EXISTS子句与子查询中的TOP子句一起发布

Question

我在下面有一个查询子查询（目的是从货运价= 2的订单表中搜索前2个订单，并获得这些订单的客户）

1. 使用IN子句的工作查询

    SELECT *
    FROM Customers C
    WHERE C.CUST_ID IN (
      SELECT TOP 2 CUST_ID
      FROM Orders O
      where FREIGHT_CHARGES = 2
    ) 

但是我想将IN子句转换为更有效的EXISTS子句

2. 代码不适用于exists子句

    SELECT *
    FROM Customers C
    WHERE EXISTS (
        SELECT TOP 2 CUST_ID
        FROM Orders O
        where FREIGHT_CHARGES = 2 AND C.CUST_ID = O.CUST_ID
    )

在第二个查询的情况下，我没有检索前2个cust_id，但所有记录。

[请让我知道任何实现以获得期望结果的方法。

Answer 1

SELECT *
FROM Customers C
  WHERE EXISTS (
    SELECT 1
    FROM Orders O
    JOIN Orders O2
      ON O.FREIGHT_CHARGES = O2.FREIGHT_CHARGES 
      AND O.CUST_ID = O.CUST_ID
      AND O.ID <> O2.ID
    where O.FREIGHT_CHARGES = 2 AND C.CUST_ID = O.CUST_ID
   )

或

SELECT *
FROM Customers C
WHERE EXISTS (
    SELECT 1
    FROM Orders O
    where FREIGHT_CHARGES = 2 AND C.CUST_ID = O.CUST_ID
    GROUP BY FREIGHT_CHARGES, CUST_ID
    HAVING COUNT(1) > 1
)

您必须测试您的情况下哪个更好

Answer 2

SELECT
    TOP 2
    C.*
FROM Customers C
INNER JOIN Orders O ON C.CUST_ID = O.CUST_ID
WHERE O.FREIGHT_CHARGES = 2
ORDER BY C.CUST_ID -- Use 'ORDER BY' clause if 'C.CUST_ID' column does not have clustered index or you can use any other column

请让我知道您的反馈。

Answer 3

就您而言，EXISTS将不是您的理想选择。

请检查此链接：https://www.sqlservertutorial.net/sql-server-basics/sql-server-exists/