How to get the max count of an attribute with 3 tables? - sql

Question

I need to query which author sold the most books and how many books the author sold.
select a.firstname ||''|| a.lastname as fullname,
max(count(datesold))
from author a,
transaction t,
book b
where a.authorid = b.authorid
and b.bookid = t.bookid
group by
a.firstname,
a.lastname;
It gave me an error of not a single-group group function.
Any idea what is the issue here?

Answer 1

With some sample data
SQL> with
2 author (authorid, firstname, lastname) as
3 (select 1, 'Stephen', 'King' from dual union all
4 select 2, 'Jo' , 'Nesbo' from dual),
5 book (bookid, authorid) as
6 (select 100, 1 from dual union all
7 select 200, 1 from dual union all
8 select 300, 2 from dual
9 ),
10 transaction (trans_id, bookid) as
11 (select 1, 100 from dual union all
12 select 2, 100 from dual union all
13 select 3, 100 from dual union all
14 select 4, 300 from dual
15 ),
query uses the RANK analytic function which ranks rows by number of rows in the transaction table (it says how many books were sold). Finally, fetch row(s) that rank as highest:
16 temp as
17 (select a.firstname || ' ' || a.lastname AS fullname,
18 count(t.bookid) cnt,
19 rank() over (order by count(t.bookid) desc) rnk
20 from author a join book b on a.authorid = b.authorid
21 join transaction t on t.bookid = b.bookid
22 group by a.firstname, a.lastname
23 )
24 select fullname, cnt
25 from temp
26 where rnk = 1;
FULLNAME CNT
------------- ----------
Stephen King 3
SQL>

Answer 2

You can use:
select MAX(a.firstname ||' '|| a.lastname) as fullname,
COUNT(datesold)
from author a
INNER JOIN book b
ON (a.authorid = b.authorid)
INNER JOIN transaction t
ON (b.bookid = t.bookid)
GROUP BY
a.authorid
ORDER BY
COUNT(datesold) DESC
FETCH FIRST ROW ONLY;
Do not aggregate by firstname and lastname as there are many people in the world with identical names and you do not want to count everyone with the same name as a single person.
Which, for the sample data:
CREATE TABLE author (authorid, firstname, lastname, dateofbirth) AS
SELECT 1, 'Alice', 'Adams', DATE '1900-01-01' FROM DUAL UNION ALL
SELECT 2, 'Alice', 'Adams', DATE '1910-01-01' FROM DUAL UNION ALL
SELECT 3, 'Betty', 'Baron', DATE '1920-01-01' FROM DUAL UNION ALL
SELECT 4, 'Carol', 'Corrs', DATE '1930-01-01' FROM DUAL UNION ALL
SELECT 5, 'Carol', 'Corrs', DATE '1940-01-01' FROM DUAL;
CREATE TABLE book (bookid, authorid) AS
SELECT 1, 1 FROM DUAL UNION ALL
SELECT 2, 2 FROM DUAL UNION ALL
SELECT 3, 3 FROM DUAL UNION ALL
SELECT 4, 4 FROM DUAL UNION ALL
SELECT 5, 5 FROM DUAL;
CREATE TABLE transaction (bookid, datesold) AS
SELECT 1, DATE '1970-01-01' FROM DUAL UNION ALL
SELECT 1, DATE '1970-01-02' FROM DUAL UNION ALL
SELECT 1, DATE '1970-01-03' FROM DUAL UNION ALL
SELECT 1, DATE '1970-01-04' FROM DUAL UNION ALL
SELECT 3, DATE '1970-01-01' FROM DUAL UNION ALL
SELECT 4, DATE '1970-01-01' FROM DUAL UNION ALL
SELECT 4, DATE '1970-01-02' FROM DUAL UNION ALL
SELECT 5, DATE '1970-01-01' FROM DUAL UNION ALL
SELECT 5, DATE '1970-01-02' FROM DUAL UNION ALL
SELECT 5, DATE '1970-01-03' FROM DUAL;
Outputs:
FULLNAME
COUNT(DATESOLD)
Alice Adams
4
db<>fiddle here