a database table. It’s also a sort of a unique identifier such as a phone number, personal social security number etc. A database must have just one primary key. A table in a database comprises a column or a set of columns that contain values that uniquely identify each row in the table.

This column or set of columns is called the primary key of the table which must contain values, and cannot contain null values. Without a primary key, the related database won’t work.

A primary key is also created by defining a PRIMARY KEY restriction when creating or modifying a table. However, in the Structured Query Language Standard, a primary key may consist of one or multiple columns, while each column is implicitly defined as NOT NULL.

If you define a PRIMARY KEY restriction on more than one column, it may end in duplication of values within one column, that’s why each combination of values must be unique for all the columns.

The Properties of a Primary Key:

1. Each table must have just the primary key.

2. A primary key must not contain NULL values.

3. It may also consist of one or more columns.

4. All the columns must be defined as NOT NULL.

5. A primary key is grouped as a unique index by default.

Unique Key

A unique key consists of one or more than one field/column of a table that distinctively identifies a record in a database table. The UNIQUE KEY restriction makes sure all the values in a column are unique within the database.

The key can also consist of more than one column. But, a unique key can accept only one null value. Two rows cannot have the same values in a database table.

A unique key is a bit 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 related database system, it checks for integrity of values before assigning the restriction to stop two records from having the same values in a particular column.

Notable Difference Between Primary Key and Unique Key

  • Both keys are entity integrity restrictions that are similar in many aspects. But, they have their fair share of differences when it comes to programming.
  • A primary key is used to uniquely identify a record/row in a database table, while the unique key is used to uniquely identify all possible rows in a table and not only the currently existing rows.
  • A primary key doesn’t accept NULL values in a table while the other key can allow NULL values with an exception of only one NULL in a table.
  • A Primary key is a set of one or more columns/fields of a database table that uniquely identify a record in a table. Whereas, a unique key prevents two records from having identical values in a column.
  • Fantastically, there can only be one PRIMARY KEY for a given table, while the unique key can be more than one for a table.

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