Tuesday, March 04, 2003

What is Oracle's Mysterious MERGE Keyword for?

Oracle 9i supports a new SQL statement called MERGE. It's fully documented but let me provide an example that I've just been working on.

The advantage of MERGE is that a single SQL statement handles two cases:

  • INSERT if the record doesn't exist.
  • UPDATE if the record exists.

The intended use of MERGE was to modify one database table based on another table (or view). However, through a little creativity we can use MERGE for more mundane tasks - like the standard administrative Add/Edit a single-record functions.

The following SQL shows how to use the MERGE statement to insert or update a single record.

MERGE INTO 
	dr_fulltext A
USING
	(select 'Test41' u_object, 'EN-US' u_locale from dual) B
ON 
	(A.u_object = B.u_object AND A.u_locale = B.u_locale)
WHEN MATCHED THEN 
	UPDATE SET A.keywords = '33333333333'
WHEN NOT MATCHED THEN 
	INSERT (
	    A.u_object, A.u_locale, A.keywords
	) values (
	    'Test41', 'EN-US', '222222'
	);

Since the Merge clause is designed for getting information from a table I needed to finagle the USING class in order to create what is essentially a virtual table of one record. The ON clause checks to see if that single record (from the B table) exists in the A table. If it doesn't exist the INSERT statement is run. If it does, then the UPDATE statement is run.

The parentheses around the ON clause seem to be critical but I don't know why. If the parentheses are left off, then an "ORA-00969: missing ON keyword" error results.

The alias names in the UPDATE and INSERT are logically not needed, but Oracle demands clarity because the Keywords field is used twice. If the alias name (A) is not supplied Oracle responds with an "ORA-00957: duplicate column name" error.

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