Without the UNIQUE option, only the first duplicate transaction observation updates the master.
If the duplicates in the transaction data set are not consecutive, the search begins at the beginning of the index each time, so that each duplicate is applied to the master.
If any transaction observations do not match master observations, they become new observations in the output data set.
Dropping, keeping, and renaming variables is often useful when you update a data set.
Renaming like-named variables prevents the second value that is read from over-writing the first one.
I like to think of doing such a thing as "making a data set shorter." Although there are other ways, we'll investigate two common ways of reducing the number observations in a data set here.
We can use an IF-THEN-DELETE statement to exclude observations from the output data set.