My Adventures in Coding

January 5, 2016

SQL Server – How to write an Upsert using MERGE

Filed under: SQL,SQL Server — Brian @ 8:35 pm
Tags: , , , ,

Normally, when you want an application to INSERT a row to a table if it does not exist or UPDATE it if it does exist, your application must first do a SELECT to check if the row exists, which is the standard SELECT-UPDATE-INSERT pattern. The down side to this pattern is it requires two database round trips instead of one.

Now, over the years I have worked with document stores such as MongoDB and really enjoyed the freedom to be able to make a single call to the database and be able to ADD/REPLACE a document, without having to check if it exists.

Fortunately in SQL Server 2008, the MERGE function was introduced.

MERGE allows you to make only a single database round trip when you want to INSERT a row if it does not exist, or UPDATE it if it does. The following is a simple example showing how to use the MERGE statement.

Quick Start

So if you just want a quick example to get you started then here you go. In this example the table “ClientData” is the one being updated.

  • MERGE – specifies the table we will be inserting a row into or updating
  • USING – defines the condition we will be using to check if the row exists or not
  • WHEN MATCHED THEN – SQL statement to run when the row exists
  • WHEN NOT MATCHED – SQL statement to run when the row does not exist
MERGE dbo.ClientData AS [Target] 
USING (SELECT 12345 AS clientId, 'Some' AS data) AS [Source] ON [Target].clientId = [Source].clientId 
WHEN MATCHED THEN UPDATE SET [Target].data = [Source].data, [Target].updatedDateUtc = GetUtcDate() 
WHEN NOT MATCHED THEN INSERT (clientId, data) VALUES ([Source].clientId, [Source].data);

How it Works

First lets create a table to use for our test of the Merge statement:

CREATE TABLE dbo.ClientData(
	ClientId [bigint] NOT NULL,
	Data [varchar](20) NOT NULL,
	UpdatedDateUtc [datetime] NOT NULL DEFAULT (getutcdate()),
 CONSTRAINT [PK_ClientData_ClientId] PRIMARY KEY CLUSTERED (
	ClientId ASC
)) ON [PRIMARY]
GO

You can verify the table has been created and see that it is empty:

SELECT * FROM dbo.ClientData

Now, run the following Merge statement for the first time, where no matching row in the table:

MERGE dbo.ClientData AS [Target]
USING (SELECT 12345 AS clientId) AS [Source] 
ON [Target].clientId = [Source].clientId
WHEN MATCHED THEN  UPDATE SET [Target].data = 'Update', [Target].updatedDateUtc = GetUtcDate()
WHEN NOT MATCHED THEN  INSERT (clientId, data) VALUES ([Source].clientId, 'Insert');

As you can see, the INSERT statement was executed:

SELECT * FROM dbo.ClientData

Now let’s run the exact same merge statement a second time and see what happens:

MERGE dbo.ClientData AS [Target]
USING (SELECT 12345 AS clientId) AS [Source] 
ON [Target].clientId = [Source].clientId
WHEN MATCHED THEN  UPDATE SET [Target].data = 'Update', [Target].updatedDateUtc = GetUtcDate()
WHEN NOT MATCHED THEN  INSERT (clientId, data) VALUES ([Source].clientId, 'Insert');

Now, you can see the the UPDATE statement was executed since the “data” field has been updated to the text “Update”:

select * from dbo.ClientData

If you are curious about the performance difference between MERGE and SELECT-INSERT-UPDATE here is a performance comparison.

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