Unique Constraint is used to ensure that values in a column are unique across all rows in a table in SQL. It guarantees that no two rows can have the same value in the specified column(s).



SQL constraints are used to set rules or restrictions on the data in a table. They ensure the integrity and consistency of the data by limiting the type of data that can be inserted, updated, or deleted in a table.

A unique constraint is a type of constraint that ensures the uniqueness of values in one or more columns of a table. It allows only unique and non-null values to be entered into the specified column(s). If a unique constraint is defined on a column, it will not allow duplicate values in that particular column.

Key Points:
- The unique constraint can be applied to one or more columns in a table.
- It can be defined at the time of creating a table or can be added later using the ALTER TABLE statement.
- The unique constraint can also be applied to a combination of columns, ensuring the uniqueness of the combination of values across the specified columns.
- If a unique constraint is violated while inserting or updating data, the database will throw an error and reject the operation.
- Unique constraints can be used to enforce business rules, such as ensuring that usernames or email addresses are unique in a user table.
- Unlike the primary key constraint, a unique constraint allows NULL values. However, it still enforces uniqueness among non-NULL values.

Example:

Consider a table called "Employees" with columns "EmployeeID" and "Email". If we want to ensure that no two employees have the same email address, we can apply a unique constraint on the "Email" column.

SQL Syntax:

CREATE TABLE Employees (
EmployeeID INT PRIMARY KEY,
Email VARCHAR(255) UNIQUE,
-- Other columns...
);

In the above example, the UNIQUE constraint is used to ensure that each email address in the "Email" column is unique across all rows in the "Employees" table.