Tuesday, August 01, 2017

The DB2 12 for z/OS Blog Series - Part 14: Improved MERGE SQL Statement

A very common requirement for application developers is to be able to read through new data – from a table, a file or as entered by an end user – and either INSERT the data if it does not already exist or UPDATE data that does exist with new values.

The ANSI SQL standard defines the MERGE statement for this purpose. The purpose of the MERGE statement is to take two “tables” and merge the data into one table. DB2 for z/OS has supported the MERGE statement since Version 9, but it is more functional now as of Version 12.

Prior to DB2 12, the MERGE statement could not accept a table reference as a way of supplying source data. Input to the MERGE can only be a host variable array or a list of values. This limitation caused MERGE to be somewhat lightly implemented.

Well, Version 12 does away with this limitation – and adds even more features. So you can now write a MERGE statement where data from one table is merged with data from another table. Remember, merge takes the data and compares it and when the comparison is matched does one thing… and when the comparison is not matched it does another. So you can UPDATE when matched and INSERT when not matched.

Consider the following SQL:

MERGE INTO EMP Tgt
USING (SELECT EMPNO, FNAME, LNAME, ADDRESS, SALARY FROM NEW_EMP) Src
ON (Tgt.EMPNO = Src.EMPNO)
WHEN MATCHED THEN
  UPDATE SET (Tgt.FNAME, Tgt.LNAME, Tgt.ADDRESS, Tgt.SALARY) =
  (Src.FNAME, Src.LNAME, Src.ADDRESS, Src.SALARY)
WHEN NOT MATCHED THEN
  INSERT (EMPNO, FNAME, LNAME, ADDRESS, SALARY)
  VALUES (Src.EMPNO, Src.FNAME, Src.LNAME, Src.ADDRESS, Src.SALARY)
ELSE IGNORE;

This MERGE statement takes a table containing new/revised employee data and inserts the data when a match is not found and updates the data if it is found. Note that this is a simple MERGE that assumes that all the columns (in this case) are provided if the data is to be updated.

More complex MERGE statements are possible as of DB2 12 because you can now provide additional matching condition options and additional predicates on the matching conditions (instead of just matched/not matched). It is also possible to issue a SIGNAL statement to return an error when a matching condition evaluates to True.

When you use the new functionality of the MERGE statement in DB2 12+, the operations is atomic; this means that the source rows are processed as a set of rows by each WHEN clause. If an error occurs for any source row, processing stops and no target rows are modified.


But the bottom line here is that the MERGE statement has been significantly improved and is a powerful way of processing data using only SQL as of DB2 12 for z/OS. 

4 comments:

mohit gupta said...

which certification is best 610 611 730 and 731 for ibm ?

Craig S. Mullins said...
This comment has been removed by the author.
Craig S. Mullins said...

Each certification level is for a different skill set and which one is best depends upon what you are trying to achieve/certify. The fundamentals certification is the base for the rest of the certifications and is the best place to start. From there, take the path that best suits you for development, DBA or sysadmin and platform z/OS Linux Unix Windows...

Craig S. Mullins said...

Here is a good link for reviewing all of the IBM certification tests available (for DB2 and other IBM software): https://www-03.ibm.com/certify/tests/test_index.shtml