An Essential Guide to SQL Server UNIQUE Constraint

Summary: in this tutorial, you will learn how to use the SQL Server UNIQUE constraint to ensure the uniqueness of data contained in a column or a group of columns.

Introduction to SQL Server `UNIQUE` constraint

SQL Server UNIQUE constraints allow you to ensure that the data stored in a column, or a group of columns, is unique among the rows in a table.

The following statement creates a table whose data in the email column is unique among the rows in the hr.persons table:

CREATE SCHEMA hr;
GO

CREATE TABLE hr.persons(
    person_id INT IDENTITY PRIMARY KEY,
    first_name VARCHAR(255) NOT NULL,
    last_name VARCHAR(255) NOT NULL,
    email VARCHAR(255) UNIQUE
);
Code language: SQL (Structured Query Language) (sql)

In this syntax, you define the UNIQUE constraint as a column constraint. You can also define the UNIQUE constraint as a table constraint, like this:

CREATE TABLE hr.persons(
    person_id INT IDENTITY PRIMARY KEY,
    first_name VARCHAR(255) NOT NULL,
    last_name VARCHAR(255) NOT NULL,
    email VARCHAR(255),
    UNIQUE(email)
);
Code language: SQL (Structured Query Language) (sql)

Behind the scenes, SQL Server automatically creates a UNIQUE index to enforce the uniqueness of data stored in the columns that participate in the UNIQUE constraint. Therefore, if you attempt to insert a duplicate row, SQL Server rejects the change and returns an error message stating that the UNIQUE constraint has been violated.

The following statement inserts a new row into the hr.persons table:

INSERT INTO hr.persons(first_name, last_name, email)
VALUES('John','Doe','[email protected]');
Code language: SQL (Structured Query Language) (sql)

The statement works as expected. However, the following statement fails due to the duplicate email:

INSERT INTO hr.persons(first_name, last_name, email)
VALUES('Jane','Doe','[email protected]');
Code language: SQL (Structured Query Language) (sql)

SQL Server issued the following error message:

Violation of UNIQUE KEY constraint 'UQ__persons__AB6E616417240E4E'. Cannot insert duplicate key in object 'hr.persons'. The duplicate key value is ([email protected]).Code language: Shell Session (shell)

If you don’t specify a separate name for the UNIQUE constraint, SQL Server will automatically generate a name for it. In this example, the constraint name is UQ__persons__AB6E616417240E4E, which is not quite readable.

To assign a particular name to a UNIQUE constraint, you use the CONSTRAINT keyword as follows:

CREATE TABLE hr.persons (
    person_id INT IDENTITY PRIMARY KEY,
    first_name VARCHAR(255) NOT NULL,
    last_name VARCHAR(255) NOT NULL,
    email VARCHAR(255),
    CONSTRAINT unique_email UNIQUE(email)
);
Code language: SQL (Structured Query Language) (sql)

The following are the benefits of assigning a UNIQUE constraint a specific name:

It easier to classify the error message.
You can reference the constraint name when you want to modify it.

`UNIQUE` constraint vs. `PRIMARY KEY` constraint

Although both UNIQUE and PRIMARY KEY constraints enforce the uniqueness of data, you should use the UNIQUE constraint instead of PRIMARY KEY constraint when you want to enforce the uniqueness of a column, or a group of columns, that are not the primary key columns.

Different from PRIMARY KEY constraints, UNIQUE constraints allow NULL. Moreover, UNIQUE constraints treat the NULL as a regular value, therefore, it only allows one NULL per column.

The following statement inserts a row whose value in the email column is NULL:

INSERT INTO hr.persons(first_name, last_name)
VALUES('John','Smith');
Code language: SQL (Structured Query Language) (sql)

Now, if you try to insert one more NULL into the email column, you will get an error:

INSERT INTO hr.persons(first_name, last_name)
VALUES('Lily','Bush');
Code language: SQL (Structured Query Language) (sql)

Here is the output:

Violation of UNIQUE KEY constraint 'UQ__persons__AB6E616417240E4E'. Cannot insert duplicate key in object 'hr.persons'. The duplicate key value is (<NULL>).
Code language: SQL (Structured Query Language) (sql)

`UNIQUE` constraints for a group of columns

To define a UNIQUE constraint for a group of columns, you write it as a table constraint with column names separated by commas as follows:

CREATE TABLE table_name (
    key_column data_type PRIMARY KEY,
    column1 data_type,
    column2 data_type,
    column3 data_type,
    ...,
    UNIQUE (column1,column2)
);
Code language: SQL (Structured Query Language) (sql)

The following example creates a UNIQUE constraint that consists of two columns person_id and skill_id:

CREATE TABLE hr.person_skills (
    id INT IDENTITY PRIMARY KEY,
    person_id int,
    skill_id int,
    updated_at DATETIME,
    UNIQUE (person_id, skill_id)
);
Code language: SQL (Structured Query Language) (sql)

Add `UNIQUE` constraints to existing columns

When you add a UNIQUE constraint to an existing column or a group of columns in a table, SQL Server first examines the existing data in these columns to ensure that all values are unique. If SQL Server finds the duplicate values, then it returns an error and does not add the UNIQUE constraint.

The following shows the syntax of adding a UNIQUE constraint to a table:

ALTER TABLE table_name
ADD CONSTRAINT constraint_name 
UNIQUE(column1, column2,...);
Code language: SQL (Structured Query Language) (sql)

Suppose you have the following hr.persons table:

CREATE TABLE hr.persons (
    person_id INT IDENTITY PRIMARY KEY,
    first_name VARCHAR(255) NOT NULL,
    last_name VARCHAR(255) NOT NULL,
    email VARCHAR(255),
    phone VARCHAR(20),
);  
Code language: SQL (Structured Query Language) (sql)

The following statement adds a UNIQUE constraint to the email column:

ALTER TABLE hr.persons
ADD CONSTRAINT unique_email UNIQUE(email);
Code language: SQL (Structured Query Language) (sql)

Similarly, the following statement adds a UNIQUE constraint to the phone column:

ALTER TABLE hr.persons
ADD CONSTRAINT unique_phone UNIQUE(phone); 
Code language: SQL (Structured Query Language) (sql)

Delete `UNIQUE` constraints

To define a UNIQUE constraint, you use the ALTER TABLE DROP CONSTRAINT statement as follows:

ALTER TABLE table_name
DROP CONSTRAINT constraint_name;
Code language: SQL (Structured Query Language) (sql)

The following statement removes the unique_phone constraint from the hr.person table:

ALTER TABLE hr.persons
DROP CONSTRAINT unique_phone;
Code language: SQL (Structured Query Language) (sql)

Modify `UNIQUE` constraints

SQL Server does not have any direct statement to modify a UNIQUE constraint, therefore, you need to drop the constraint first and recreate it if you want to change the constraint.

In this tutorial, you have learned how to use the SQL Server UNIQUE constraint to make sure that the data contained in a column or a group of columns is unique.

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Introduction to SQL Server UNIQUE constraint

UNIQUE constraint vs. PRIMARY KEY constraint

UNIQUE constraints for a group of columns

Add UNIQUE constraints to existing columns

Delete UNIQUE constraints