Earlier, we discussed the concept of creating a unique nonclustered index with the IGNORE_DUP_KEY option in SQL Server. Today, we will explore when and how this option can be useful in practical scenarios.
Let’s consider a situation where we have two tables. The first table contains all the data, while the second table has only a partial set of data. Our goal is to insert all the data from the first table into the second table, but we want to ignore any duplicate values based on a specific column.
To achieve this, we can use the IGNORE_DUP_KEY option when creating a nonclustered index on the second table. This option allows us to insert data from the first table into the second table, even if there are duplicate values in the first table. The duplicate values will be ignored and not inserted into the second table.
Let’s walk through an example to better understand how this feature works. First, we need to create a table with some duplicate records based on the FirstName column:
USE tempdb
GO
CREATE TABLE DupData (
ID INT,
FirstName VARCHAR(100),
LastName VARCHAR(100),
City VARCHAR(100)
)
GO
INSERT INTO DupData (ID, FirstName, LastName, City)
VALUES (1, 'John', 'Doe', 'LA');
INSERT INTO DupData (ID, FirstName, LastName, City)
VALUES (2, 'Joe', 'Doe', 'LA');
INSERT INTO DupData (ID, FirstName, LastName, City)
VALUES (3, 'John', 'Doe', 'LA');
INSERT INTO DupData (ID, FirstName, LastName, City)
VALUES (4, 'Joseph', 'Doe', 'LA');
GO
Now, let’s explore two options for inserting data from the DupData table into a new table called DupIndex:
Option 1: With IGNORE_DUP_KEY set OFF
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 will throw an error
-- Error will rollback transaction
INSERT INTO DupIndex (ID, FirstName, LastName, City)
SELECT ID, FirstName, LastName, City
FROM DupData
ORDER BY ID
GO
-- Table will contain NO rows
SELECT * FROM DupIndex
GO
DROP TABLE DupIndex
GO
Option 2: With IGNORE_DUP_KEY set ON
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 will throw a warning
-- Warning will not insert duplicate row
INSERT INTO DupIndex (ID, FirstName, LastName, City)
SELECT ID, FirstName, LastName, City
FROM DupData
ORDER BY ID
GO
-- Table will contain three rows
SELECT * FROM DupIndex
GO
-- Clean up
DROP TABLE DupIndex
DROP TABLE DupData
GO
As you can see, when the IGNORE_DUP_KEY option is set to OFF, any attempt to insert duplicate values into the DupIndex table will result in an error, and the transaction will be rolled back. On the other hand, when the IGNORE_DUP_KEY option is set to ON, the duplicate values will be ignored, and a warning message will be generated. The process will continue, and the non-duplicate rows will be inserted into the table.
It’s important to note that the use of the IGNORE_DUP_KEY option should be based on your specific business logic requirements. This feature can be particularly useful when dealing with INSERT statements and transactions, where you want to insert data from one table to another while ignoring duplicate values.
Now that you understand the concept and usage of the IGNORE_DUP_KEY option, think about the type of situations in which you might find this feature beneficial. Share your thoughts and let us know how you would utilize this option in your SQL Server projects.