MERGE in SQL SERVER -INSERT, UPDATE, DELETE in single execution.

To do multiple DML operations in SQL SERVER, we use MERGE, that a strong feature of SQL SERVER, In earlier versions of SQL SERVER we had to write separate statement for INSERT, UPDATE, DELETE, now by using MERGE we can perform INSERT, UPDATE, DELETE in one statement by checking data, if data matched, then update otherwise insert or in the particular condition we can delete also.

Note: Merge statement read entire data and processed only one.

The Merge query is given below:

Suppose there are two tables, Table1 and table2, table1 contains data about books like ISBN, Title, quantity, price, etc.. and table2 contains updated data of book so here we need to update Table1 from Table2, The table may have new records or updated records(like quantity, price, etc.) or it may be some records should be deleted from Table1.

CREATE  TABLE TABLE1

(

biBookId BIGINT IDENTITY(1,1),

vIsbn VARCHAR(20),

iQty INT,

dcPrice DECIMAL(18,2)

)

CREATE  TABLE TABLE2

(

biBookId BIGINT IDENTITY(1,1),

vIsbn VARCHAR(20),

iQty INT,

dcPrice DECIMAL(18,2)

)

/*insert some data in table1*/

INSERT INTO TABLE1(vIsbn,iQty,dcPrice) VALUES('9987876543343',10,0.00)

INSERT INTO TABLE1(vIsbn,iQty,dcPrice) VALUES('8898576476532',1,21.30)

INSERT INTO TABLE1(vIsbn,iQty,dcPrice) VALUES('9898765456763',50,4.00)

INSERT INTO TABLE1(vIsbn,iQty,dcPrice) VALUES('8987655663452',0,5.00)

/*insert some data in table2*/

INSERT INTO TABLE2(vIsbn,iQty,dcPrice) VALUES('9987876543343',20,0.00)

INSERT INTO TABLE2(vIsbn,iQty,dcPrice) VALUES('8898576476532',1,31.30)

INSERT INTO TABLE2(vIsbn,iQty,dcPrice) VALUES('9898765456763',5,4.00)

INSERT INTO TABLE2(vIsbn,iQty,dcPrice) VALUES('8987655663452',0,15.00)

INSERT INTO TABLE2(vIsbn,iQty,dcPrice) VALUES('8987655663451',11,25.00)

/*data in table1 before merge*/

select * from TABLE1

/*MERGE statement*/

MERGE TABLE1 AS tbl1

USING (SELECT  vIsbn,iQty,dcPrice FROM TABLE2) as tbl2

ON tbl1.vIsbn=tbl2.vIsbn

WHEN MATCHED AND tbl2.iQty=0 THEN DELETE

WHEN MATCHED THEN UPDATE

SET tbl1.iQty=tbl2.iQty,

tbl1.dcPrice=tbl2.dcPrice

WHEN NOT MATCHED THEN

INSERT(vIsbn,iQty,dcPrice)

VALUES(tbl2.vIsbn, tbl2.iQty, tbl2.dcPrice);

GO

SELECT * FROM TABLE1

DROP TABLE TABLE1

DROP TABLE TABLE2