Why we use group by in sql

Question

When we make a SQL request like this:

SELECT attr1, attr2, AGGR(*)
  FROM SomeTable
 GROUP BY attr1, attr2

for some aggregate function, we have to include a GROUP BY clause and list attr1 and attr2 in it. My question is: why doesn't the SQL DBMS do it itself?

Answer 1

Because you may want to group by attr1, attr2 and attr3, even though you have no interest in the actual attr3 value itself.

In other words, you may be required to group by the non-aggegated columns but this in only at a minimum. There's nothing preventing you from grouping by columns not actually included in the query.

And, since the query evaluator doesn't know what you wanted (or it wants you to follow the relevant standard), it may think it safer to insist you explicitly state it.