count() without GROUP BY

Question

So I tried running this simple code

select COUNT(Ename),BOSS
from EMPLOYEE
WHERE BOSS=7839

but seems I did not get the output unless I use a "group by" boss below. I am not able to understand why can't I get output without "group by".

Can anybody explain in laymen terms. Would be much appreciated.

Answer 1

Count is an aggregate function. When you use Count (or any other aggregate function such as Count, Sum, Max, Min) next to select, then every other column item must be in group by. If you only use

select COUNT(Ename) -- there is no BOSS
from EMPLOYEE

then you don't have to use Group By. Lets say you have 5 columns: 1-COUNT(Ename), 2- BOSS, 3- column3, 4- column4, 5-column5. Then you would have to write select code as below:

select COUNT(Ename),BOSS, column3, column4, column5
from EMPLOYEE
WHERE BOSS=7839
GROUP BY
   BOSS, column3, column4, column5