获取多个表格中每个租户的行数。

Question

比如说我有2张桌子。Tenants 和 Wargles. Wargles 有一个外键向 Tenants 叫做 TenantId. 如果我想知道每个租户的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，让我们假设我有另一张表。Fiddles，因为 Wargles 有一个FK走向 Tenants. 我怎样才能在上面的查询中再加一列，这样我就能得到每个租户的wargles数和fiddles数？

我试过用这个方法。

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

但这行不通 因为WargleCount和FiddleCount的数字是一样的 是两张表的行的乘积

Answer 1

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

Answer 2

最有效的方法可能是使用相关的子查询。

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，这可以利用关于以下方面的索引： Wargles(TenantId) 和 Fiddles(TenantId).

Answer 3

在你的情况下，作为可扩展的解决方案，我建议使用Scalar Function。

/* 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);

函数

/*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

选择声明

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