This makes sense, because if we try to
SELECT a column not included in the
GROUP BY clause and there are multiple possible values for that column in a group, the database will not know which value to return.
The SQL standard requires that selected columns be functionally dependent on those in the
GROUP BY clause. This means that a primary key can be used in the
GROUP BY clause in place of a full list of selected columns, where the selected columns are dependent on that key.