using SELECT *. SQL is decoupled
strange things might happen when you are recompiling your programms. If you don't use external defined DS for fetch, the programms won't compile. Using external DS your programm could fail, if your environments have diffrent versions. If you are using the same programm with diffrent stages of your database, you will get problems.

You can add columns without affect existing
that's not the one and only change happening to a database! you could alter columns (e.g. date instead of numeric or alpha), remove (unused) columns, normalize a table (splitting one into two tables), add redundance for performance reason. For all these cases you could hide this in the external schema of your database. As an additional thought: normally there are columns not to be of interest for the application, or must not be changed by the user (timestamp last change, created by user...). Maybe you want to have a look what "external schema is meaning" (https://www.tutorsland.com/forum/comp/dbms/what-do-you-mean-by-conceptual-logical-and-physical-schemas.html).
Using a view layer you could make all changes to your database, as long as you could provide an equivalent view as before.

@public exclude:
Most relational databases you wuold have to connect to the database, providing a user and its normal practice for applications to connect with the database owner; its only db2 on as400 connecting by automatic to the local database.


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