This question is an attempt to understand/question implementation of SQL in PostgreSQL.
While reading PostgreSQL v10 docs (PDF version), I noticed a footnote at end of **2.5. Querying a Table** stating:
> the implementation of DISTINCT automatically orders the rows and so ORDER BY is unnecessary. But this is not required by the SQL standard, and current PostgreSQL does not guarantee that DISTINCT causes the rows to be ordered
Let's call this error/mistake a "UDFS" (Unintended Deviation From Standards). It is possible there are other such errors in PostgreSQL's SQL implementation. (I'm glad PostgreSQL rectified it).
I think: reduction in number of rows in output when `GROUP BY` clause is used is another UDFS.
Does the SQL Standard (can't buy, and I probably won't understand it) explicitly state that **only the final row of an aggregate should be displayed**? Why not first row of an aggregate group, why not n-th row?
What if important (i.e. "uniquely identifying" for "human eyes") data was contained in first row of a group. Why does the underlying program (psql server, I suppose) **decide** that preceding rows are not worth displaying?
I don't mean to question the developers but perhaps there are bigger reasons why things are the way they are.
Cheers.
P.S. I am a SQL novice, this thread lead me to think like this. In it's context: Why do we need to calculate aggregate for each row? Isn't it *wrong*?
