Question

我无法直观地看到子查询如何以连接的形式出现。特别是，以下SQL：

SELECT DISTINCT e.EmployeeID, e.LastName, e.FirstName
FROM Employees e 
INNER JOIN Orders o ON e.EmployeeID = o.EmployeeID
WHERE EXISTS
(
  SELECT c.Country
  FROM Customers c
  WHERE c.Country = e.Country
)

如果我想要将带有子查询的语句转换为连接，我会收到一些关于该怎么做的提示，我将不胜感激。

Answer 1

您需要注意重复项，但EXISTS的转换非常直接：

SELECT DISTINCT e.EmployeeID, e.LastName, e.FirstName
FROM Employees e INNER JOIN
     Orders o
     ON e.EmployeeID = o.EmployeeID INNER JOIN
     (SELECT DISTINCT c.Country
      FROM Customers c
     ) c
     ON  c.Country = e.Country

Answer 2

这看起来很干净。事实上，它不是：

SELECT DISTINCT e.EmployeeID, e.LastName, e.FirstName
FROM Employees e
JOIN Orders o ON o.EmployeeID = e.EmployeeID
JOIN Customers c ON c.Country = e.Country
        ;

重点是，如果orders和Customers有多个匹配行，则这些行将导致合并Employee记录的单独副本。这些后来必须被DISTINCT抑制（优化者可能会捕获这个，或者它可能不会）

关键是：你不需要一个独特的，因为你只是从员工中选择列：

SELECT e.EmployeeID, e.LastName, e.FirstName
FROM Employees e 
WHERE EXISTS( 
  SELECT * FROM Orders o
  WHERE o.EmployeeID = e.EmployeeID
  )
AND EXISTS(
  SELECT * FROM Customers c
  WHERE c.Country = e.Country
  );

或者，或多或少丑陋的风格：

SELECT e.EmployeeID, e.LastName, e.FirstName
FROM Employees e 
WHERE EXISTS(
  SELECT c.Country
  FROM Orders o
  JOIN Customers c
  ON o.EmployeeID = e.EmployeeID
  AND c.Country = e.Country
);

使用Join而不是Subquery重写SQL代码

2 个答案: