SQL：子查询的列太多

Question

我正在尝试使用 postgresql 进行查询。 该数据库包含两个关系：“kingdom”，其中包括一些英国国王，以及“dinasty”，其中包含一些来自斯图尔特王朝的人

“王国”关系包括国王的名字以及他的王国开始和结束的时间。 “阴”的关系包括姓名、性别、出生和死亡。

我要查询的是在他去世时最年长的国王。

根据我的查询，我在第 3 行（未输入）收到此错误：subquery has too many columns

这是查询：

SELECT kingdom.king, dinasty.birth, dinasty.death
FROM kingdom, dinasty
WHERE kingdom.king = dinasty.name AND kingdom.king NOT IN
(
    SELECT DISTINCT R1.king, R1.birth, R1.death
    FROM
    (
        SELECT DISTINCT R1.king, D1.birth, D1.death
        FROM kingdom AS R1, dinasty AS D1, dinasty AS D2
        WHERE R1.king=D1.name
    ) AS R1, 
    (
        SELECT DISTINCT R1.king, D1.birth, D1.death
        FROM kingdom AS R1, dinasty AS D1, dinasty AS D2
        WHERE R1.king=D1.name
    ) AS R2
    WHERE R1.death-R1.birth < R2.death-R2.birth
);

NOT IN 里面的内容是正确的。

Answer 1

您在子查询中投影 三个 列，但在 IN 子句中比较单个 一个。在子查询中为 IN 选择所需的列 (r1.king)：

SELECT kingdom.king, dinasty.birth, dinasty.death
FROM kingdom, dinasty
WHERE kingdom.king = dinasty.name AND kingdom.king NOT IN
(
    SELECT DISTINCT R1.king
    FROM
    (
        SELECT DISTINCT R1.king, D1.birth, D1.death
        FROM kingdom AS R1, dinasty AS D1, dinasty AS D2
        WHERE R1.king=D1.name
    ) AS R1, 
    (
        SELECT DISTINCT R1.king, D1.birth, D1.death
        FROM kingdom AS R1, dinasty AS D1, dinasty AS D2
        WHERE R1.king=D1.name
    ) AS R2
    WHERE R1.death-R1.birth < R2.death-R2.birth
);

Answer 2

如已回答，您的列数不匹配，但有一种更简单的方法来编写。

在编写查询时，最好分阶段进行思考。首先，您需要知道每位国王去世时的年龄：

SELECT *, death-birth AS lived_for FROM dinasty

现在你有了它，你可以使用 DISTINCT ON 来为每个王国找到寿命最长的国王

SELECT DISTINCT ON( name ) name, birth, death, lived_for
  FROM (
      SELECT *, death-birth AS lived_for FROM dinasty
    ) a
  ORDER BY name, lived_for DESC
;

distinct on 将占据每个不同值的第一行，因此将其与正确的 ORDER BY 配对很重要。首先我们按王朝的名称排序，然后按国王的寿命按降序排序。这意味着每个朝代显示的第一个国王将是寿命最长的国王，这就是 DISTINCT ON 将为每个朝代保留的记录。

请注意，我还删除了 JOIN 到 kindgom，但如果需要，您可以重新添加：

SELECT k.*, oldest.*
  FROM (
    SELECT DISTINCT ON( name ) name, birth, death, lived_for
      FROM (
          SELECT *, death-birth AS lived_for FROM dinasty
        ) a
      ORDER BY name, lived_for DESC
    ) oldest
    JOIN kingdom k ON k.king = oldest.name
;

最后，如果您需要在子选择中使用多个列，您可以使用 ROW() 构造：

SELECT ...
  FROM table_a
  WHERE ROW(f1, f2, f3) NOT IN (SELECT f1a, f2a, f3a FROM ... )
;