Traditionally, the MERGE is considered to be a better choice than Update & Insert (UPSERT) due to it's strength and flexibility.
UPSERT
The Update and Insert part need to reference:
The same table
The same row
Should use the PI for a respective operation
PI shouldn't be an Identity Column
MERGE
The Insert portion may specify different partitions
The Update can modify partitioning fields
Partitioning fields need not be specified in WHERE of Update portion
For additional details, please refer:
http://forums.teradata.com/forum/database/merge-vs-upsert-0
-Wasif
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