Question

Does DISTINCT in a simple query with an aggregate function have any effect?

select DISTINCT salesperson, SUM(sales_amt) from sales GROUP BY salesperson

I realize there are more complicated queries where DISTINCT can have an affect, such as:

select salesperson, SUM(DISTINCT sales_amt) from sales GROUP BY salesperson

(support for that syntax by platform may vary)

But I want to confirm that in the simple query example, DISTINCT is redundant.

EDIT: fixed missing GROUP BY salesperson

Answer 1

Assuming you are missing GROUP BY salesperson (it’s invalid in SQL Server if you omit the group by), the DISTINCT is redundant in your first query. The GROUP BY effectively performs a DISTINCT here by aggregating salesperson.

select DISTINCT salesperson, SUM(sales_amt) from sales GROUP BY salesperson

And you are, as you’ve noted, correct that placement of the DISTINCT inside the aggregate SUM() may produce a different rowset.