Difference Between Primary Key and Unique Key

Primary Key in DBMS

Primary Key (non-null key) is a column of the table that identifies every record that should be unique in the table. The primary Key column should not contain any duplicate values(every column value in the primary key column should be unique), therefore there should only be one primary key column in the table. Primary keys are generally used as Foreign keys in the other table.

Reasons to Use Primary Key

• According to the definition of Primary Key, the primary key column should contain unique values, so the primary key is used to identify every record in the table.
• We can use the Primary key when we don't want any duplicate or NULL values in the particular column.
• Data is organized in a sequence of clustered indexes whenever you physically organize the DBMS table.

Features of Primary Key

• Values in the primary key column are NOT NULL.
• We can create only one primary key for a table.
• Primary Key column should not contain duplicate values.
• It implements entity integration of the table.

Unique Key in DBMS

Unique Key constraint is a column or combination of columns in a table that uniquely identifies a record. Unique constraints are also referenced by the foreign key of another table. Unique Key is the same as the primary key, but the Unique key column can store NULL values in it.

Reasons to Use Unique Key

• Unique key can be used when you have to keep null values in the column.
• Unique key is to make sure that the information in the column for each table record is unique.
• Unique Key allows us to enter the NULL values in it.

Features of Unique Key

• We can construct the unique key from one or more table fields.
• We can define more than one unique key in the table.
• A unique key can be easily referenced by a foreign key.
• We can store NULL values in the Unique Key, but only one NULL is allowed null per column.

Primary Key vs Unique Key: Key Differences

Purpose:

• Primary Key: Ensures each record in a table is unique and is used to identify each row uniquely within the table. It also serves as a reference for foreign keys in related tables.
• Unique Key: Ensures that each value in a column or set of columns is unique but may not be used for identifying rows or as a reference for foreign keys.

Number of Occurrences:

• Primary Key: Only one primary key is allowed per table, and it must contain unique values for all rows.
• Unique Key: Multiple unique keys can exist within a table, each enforcing uniqueness on different columns or combinations of columns.

Null Values:

• Primary Key: Does not allow NULL values in the key columns, meaning every row must have a value in the primary key column(s).
• Unique Key: Allows NULL values in the key columns, as long as the non-NULL values remain unique.

Syntax:

• Primary Key:

• Unique Key:

Example:

• Primary Key:

• Unique Key:

Difference Between Primary Key and Unique Key: Comparison Chart

Example of Primary Key

The following example describes that there are database sales, and there are two tables in that database:

1. Customers
2. Item

Customers Tables have five attributes:

1. Customer ID
2. Customer Name
3. Phone Number
4. Email Address
5. Item Id

Item Tables have four attributes:

1. Item ID
2. Item Name
3. Item Price
4. Items Left

In Customer Table, Customer Id can never contain a duplicate or null value because when the new customer data is entered, it will generate a new customer_id for that customer. So, the customer_id column contains unique values. So can be considered as a primary key.

In Items Table, Item_Id never contains a duplicate or null value because every item has its unique item id. You can easily identify each row of a table with item id. So it is considered as a primary key.

Example of Unique Key

The following example describes that there is a table called Students, in that Students table there are 6 attributes:

1. Student_ID
2. Student_Name
3. Phone Number
4. Roll Number
5. Batch Number
6. Email Address

In this Student Table, Student_Id can have unique constraints where entries are in the column. So, we can consider Student_ID as a Unique Key.