-----------------
MERGE statement
-----------------

  Function:
    Read data from the source and INSERT or UPDATE in the target table depending on a
    condition.

  Author:
    Adriano dos Santos Fernandes <adrianosf@uol.com.br>

  Format:
	<merge statement> ::=
		MERGE
			INTO <table or view> [ [AS] <correlation name> ]
			USING <table or view or derived table> [ [AS] <correlation name> ]
			ON <condition>
			[ <merge when matched> ]
			[ <merge when not matched> ]

	<merge when matched> ::=
		WHEN MATCHED THEN
			UPDATE SET <assignment list>

	<merge when not matched> ::=
		WHEN NOT MATCHED THEN
			INSERT [ <left paren> <column list> <right paren> ]
				VALUES <left paren> <value list> <right paren>

  Syntax rules:
	1. At least one of <merge when matched> and <merge when not matched> should be specified
	   and each one should not be specified more than one time.

  Scope:
    DSQL, PSQL

  Examples:
	1.
		MERGE
			INTO customers c
			USING (SELECT * FROM customers_delta WHERE id > 10) cd
			ON (c.id = cd.id)
			WHEN MATCHED THEN
				UPDATE SET
					name = cd.name
			WHEN NOT MATCHED THEN
				INSERT (id, name)
					VALUES (cd.id, cd.name)

  Notes:
	A right join is made between INTO and USING tables using the condition.
	UPDATE is called when a record exist in the left table (INTO), otherwise
	INSERT is called.

	If no record is returned in the join, INSERT is not called.