Using MERGE in SQL Server to insert, update and delete at the same time

By: Arshad Ali | Read Comments (4) | Print Arshad is a SQL and BI Developer focusing on Data Warehousing projects @ Microsoft India R&D Pvt Ltd. Related Tips: More

Solution
Beginning with SQL Server 2008, now you can use MERGE SQL command to perform these operations in a single statement. This new command is similar to the UPSERT (fusion of the words UPDATE and INSERT.) command of Oracle; it inserts rows that don’t exist and updates the rows that do exist. With the introduction of the MERGE SQL command, developers can more effectively handle common data warehousing scenarios, like checking whether a row exists, and then executing an insert or update or delete.

The MERGE statement basically merges data from a source result set to a target table based on a condition that you specify and if the data from the source already exists in the target or not. The new SQL command combines the sequence of conditional INSERT, UPDATE and DELETE commands in a single atomic statement, depending on the existence of a record. The new MERGE SQL command looks like as below:

MERGE <target_table> [AS TARGET]
USING <table_source> [AS SOURCE]
ON <search_condition>
[WHEN MATCHED 
THEN <merge_matched> ]
[WHEN NOT MATCHED [BY TARGET]
THEN <merge_not_matched> ]
[WHEN NOT MATCHED BY SOURCE
THEN <merge_ matched> ];

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.

Putting it all together

In this example I will take a Products table as target table and UpdatedProducts as a source table containing updated list of products. I will then use the MERGE SQL command to synchronize the target table with the source table.

• First Let's create a target table and a source table and populate some data to these tables.

--Create a target table
CREATE TABLE Products
(
ProductID INT PRIMARY KEY,
ProductName VARCHAR(100),
Rate MONEY
) 
GO
--Insert records into target table
INSERT INTO Products
VALUES
(1, 'Tea', 10.00),
(2, 'Coffee', 20.00),
(3, 'Muffin', 30.00),
(4, 'Biscuit', 40.00)
GO
--Create source table
CREATE TABLE UpdatedProducts
(
ProductID INT PRIMARY KEY,
ProductName VARCHAR(100),
Rate MONEY
) 
GO
--Insert records into source table
INSERT INTO UpdatedProducts
VALUES
(1, 'Tea', 10.00),
(2, 'Coffee', 25.00),
(3, 'Muffin', 35.00),
(5, 'Pizza', 60.00)
GO
SELECT * FROM Products
SELECT * FROM UpdatedProducts
GO

• Next I will use the MERGE SQL command to synchronize the target table with the refreshed data coming from the source table.

--Synchronize the target table with
--refreshed data from source table
MERGE Products AS TARGET
USING UpdatedProducts AS SOURCE 
ON (TARGET.ProductID = SOURCE.ProductID) 
--When records are matched, update 
--the records if there is any change
WHEN MATCHED AND TARGET.ProductName <> SOURCE.ProductName 
OR TARGET.Rate <> SOURCE.Rate THEN 
UPDATE SET TARGET.ProductName = SOURCE.ProductName, 
TARGET.Rate = SOURCE.Rate 
--When no records are matched, insert
--the incoming records from source
--table to target table
WHEN NOT MATCHED BY TARGET THEN 
INSERT (ProductID, ProductName, Rate) 
VALUES (SOURCE.ProductID, SOURCE.ProductName, SOURCE.Rate)
--When there is a row that exists in target table and
--same record does not exist in source table
--then delete this record from target table
WHEN NOT MATCHED BY SOURCE THEN 
DELETE
--$action specifies a column of type nvarchar(10) 
--in the OUTPUT clause that returns one of three 
--values for each row: 'INSERT', 'UPDATE', or 'DELETE', 
--according to the action that was performed on that row
OUTPUT $action, 
DELETED.ProductID AS TargetProductID, 
DELETED.ProductName AS TargetProductName, 
DELETED.Rate AS TargetRate, 
INSERTED.ProductID AS SourceProductID, 
INSERTED.ProductName AS SourceProductName, 
INSERTED.Rate AS SourceRate; 
SELECT @@ROWCOUNT;
GO

When the above is run this is the output.  There were 2 updates, 1 delete and 1 insert.

If we select all records from the Products table we can see the final results.  We can see the Coffee rate was updated from 20.00 to 25.00, the Muffin rate was updated from 30.00 to 35.00, Biscuit was deleted and Pizza was inserted.

Notes

• The MERGE SQL statement requires a semicolon (;) as a statement terminator. Otherwise Error 10713 is raised when a MERGE statement is executed without the statement terminator.
• When used after MERGE, @@ROWCOUNT returns the total number of rows inserted, updated, and deleted to the client.
• At least one of the three MATCHED clauses must be specified when using MERGE statement; the MATCHED clauses can be specified in any order. However a variable cannot be updated more than once in the same MATCHED clause.
• Of course it’s obvious, but just to mention, the person executing the MERGE statement should have SELECT Permission on the SOURCE Table and INSERT, UPDATE and DELETE Permission on the TARGET Table.
• MERGE SQL statement improves the performance as all the data is read and processed only once whereas in previous versions three different statements have to be written to process three different activities (INSERT, UPDATE or DELETE) in which case the data in both the source and target tables are evaluated and processed multiple times; at least once for each statement.
• MERGE SQL statement takes same kind of locks minus one Intent Shared (IS) Lock that was due to the select statement in the ‘IF EXISTS’ as we did in previous version of SQL Server.
• For every insert, update, or delete action specified in the MERGE statement, SQL Server fires any corresponding AFTER triggers defined on the target table, but does not guarantee on which action to fire triggers first or last. Triggers defined for the same action honor the order you specify.

Next Steps

• Review “MERGE (Transact-SQL)” on MSDN.
• Review Chad Boyd' blog.

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Comments and Feedback:

Tuesday, March 10, 2009 - 10:54:55 AM - jcelko
Very nice job!  The only thing I would add is an example with more predicates in the WHEN clause --  WHEN MATCHED AND SOURCE. foobar > TARGET.foobar THEN .. WHEN MATCHED AND SOURCE. foobar <= TARGET.foobar THEN .. You can get a lot of power in one statement and avoid procedural coding.

Wednesday, March 11, 2009 - 1:33:23 PM - arshad0384
First of all thanks for your appreciation and thanks again for addition, this will help the readers.

Tuesday, September 28, 2010 - 1:46:27 PM - kv
How do we use the merge function when the source and target are on different databases?

Thursday, March 10, 2011 - 5:09:03 PM - Don
Best example and explanation of a multi-matched/not matched merge I have encountered. Well Done!

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