Question

我试图找到两台或多台不同电脑（PC和/或笔记本电脑）的制造商，其中“HD”大于或等于10。

产品有：制造商＆amp;模型

PC和PC都是笔记本电脑有：型号＆amp; HD

这是我到目前为止所写的：

SELECT DISTINCT maker
FROM
    (
        (
            SELECT model, maker
            FROM
            (SELECT model FROM laptop WHERE hd >= 10) AS lap2
            NATURAL JOIN product
        ) as ta
        UNION ALL
        (
            SELECT model, maker
            FROM
            (SELECT model FROM pc WHERE hd >= 10) AS pc2
            NATURAL JOIN product
        ) AS tb
    ) AS t1
    JOIN t1 as t2
    ON (t1.model != t2.model and t1.model > t2.model)

然而，它为我提供了一个可爱的错误：

1064 - 您的SQL语法出错;查看与您的MySQL服务器版本对应的手册，以便在'AS tb附近使用正确的语法AS t1 JOIN t1 as t2 ON（t1.model！= t2.model和t1.model＆gt; t2.model）'at line 16

编辑：

工件：

SELECT model, maker
FROM
(SELECT model FROM laptop WHERE hd >= 10) AS lap2
NATURAL JOIN product

SELECT model, maker
FROM
(SELECT model FROM pc WHERE hd >= 10) AS pc2
NATURAL JOIN product

Answer 1

由于你的括号是平衡的，我不认为“太多括号”是问题所在。

问题出在

JOIN t1 AS t2

您有t1作为UNION查询;你不能轻易地将t1作为一个表而不会让所有人感到困惑。

您可以写：

 JOIN t3 AS t2

其中t3是DBMS中的表（或视图）。

顺便提一下，你的ON条件不需要两个条件：

 ON (t1.model != t2.model AND t1.model > t2.model)

使用：

就足够了

ON (t1.model > t2.model)

复杂题外话题

如果你想在UNION上进行自联接，你必须两次编写UNION，或者（如果MySQL支持它，我认为它没有）CTE（公用表表达式），在主SQL语句之前称为WITH子句：

WITH t1(model, maker) AS
    (SELECT model, maker
       FROM (SELECT model FROM laptop WHERE hd >= 10) AS lap2
       NATURAL JOIN product
     UNION ALL
     SELECT model, maker
       FROM (SELECT model FROM pc WHERE hd >= 10) AS pc2
       NATURAL JOIN product
    )
SELECT DISTINCT t1a.maker
  FROM t1 AS t1a
  JOIN t1 AS t1b ON (t1a.model > t1b.model)

我不太清楚ON状况意味着什么;它似乎有利于斑马而不是土豚，但这就是全部。但是，这在语法上（近似）是正确的。

如果没有WITH子句，则将表达式写出两次：

SELECT DISTINCT t1a.maker
  FROM (SELECT model, maker
           FROM (SELECT model FROM laptop WHERE hd >= 10) AS lap2
           NATURAL JOIN product
         UNION ALL
         SELECT model, maker
           FROM (SELECT model FROM pc WHERE hd >= 10) AS pc2
           NATURAL JOIN product
       ) AS t1a
  JOIN (SELECT model, maker
           FROM (SELECT model FROM laptop WHERE hd >= 10) AS lap2
           NATURAL JOIN product
         UNION ALL
         SELECT model, maker
           FROM (SELECT model FROM pc WHERE hd >= 10) AS pc2
           NATURAL JOIN product
       ) AS t1b
    ON (t1a.model > t1b.model)

简单回答

当然，这只是对您的查询的解释。你的问题似乎更简单：

SELECT Maker
  FROM (SELECT model, maker
           FROM (SELECT model FROM laptop WHERE hd >= 10) AS lap2
           NATURAL JOIN product
         UNION ALL
         SELECT model, maker
           FROM (SELECT model FROM pc WHERE hd >= 10) AS pc2
           NATURAL JOIN product
       )  AS t  -- per comment (and SQL standard, but it really is superfluous!)
 GROUP BY Maker
HAVING COUNT(*) > 1;

括号太多

编辑：

1 个答案:

复杂题外话题

简单回答