In this post, I am sharing a demonstration on how to write merge statement using CTE of PostgreSQL.

PostgreSQL 9.1 has a new feature to write CTE and to use the CTE you can also merge INSERT/UPDATE in an expression.

The merge statement is always a discussion point for Database Developers of MySQL/PostgreSQL.
Below is a full demonstration to insert a record if not exist and update a record if they exist.

Let’s first create a sample table:

CREATE TABLE tbl_TestUpsert 
(
	Rno INTEGER
	,Name VARCHAR(100)
);

Now create a Stored Function:

CREATE OR REPLACE FUNCTION fn_InserUpdateMergeDemo
(
	InputRno INTEGER  
	,InputName VARCHAR(100)
)
RETURNS integer AS
$BODY$	
DECLARE 	
BEGIN	
	WITH  to_be_upserted 
	(Rno,Name) AS 
	(VALUES(InputRno,InputName))
	,updated AS 
	(
		UPDATE tbl_TestUpsert
		SET Rno=InputRno,Name=InputName
		FROM to_be_upserted
		WHERE tbl_TestUpsert.Rno = to_be_upserted.Rno
		RETURNING tbl_TestUpsert.Rno
	)
	INSERT INTO tbl_TestUpsert(Rno,Name)
	SELECT Rno,Name
	FROM to_be_upserted
	WHERE Rno NOT IN (SELECT Rno FROM updated);	
	
	RETURN 0;
	EXCEPTION WHEN OTHERS THEN	
	RAISE;	
END;
$BODY$
LANGUAGE plpgsql VOLATILE
COST 100;

As you can see in the above function that CTE is created with two input parameters and merge INSERT/UPDATE using an expression.

Now call this function with sample data:

SELECT *FROM fn_InserUpdateMergeDemo(1,'Anvesh');
SELECT *FROM fn_InserUpdateMergeDemo(2,'Neevan');
SELECT *FROM fn_InserUpdateMergeDemo(3,'Ronika');
SELECT *FROM fn_InserUpdateMergeDemo(2,'Roy');

You can find the two records have same rno column and “Neevan” updated by “Roy”.
Check this result: