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Using SQL Server 2008's
MERGE statement
By Tim Chapman
September 24, 2007, 12:09 PM PDT

Takeaway: SQL Server 2008’s new MERGE construct allows you to insert, update, or delete
data based on certain join conditions in the same statement. Tim Chapman shows you how
MERGE works with a hands-on example.

SQL Server 2008’s new MERGE statement allows you to insert, update, or delete data based on
certain join conditions in the same statement. In previous versions of SQL Server, you have to
create separate statements if you need to insert, update, or delete data in one table based on
certain conditions in another table. With MERGE, you can include the logic for these data
modifications in one statement.

How MERGE works
The MERGE statement basically works as separate insert, update, and delete statements all
within the same statement. You specify a “Source” record set and a “Target” table, and the join
between the two. You then specify the type of data modification that is to occur when the records
between the two data are matched or are not matched. MERGE is very useful, especially when it
comes to loading data warehouse tables, which can be very large and require specific actions to
be taken when rows are or are not present.

MERGE example
I will simulate sales feeds being received in the database and loaded to a reporting table that
records daily sales statistics. In a typical scenario, the records would be loaded into a staging table
(SalesFeed in this example), and then a series of transformations or DDL statements would be
executed on the reporting table (SalesArchive in this example) to update the daily sales data. The
MERGE statement allows you to use one statement to update the SalesArchive table rather than
use several different DDL statements, which potentially could reduce the time it takes to make the
updates occur, since only one lookup is done on the data rather than several.

The following script creates the SalesArchive and SalesFeed tables:

CREATE TABLE SalesArchive
(
CustomerID INT PRIMARY KEY,
SalesDate INT,
TotalSalesAmount MONEY,
TotalSalesCount SMALLINT,
CreationDate DATETIME CONSTRAINT df_CreationDate DEFAULT(GETDATE()),
UpdatedDate DATETIME CONSTRAINT df_UpdatedDate DEFAULT(GETDATE())
)

http://www.techrepublic.com/blog/datacenter/using-sql-server-2008s-merge-statement/194[08/29/2012 3:20:12 PM]


CREATE TABLE SalesFeed
(
CustomerID INT,
Product VARCHAR(10),
SaleAmount MONEY
)

The script below loads some data into the SalesFeed table. The way in which I am inserting data
into this table is new to SQL Server 2008; it allows you to specify many values to be inserted
using the VALUES clause of the INSERT statement.

INSERT INTO SalesFeed
(CustomerID, Product, SaleAmount)
VALUES
(1,'PoolTable', 1000),
(2,'BigScreen', 955),
(3,'Computer', 590),
(4,'BigScreen', 880),
(5,'Computer', 700)

I have a few rows of data in my SalesFeed table and no data in my SalesArchive table. Now it is
time for me to create my MERGE statement to add data to this table. Below is the MERGE script.

MERGE SalesArchive AS SA
USING (
SELECT
CustomerID,
LoadDate = MIN(CONVERT(VARCHAR(8), GETDATE(), 112)),
TotalSalesAmount = SUM(SaleAmount),
TotalSalesCount = COUNT(*)
FROM SalesFeed
GROUP BY CustomerID
) AS SalesFeedCTE (CustomerID, LoadDate, TotalSalesAmount, TotalSalesCount)
ON
(
SA.CustomerID = SalesFeedCTE.CustomerID AND SA.SalesDate = SalesFeedCTE.LoadDate
)
WHEN NOT MATCHED THEN
INSERT (CustomerID, SalesDate, TotalSalesAmount, TotalSalesCount, CreationDate, UpdatedDate)
VALUES( SalesFeedCTE.CustomerID, SalesFeedCTE.LoadDate, SalesFeedCTE.TotalSalesAmount,
SalesFeedCTE.TotalSalesCount, GETDATE(), GETDATE())
WHEN MATCHED THEN
UPDATE
SET SA.TotalSalesAmount = SA.TotalSalesAmount + SalesFeedCTE.TotalSalesAmount,
SA.TotalSalesCount = SA.TotalSalesCount + SalesFeedCTE.TotalSalesCount,
SA.UpdatedDate = GETDATE();

At first glance, it looks reasonably complicated, but it’s not too bad once you get used to it. The
table immediately following the MERGE statement is the table that will be modified; this is known
as the TARGET table. In the USING statement, data from the SalesFeed table is being
aggregated inside of a subquery based on the CustomerID; this portion is known as the SOURCE.
This aggregation allows me to guarantee that there will be only one record per customer to update
my SalesArchive table.

The ON clause of the MERGE statement is where I specify: the joining between the SOURCE, the
aggregated data from the subquery, and the TARGET, the SalesArchive table.

The WHEN NOT MATCHED clause is where I specify what action I want to occur when the
records from the SOURCE are not found in the TARGET. In this scenario, I want to insert those
records into the SalesArchive table.

The WHEN MATCHED clause is where I specify what I need to occur when the records from the
SalesArchive table and the subquery of the SalesFeed table are found. In this scenario, I want to
update what is currently in the table for that day, such as the TotalSalesAmount, the
TotalSalesCount, and the UpdatedDate.

With this scenario, if another sales feed comes into the database, only one statement will need to
be run for that feed. Any new customer sales will be added to the database, and any existing
sales will be updated with the new sales information.

Tim Chapman a SQL Server database administrator and consultant who works for a bank in
Louisville, KY. Tim has more than eight years of IT experience, and he is a Microsoft certified
Database Developer and Administrator. If you would like to contact Tim, please e-mail him at



chapman.tim@gmail.com.
—————————————————————————————–

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12 Join the conversation! Add Your Opinion
Comments Follow via:

Staff Picks Top Rated Most Recent My Contacts See All Comments

Can you merge tables across DB's same server? 0
NewToTheGame Updated - 14th Jul 2010 Votes

Trying to find out if it is possible to merge table data from table on another DB on
same server

What syntax is required?

All web examples us tables in same DB

View in thread

Re: Performance 0
chapman.tim@... 25th Sep 2007 Votes

You're really not going to get around the cost incurred to run the separate
insert/update/delete statements. I haven't ran any significant tests for this myself, but I
don't think that the advantages... Read Whole Comment +

View in thread

Performance 0
richnorgaard@... 25th Sep 2007 Votes

While I agree the table may be scanned once, the MERGE statement still invokes



both an INSERT and UPDATE. The optimizer may build a special plan to leverage
the lookup, but in the end the INSERT and... Read Whole Comment +

View in thread

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