Question

Given scenario:

table fd  
(cust_id, fd_id) primary-key and amount  

table loan  
(cust_id, l_id) primary-key and amount

I want to list all customers who have a fixed deposit with an amount less than the sum of all their loans.

Query:

SELECT cust_id
  FROM fd
    WHERE amount
     <
    (SELECT sum(amount)
        FROM loan
          WHERE fd.cust_id = loan.cust_id);

OR should we use

SELECT cust_id
  FROM fd
    WHERE amount
     <
    (SELECT sum(amount)
        FROM loan
          WHERE fd.cust_id = loan.cust_id group by cust_id);

A customer can have multiple loans but one FD is considered at a time.

Answer 1

GROUP BY can be omitted in this case, because there is only (one) aggregate function(s) in the SELECT list and all rows are guaranteed to belong to the same group of cust_id ( by the WHERE clause).

The aggregation will be over all rows with matching cust_id in both cases. So both queries are correct.

---

This would be a cleaner another way to implement the same thing:

SELECT fd.cust_id
FROM   fd
JOIN   loan USING (cust_id)
GROUP  BY fd.cust_id, fd.amount
HAVING fd.amount < sum(loan.amount)

There is one difference: rows with identical (cust_id, amount) in fd only appear once in the result of my query, while they would appear multiple times in the original.

Either way, if there is no matching row with a non-null amount in table loan, you get no rows at all. I assume you are aware of that.