Oracle perform a count for each row on a table - sql

Question

I have a doubt regarding a query in oracle SQL.
I have a groups table, that I can query as:
SELECT *
FROM groups g
WHERE g.owner = 123;
This would give something like:
groupId
owner
1
123
2
123
3
123
4
123
I also have another table of administrators, that I can query as:
SELECT *
FROM admins a
ORDER BY groupId;
This would get administrators as:
adminId
userName
groupId
1
myadmin1
1
2
myAdmin2
1
3
myAdmin3
1
4
myAdmin4
2
5
myAdmin5
3
6
myAdmin6
3
That basically means that a group can have multiple administrators. I would like to count the number of administrators for each group. A result such as:
groupId
owner
adminCount
1
123
3
2
123
1
3
123
2
4
123
0
However, I cannot make a count of each administrator in the table and then make a join, as it is a table with a lot of rows.
I would like to perform the count query
SELECT count(*)
FROM admins a
WHERE groupId = 1;
for each row of the groups query, such that I get the desired result without performing a count of each administrator in the table, just the ones that belong to the groups from a specific owner.
Does someone know how can I count it without counting all the rows in the administrators table?
Thanks

Answer 1

The easiest and most readable variant is to use outer apply (or lateral(+)):
select *
from groups g
outer apply (
select count(*) as adminCount
from admins a
where a.groupId=g.groupId
);
Or you can get the same results using subqueries (moreover, in fact Oracle optimizer can decide to transform outer-apply/lateral to this variant, since it has "lateral view decorrelation" transformation):
select g.groupId,g.owner, nvl(a.adminCount,0) as adminCount
from groups g
left join (
select x.groupId, count(*) as adminCount
from admins x
group by x.groupId
) a
on a.groupId=g.groupId;
or even group-by with join:
select g.groupId,g.owner, count(a.groupId) as adminCount
from groups g
left join admins a
on g.groupId=a.groupId
group by g.groupId,g.owner
https://dbfiddle.uk/a-Q_abg8

Answer 2

You could use analytic function COUNT() OVER() ...
Select Distinct
g.GROUP_ID,
g.OWNER,
Count(a.ADMIN_ID) OVER(Partition By g.GROUP_ID) "COUNT_ADMINS"
From groups g
Left Join admins a ON(a.GROUP_ID = g.GROUP_ID)
Where g.OWNER = 123
Order By g.GROUP_ID
... this requires the Distinct keyword which could be performance costly with big datasets. I don't expect that user groups and admins are that big.
WIth your sample data:
WITH
groups (GROUP_ID, OWNER) AS
(
Select 1, 123 From Dual Union ALL
Select 2, 123 From Dual Union ALL
Select 3, 123 From Dual Union ALL
Select 4, 123 From Dual
),
admins (ADMIN_ID, ADMIN_USER_NAMAE, GROUP_ID) AS
(
Select 1, 'myadmin1', 1 From Dual Union All
Select 2, 'myadmin2', 1 From Dual Union All
Select 3, 'myadmin3', 1 From Dual Union All
Select 4, 'myadmin4', 2 From Dual Union All
Select 5, 'myadmin5', 3 From Dual Union All
Select 6, 'myadmin6', 3 From Dual
)
... the result is
GROUP_ID
OWNER
COUNT_ADMINS
1
123
3
2
123
1
3
123
2
4
123
0