How to include "zero" / "0" results in COUNT aggregate?
You must use LEFT JOIN
instead of INNER JOIN
SELECT person.person_id, COUNT(appointment.person_id) AS "number_of_appointments"
FROM person
LEFT JOIN appointment ON person.person_id = appointment.person_id
GROUP BY person.person_id;
if you do the outer join (with the count), and then use this result as a sub-table, you can get 0 as expected (thanks to the nvl function)
Ex:
select P.person_id, nvl(A.nb_apptmts, 0) from
(SELECT person.person_id
FROM person) P
LEFT JOIN
(select person_id, count(*) as nb_apptmts
from appointment
group by person_id) A
ON P.person_id = A.person_id
USE join to get 0 count in the result using GROUP BY.
simply 'join' does Inner join in MS SQL so , Go for left or right join.
If the table which contains the primary key is mentioned first in the QUERY then use LEFT join else RIGHT join.
EG:
select WARDNO,count(WARDCODE) from MAIPADH
right join MSWARDH on MSWARDH.WARDNO= MAIPADH.WARDCODE
group by WARDNO
.
select WARDNO,count(WARDCODE) from MSWARDH
left join MAIPADH on MSWARDH.WARDNO= MAIPADH.WARDCODE group by WARDNO
Take group by from the table which has Primary key and count from the another table which has actual entries/details.
You want an outer join for this (and you need to use person as the "driving" table)
SELECT person.person_id, COUNT(appointment.person_id) AS "number_of_appointments"
FROM person
LEFT JOIN appointment ON person.person_id = appointment.person_id
GROUP BY person.person_id;
The reason why this is working, is that the outer (left) join will return NULL
for those persons that do not have an appointment. The aggregate function count()
will not count NULL
values and thus you'll not get a zero.
If you want to learn more about outer joins, here is a nice tutorial: http://sqlzoo.net/wiki/Using_Null