In SQL Server, when creating a unique nonclustered index, it is important to consider the option of setting IGNORE_DUP_KEY = ON. By default, when a nonclustered index is created without any option, the IGNORE_DUP_KEY option is set to OFF. This means that when duplicate values are inserted into the index, an error is thrown regarding the duplicate value.

However, when the IGNORE_DUP_KEY option is set to ON, duplicate values can be inserted without throwing an error. Instead, a warning is displayed.

Let’s take a look at an example to better understand this concept:

USE tempdb
GO

CREATE TABLE DupIndex (
    ID INT,
    FirstName VARCHAR(100),
    LastName VARCHAR(100),
    City VARCHAR(100)
)
GO

-- Create Unique Index on Table
CREATE UNIQUE NONCLUSTERED INDEX [IX_DupIndex_FirstName] ON [dbo].[DupIndex] ([FirstName] ASC) ON [PRIMARY]
GO

INSERT INTO DupIndex (ID, FirstName, LastName, City) VALUES (1, 'John', 'Doe', 'LA');
INSERT INTO DupIndex (ID, FirstName, LastName, City) VALUES (2, 'Joe', 'Doe', 'LA');

/* The following line will throw an error:
   Msg 2601, Level 14, State 1, Line 6
   Cannot insert duplicate key row in object 'dbo.DupIndex' with unique index 'IX_DupIndex_FirstName'. */
INSERT INTO DupIndex (ID, FirstName, LastName, City) VALUES (3, 'John', 'Doe', 'LA');
GO

-- Table will contain only two rows
SELECT * FROM DupIndex
GO

DROP TABLE DupIndex
GO

-- Create the table again
USE tempdb
GO

CREATE TABLE DupIndex (
    ID INT,
    FirstName VARCHAR(100),
    LastName VARCHAR(100),
    City VARCHAR(100)
)
GO

-- Create Unique Index on Table with IGNORE_DUP_KEY set ON
CREATE UNIQUE NONCLUSTERED INDEX [IX_DupIndex_FirstName] ON [dbo].[DupIndex] ([FirstName] ASC) WITH (IGNORE_DUP_KEY = ON) ON [PRIMARY]
GO

INSERT INTO DupIndex (ID, FirstName, LastName, City) VALUES (1, 'John', 'Doe', 'LA');
INSERT INTO DupIndex (ID, FirstName, LastName, City) VALUES (2, 'Joe', 'Doe', 'LA');

/* The following line will throw a warning only and will not throw an error:
   Duplicate key was ignored. */
INSERT INTO DupIndex (ID, FirstName, LastName, City) VALUES (3, 'John', 'Doe', 'LA');
GO

-- Table will contain only two rows
SELECT * FROM DupIndex
GO

DROP TABLE DupIndex
GO

In the first part of the example, we create a table called DupIndex and then create a unique nonclustered index on the FirstName column. When we try to insert a duplicate value into the index, an error is thrown, preventing the insertion.

In the second part of the example, we recreate the table and index, but this time we set the IGNORE_DUP_KEY option to ON when creating the index. Now, when we try to insert a duplicate value, a warning is displayed instead of an error, and the duplicate key is ignored.

Understanding the importance of setting IGNORE_DUP_KEY = ON can be beneficial in scenarios where you want to allow duplicate values in a nonclustered index without causing errors. However, it is important to consider the implications of allowing duplicates and ensure it aligns with your data integrity requirements.

By utilizing the IGNORE_DUP_KEY option effectively, you can have more control over the behavior of your nonclustered indexes and tailor them to your specific needs.