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MERGE statement
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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.