Unable to MAX(COUNT) and have multiple columns - sql

Question

I have 4 tables, EMPLOYEE, DRIVER, TRIP and TRIPLEG
EMPLOYEE table has Name which I want to extract and show with MAX count, has E# which is shared with DRIVER
DRIVER table has L#(licence number) which is common with TRIP
TRIP table has T#(trip number) which is common with TRIPLEG
I'm trying to find the max number of tripleg a driver has done(In this case driver with licence number 10002:
SELECT MAX(COUNT(TRIPLEG.LEG#))
FROM TRIP, TRIPLEG
ON TRIP.T# = TRIPLEG.T#
WHERE TRIP.L# = 10002
GROUP BY TRIP.T#
COUNT(TRIPLEG.LEG#) gives me https://i.imgur.com/AYAovov.png,
so I did the above MAX(COUNT(TRIPLEG.LEG#)) which gives me this: https://i.imgur.com/alCFlO3.png
I'm unable to proceed as I tried SELECTING more columns(TRIP.T#) like
SELECT TRIP.T#, MAX(COUNT(TRIPLEG.LEG#))
FROM TRIP, TRIPLEG
ON TRIP.T# = TRIPLEG.T#
WHERE TRIP.L# = 10002
GROUP BY TRIP.T#
Gives me an error: ORA-00937: not a single-group group function
Any advice? Need to be able to start small and selecting before I can join more tables to get the Employee name displayed beside the MAX tripleg count
Thanks in advance
Essentially I want something like: (only 1 row, which is the MAX triplegs done (5))
NAME MAX(COUNT(TRIPLEG.LEG#))
-----------------------------------
BOB 5

Answer 1

I don't have your table so I'll use Scott's EMP and DEPT (as you use Oracle, so I presume you're familiar with them).
This works:
SQL> select d.dname, count(e.empno) cnt
2 from emp e join dept d on e.deptno = d.deptno
3 where d.deptno in (10, 20)
4 group by d.dname;
DNAME CNT
-------------- ----------
ACCOUNTING 3
RESEARCH 5 --> MAX count is this
SQL>
Nested count works if there are no other columns in SELECT column list (you already know that), and it returns desired value:
SQL> select max(count(e.empno)) cnt
2 from emp e join dept d on e.deptno = d.deptno
3 where d.deptno in (10, 20)
4 group by d.dname;
CNT
----------
5
SQL>
But, this won't work (you know that too):
select d.dname, max(count(e.empno)) cnt
from emp e join dept d on e.deptno = d.deptno
where d.deptno in (10, 20)
group by d.dname;
To fix it, use CTE (Common Table Expression a.k.a. the WITH factoring clause) or an inline view; I'll show you the first option, with yet another addition: I'll rank counts and find the "highest" one, and use it later to select only desired row.
SQL> with tcnt as
2 (select d.deptno,
3 d.dname,
4 count(e.empno) cnt,
5 rank() over (order by count(e.empno) desc) rnk --> rank them DESC
6 from emp e join dept d on e.deptno = d.deptno
7 where d.deptno in (10, 20)
8 group by d.dname, d.deptno
9 )
10 select t.deptno, t.dname, t.cnt
11 from tcnt t
12 where rnk = 1; --> fetch only row(s) with highest rank
DEPTNO DNAME CNT
---------- -------------- ----------
20 RESEARCH 5
SQL>
Finally, add some more columns from other tables:
SQL> with tcnt as
2 (select d.deptno,
3 d.dname,
4 count(e.empno) cnt,
5 rank() over (order by count(e.empno) desc) rnk
6 from emp e join dept d on e.deptno = d.deptno
7 where d.deptno in (10, 20)
8 group by d.dname, d.deptno
9 )
10 select t.deptno, t.dname, t.cnt, e.ename, e.job
11 from tcnt t join emp e on e.deptno = t.deptno
12 where rnk = 1;
DEPTNO DNAME CNT ENAME JOB
---------- -------------- ---------- ---------- ---------
20 RESEARCH 5 SMITH CLERK
20 RESEARCH 5 JONES MANAGER
20 RESEARCH 5 SCOTT ANALYST
20 RESEARCH 5 ADAMS CLERK
20 RESEARCH 5 FORD ANALYST
SQL>

Answer 2

Based on your tables to find the max number of tripleg a driver has done(In this case driver with licence number 10002:
Select a.tripno, max(a.trips)
From
(Select Trip.T# as tripno, count(tripleg.leg#) as trips
From tripleg join trip on tripleg.T# = trip.T#
where trip.L# = 10002
Group by tripno
) a
Group by a.tripno