This blog post is written in response to T-SQL Tuesday hosted by Jorge Segarra (aka SQLChicken).

I have been very active using these Merge operations in my development. However, I have found out from my consultancy work and friends that these amazing operations are not utilized by them most of the time. Here is my attempt to bring the necessity of using the Merge Operation to surface one more time.

MERGE is a new feature that provides an efficient way to do multiple DML operations. In earlier versions of SQL Server, we had to write separate statements to INSERT, UPDATE, or DELETE data based on certain conditions; however, at present, by using the MERGE statement, we can include the logic of such data changes in one statement that even checks when the data is matched and then just update it, and similarly, when the data is unmatched, it is inserted.

One of the most important advantages of MERGE statement is that the entire data are read and processed only once. In earlier versions, three different statements had to be written to process three different activities (INSERT, UPDATE or DELETE); however, by using MERGE statement, all the update activities can be done in one pass of database table.

I have written about these Merge Operations earlier in my blog post over here SQL SERVER – 2008 – Introduction to Merge Statement – One Statement for INSERT, UPDATE, DELETE. I was asked by one of the readers that how do we know that this operator was doing everything in single pass and was not calling this Merge Operator multiple times.

Let us run the same example which I have used earlier; I am listing the same here again for convenience.

--Let’s create Student Details and StudentTotalMarks and inserted some records.
USE tempdb
GO
CREATE TABLE StudentDetails
(
StudentID INTEGER PRIMARY KEY,
StudentName VARCHAR(15)
)
GO
INSERT INTO StudentDetails
VALUES(1,'SMITH')
INSERT INTO StudentDetails
VALUES(2,'ALLEN')
INSERT INTO StudentDetails
VALUES(3,'JONES')
INSERT INTO StudentDetails
VALUES(4,'MARTIN')
INSERT INTO StudentDetails
VALUES(5,'JAMES')
GO
CREATE TABLE StudentTotalMarks
(
StudentID INTEGER REFERENCES StudentDetails,
StudentMarks INTEGER
)
GO
INSERT INTO StudentTotalMarks
VALUES(1,230)
INSERT INTO StudentTotalMarks
VALUES(2,255)
INSERT INTO StudentTotalMarks
VALUES(3,200)
GO
-- Select from Table
SELECT *
FROM StudentDetails
GO
SELECT *
FROM StudentTotalMarks
GO
-- Merge Statement
MERGE StudentTotalMarks AS stm
USING (SELECT StudentID,StudentName FROM StudentDetails) AS sd
ON stm.StudentID = sd.StudentID
WHEN MATCHED AND stm.StudentMarks > 250 THEN DELETE
WHEN MATCHED THEN UPDATE SET stm.StudentMarks = stm.StudentMarks + 25
WHEN NOT MATCHED THEN
INSERT(StudentID,StudentMarks)
VALUES(sd.StudentID,25);
GO
-- Select from Table
SELECT *
FROM StudentDetails
GO
SELECT *
FROM StudentTotalMarks
GO
-- Clean up
DROP TABLE StudentDetails
GO
DROP TABLE StudentTotalMarks
GO

The Merge Join performs very well and the following result is obtained.

Let us check the execution plan for the merge operator. You can click on following image to enlarge it.

Let us evaluate the execution plan for the Table Merge Operator only.

We can clearly see that the Number of Executions property suggests value 1. Which is quite clear that in a single PASS, the Merge Operation completes the operations of Insert, Update and Delete.

I strongly suggest you all to use this operation, if possible, in your development. I have seen this operation implemented in many data warehousing applications.

Reference: Pinal Dave (http://blog.SQLAuthority.com)