10 Notable Differences Between Primary Key and Unique Key

Difference Between Primary Key and Unique Key: These 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 distinct 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 that identifies each record in a database table. It’s sort of a unique identifier such as a personal social security number, phone number, etc. A database must have only one primary key.

A table in a database contains a column or a set of columns that 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 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.

Properties of a Primary Key

1. Each table must have one and only one primary key, not more than one.

2. A primary key cannot contain NULL values.

3. It may consist of one or more columns.

4. All columns must be defined as NOT NULL.

5. 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 identifies a record in a database table. The UNIQUE KEY constraint makes sure all the values in a column are unique within the database.

A Unique key can also comprise 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.

Notable Distinction

1. Both primary key and unique keys are entity integrity constraints that are similar in many aspects. However, they have their fair share of differences when it comes to programming.

2. A primary key is used to uniquely identify a record/row in a database table, while the other key is used to uniquely identify all possible rows in a table and not only the currently existing rows.

3. 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.

4. 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.

5. Conceptually, there can only be one PRIMARY KEY for a given table, while the other key can be more than one for a table.

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