Unique Key in DBMS

A Unique Key in a Database Management System (DBMS) is a constraint that ensures all values in a specific column (or a set of columns) are unique. It guarantees that no two rows in the table have the same value for that column(s), helping to maintain data integrity.

Difference Between Unique Key and Primary Key

  • Primary Key: It uniquely identifies each record in a table and does not allow NULL values.

  • Unique Key: It also ensures uniqueness for the column(s) but can allow NULL values (only one NULL value is allowed in a column).

Why Do We Use a Unique Key?

The Unique Key is used to prevent duplicate data in specific columns. It ensures that the values in that column are always different across all rows in the table.

Example of a Unique Key

Let’s consider a User table:

  • Columns: UserID, Email, Phone

In this case:

  • UserID can be the Primary Key, ensuring each user is unique.

  • Email can be a Unique Key, ensuring no two users have the same email address, but allowing one user to have no email (NULL value).

  • Phone can also be a Unique Key, ensuring each user has a unique phone number.

Characteristics of a Unique Key

  1. Uniqueness: The values in the unique key column(s) must be unique across all rows.

  2. Allows NULL: Unlike primary keys, unique keys can have NULL values. However, only one NULL value is allowed.

  3. Multiple Unique Keys: A table can have multiple unique keys, but only one primary key.

  4. Non-duplicate: It prevents duplicate values, ensuring data consistency.

Best Practices for Unique Keys

  1. Use for attributes that must remain unique: For example, email addresses, usernames, or SKU numbers.

  2. Don’t overuse: Only create unique keys for columns that truly need uniqueness. Too many unique keys can complicate database design.

  3. Enforce business rules: Use unique keys to enforce specific business rules, such as having unique email addresses for users.

Conclusion

A Unique Key in DBMS is a useful way to ensure data uniqueness in a table, helping to maintain data integrity. While similar to the primary key, it allows for some flexibility, such as permitting NULL values but ensuring the rest of the data is unique. It plays an important role in keeping your database clean and consistent.