Sql 获取多个表的每个租户的行数

假设我有两个表：

Tenants

和

Wargles

Wargles

有一个外键指向

Tenants

名为

TenantId

。如果我想获得每个租户的wargles数量，我可以这样做：

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

现在，假设我有另一张表，

Fiddles

，它像

Wargles

一样对

租户有一个FK。如何在上面的查询中添加另一列，以便获得每个租户的Wargle数和Fiddle数

我试过这个：

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

但这不起作用，因为它将为WargleCount和FiddleCount（两个表中的行的乘积）提供相同的数字。
最有效的方法可能是使用相关子查询：

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;

特别是，这可以利用
Wargles（TenantId）
和
Fiddles（TenantId）
上的索引使用两个子选择

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

在您的情况下，作为可扩展解决方案，我建议使用标量函数

/* 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);

功能

/*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语句

SELECT Id AS TenantId
    ,dbo.ufnGetTenantsNo(Id, 1) AS WargleCount
    ,dbo.ufnGetTenantsNo(Id, 2) AS FiddleCount
FROM Tenants