Get the number of rows per tenant of several tables - sql

Question

Let's say I have 2 tables: Tenants and Wargles. Wargles has a Foreign Key towards Tenants called TenantId. If I want to get number of wargles per tenant, I can do this:
SELECT t.Id as TenantId, count(w.Id) as WargleCount
FROM Tenants t
JOIN Wargles w ON w.TenantId = t.Id
GROUP BY t.Id
Now, let's say I have another table, Fiddles, that, as Wargles has a FK towards Tenants. How can I add another column to the query above, so I get the number of wargles and the number of fiddles for each tenant?
I tried with this:
SELECT t.Id as TenantId, count(w.Id) as WargleCount, count(f.Id) as FiddleCount
FROM Tenants t
JOIN Wargles w ON w.TenantId = t.Id
JOIN Fiddles f ON f.TenantId = t.Id
GROUP BY t.Id
But this won't work, since it would give me the same number both for WargleCount and FiddleCount, the product of the rows from both tables.

Answer 1

Use two subselects
SELECT t.Id as TenantId,
(SELECT Count(1) FROM Fiddles F WHERE F.TenantId = T.Id) as FiddleCount,
(SELECT Count(1) FROM Wargles W WHERE W.TenantId = T.Id) as WargleCount
FROM Tenants t

Answer 2

The most efficient method is probably to use correlated subqueries:
SELECT t.Id as TenantId,
(SELECT COUNT(*)
FROM Wargles w
WHERE w.TenantId = t.Id
) as WargleCount, count(f.Id) as FiddleCount
(SELECT COUNT(*)
FROM Fiddles f
WHERE f.TenantId = t.Id
) as FiddleCount
FROM Tenants t;
In particular, this can take advantage of indexes on Wargles(TenantId) and Fiddles(TenantId).

Answer 3

In your case, as extendable solution, I would recommend Scalar Function usage.
/* SAMPLE DATA ARRANGE */
CREATE TABLE Tenants (Id INT, Title NVARCHAR(5)) ; INSERT INTO Tenants VALUES (1, 'A'), (2, 'B') , (3, 'C');
CREATE TABLE Wargles (Id INT,TenantId INT);INSERT INTO Wargles VALUES (1, 1), (2, 1) , (3, 1) , (4, 2), (5, 2) , (6, 1), (7, 3) , (8, 3);
CREATE TABLE Fiddles (Id INT,TenantId INT);INSERT INTO Fiddles VALUES (1, 1), (2, 1) , (3, 1) , (4, 2), (5, 2) , (6, 2), (7, 3) , (8, 2);
The Function
/*NEEDED CODE*/
CREATE FUNCTION dbo.ufnGetTenantsNo ( #Id AS INT , #Tb AS INT)
RETURNS INT
AS
BEGIN
DECLARE #Result INT = 0;
IF (#TB = 1)
SELECT #Result = COUNT(*)
FROM Wargles
WHERE TenantId = #Id
ELSE
SELECT #Result = COUNT(*)
FROM Fiddles
WHERE TenantId = #Id
RETURN #Result
END
GO
Select Statement
SELECT Id AS TenantId
,dbo.ufnGetTenantsNo(Id, 1) AS WargleCount
,dbo.ufnGetTenantsNo(Id, 2) AS FiddleCount
FROM Tenants