多列上的SELECT COUNT（DISTINCT ...）錯誤？

Question

我有一個表，VehicleModelYear，包含列id，年份，品牌和模型。

以下兩個查詢按預期工作：

SELECT DISTINCT make, model
FROM VehicleModelYear

SELECT COUNT(DISTINCT make)
FROM VehicleModelYear

但是，此查詢不起作用

SELECT COUNT(DISTINCT make, model)
FROM VehicleModelYear

很明顯，答案是第一個查詢返回的結果數，但只是想知道這個語法有什么問題或者為什么它不起作用。

Answer 1

SQL Server COUNT()接受以下語法

COUNT(*)
COUNT(colName)
COUNT(DISTINCT colName)

您可以擁有一個子查詢，該子查詢返回您可以計算的唯一一組make和model 。

SELECT  COUNT(*)
FROM
        (
            SELECT  DISTINCT make, model
            FROM    VehicleModelYear
        ) a

最后的“a”不是拼寫錯誤。 它是一個別名，沒有它，SQL將給出錯誤ERROR 1248 (42000): Every derived table must have its own alias 。

Answer 2

嘗試將它們組合到一個字段中：

SELECT COUNT(DISTINCT make + ' ' + model)
FROM VehicleModelYear

Answer 3

語法是有效的SQL，但尚未在SQL-Server中實現。

嘗試重寫：

; WITH cte AS
  ( SELECT DISTINCT make, model
    FROM VehicleModelYear
  )
SELECT COUNT(*) AS total
FROM cte ; 

要么：

; WITH cte AS
  ( SELECT COUNT(DISTINCT model) AS cnt
    FROM VehicleModelYear
    GROUP BY make
  )
SELECT SUM(cnt) AS total
FROM cte ; 

要么：

; WITH cte AS
  ( SELECT NULL AS n
    FROM VehicleModelYear
    GROUP BY make, model
  )
SELECT COUNT(*) AS total
FROM cte ; 

最后，修改了上面的第二個和第三個查詢，沒有子查詢：

SELECT DISTINCT
    SUM(COUNT(DISTINCT model)) OVER () AS total
FROM VehicleModelYear
GROUP BY make ;

SELECT DISTINCT COUNT(*) OVER () AS total
FROM VehicleModelYear
GROUP BY make, model ;

---

@Alexander Fedorenko的后期補充：

SELECT TOP (1)
    SUM(COUNT(DISTINCT model)) OVER () AS total
FROM VehicleModelYear
GROUP BY make ;

SELECT TOP (1) COUNT(*) OVER () AS total
FROM VehicleModelYear
GROUP BY make, model ;

要么：

; WITH cte AS
  ( SELECT DENSE_RANK() OVER(ORDER BY make, model) AS dr
    FROM VehicleModelYear
  )
SELECT MAX(dr) AS total
FROM cte ;