Well select sum(pay) from employees group by organisation is perfectly legal (although doesn't make sense because you don't know to which organisation ea

Well select sum(pay) from employees group by organisation is perfectly legal (although doesn't make sense because you don't know to which organisation each sum belongs to) Regarding: select *, sum(amount) from payslips group by organisation, pay_date Remember: you get exactly one row for each unique combination of organisation and pay_date. Assuming the following table: branch | organisation | amount | pay_date -------+--------------+--------+----------- 1 | one | 10 | 2020-01-01 2 | one | 20 | 2020-01-01 4 | one | 30 | 2020-01-01 7 | one | 40 | 2020-01-01 5 | two | 50 | 2020-01-01 13 | two | 100 | 2020-01-01 11 | two | 200 | 2020-01-01 15 | two | 300 | 2020-01-01 a "group by organisation, pay_date" yields exactly two rows. Which of the values for "branch" do you want to see for organisation "one"?