What is the meaning of GROUP BY?

Question

SQL

When using SUM, it is always required to use GROUP BY, moreover, with the same fields as in SELECT. What is the meaning of this?

Answer 1

Not always:
Read .

Answer 2

Sum is an aggregating function.
There are also other functions: COUNT is valid.
Group by groups rows together according to some criteria.
Let's say we have a table
id name
We want to count the number of entities with the same names.
we do Group by name, with the aggregating function COUNT.

Answer 3

The group by clause defines a list fold rule , where the fold itself is an aggregate function.