Question

I’m struggling to understand what this query is doing:

SELECT branch_name, count(distinct customer_name)
FROM depositor, account
WHERE depositor.account_number = account.account_number
GROUP BY branch_name

What’s the need of GROUP BY?

Answer 1

Let’s take a step away from SQL for a moment at look at the relational trainging language Tutorial D.

Because the two relations (tables) are joined on the common attribute (column) name account_number, we can use a natural join:

depositor JOIN account

(Because the result is a relation, which by definition has only distinct tuples (rows), we don’t need a DISTINCT keyword.)

Now we just need to aggregate using SUMMARIZE..BY:

SUMMARIZE (depositor JOIN account) 
   BY { branch_name } 
      ADD ( COUNT ( customer_name ) AS customer_tally )

Back in SQLland, the GROUP BY branch_name is doing the same as SUMMARIZE..BY { branch_name }. Because SQL has a very rigid structure, the branch_name column must be repeated in the SELECT clause.