Question

我正在学习 SQL SELECT 查询，并寻求一些提示：）

我找到了一个有任务的网站。当我尝试解决其中之一时，我有两种不同的解决方案。他们看起来和我一样，但不一样。

您能否解释一下为什么这些查询不相等？

SELECT DISTINCT Product.maker 
FROM Product 
JOIN Laptop ON Product.model = Laptop.model 
WHERE Laptop.speed >= 750
INTERSECT
SELECT DISTINCT Product.maker
FROM Product
JOIN PC ON Product.model = PC.model
WHERE PC.speed >= 750

AND

SELECT DISTINCT Product.maker 
FROM Product 
JOIN Laptop ON Product.model = Laptop.model
JOIN PC ON Product.model = PC.model
WHERE PC.speed >= 750 AND Laptop.speed >= 750

谢谢！

更新：

另一个解决方案

SELECT DISTINCT maker FROM Product
JOIN pc ON PC.model = Product.model AND type = 'PC' AND pc.speed > = 750
WHERE maker IN
(SELECT maker FROM Product
JOIN Laptop ON Laptop.model = Product.model AND type = 'Laptop' AND Laptop.speed > = 750)

还有一个，但最快的：

SELECT Product.maker
FROM Product
 JOIN Laptop
 ON Product.model = Laptop.model
 AND Laptop.speed > = 750
 AND maker IN(
  SELECT maker
  FROM Product
   JOIN PC
   ON Product.model = PC.model
   AND PC.speed > = 750
 )

Answer 1

相交版本允许每个联接之间的模型不同，但是单个查询（第二个）版本要求模型相同。

要更正第二个版本，请使用left join s并将speed上的条件移到联接条件中：

SELECT DISTINCT Product.maker 
FROM Product 
LEFT JOIN Laptop ON Product.model = Laptop.model
  AND Laptop.speed >= 750
LEFT JOIN PC ON Product.model = PC.model
  AND PC.speed >= 750

SQL-为什么INTERSECT不等于double JOIN？

1 个答案: