For this schema:http://github.com/metaperl/DBIx--Shootout/raw/7c090193e788f6d2825108c...

How would you get the top 5 salaries (and their department at the
time) such that the top 5 lists any employee only once?

My fear with my current query is that the GROUP BY will arbitrarily
reduce multiple instances of an employee to one row, without choosing
the row with the highest salary:

-- top 5 salaries, distinct by employees, and their department at the
time
SELECT
  *
FROM
  salaries INNER JOIN dept_emp USING (emp_no) INNER JOIN departments
USING (dept_no)
WHERE
  salaries.to_date <= dept_emp.to_date
GROUP BY
  emp_no
ORDER BY
  salary DESC
LIMIT 5 ;

I'm not sure what GROUP BY emp_no is supposed to mean (in this
context).

create table T (x int not null, y int not null, primary key (x,y));
insert into T (x,y) values (1,1), (1,2);

What would be the result of:

select x, y from T group by x;

Start by figuring out how to get the top 5 salaries from the salary
table

SELECT * FROM salaries ORDER BY salary DESC LIMIT 5;

but this might list the same employee multiple times... take
Microsoft. Bill Gates might have 5 different salaries, but they might
always the highest. I only want 1 salary from any particular employee.
And I want it to be his highest.

SELECT emp_no,
        max(salary)
FROM salaries
GROUP BY emp_no
ORDER BY 2 DESC
LIMIT 5