Question

我正在尝试使用postgresql进行查询。该数据库包含两个关系：“王国”，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晚宴的人

关系“王国”包括国王的名字，当他的王国开始和结束时。 “dinasty”的关系包括姓名，性别，出生和死亡。

我想要查询的是他去世时最年长的国王。

根据我的查询，我在第3行（不是在）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内部的内容是正确的。

Answer 1

您正在子查询中投射三个列，但在IN子句中比较它们中的一个一个。仅为子查询中的r1.king选择所需的列（IN）：

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

如上所述，您的列数并不匹配，但是编写此列的方式要简单得多。

在撰写查询时，最好分阶段进行思考。首先，你需要知道每个国王死后的年龄：

SELECT *, death-birth AS lived_for FROM dinasty

现在你已经拥有了它，你可以使用DISTINCT ON找到每个王国最长寿的国王

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将取每个不同值的第一行，因此将它与正确的ORDER BY配对非常重要。首先，我们按照dinasty的名字命令，然后国王以降序为生活多长时间。这意味着每个晚宴上显示的第一个国王将是最长寿的，这就是DISTINCT ON将为每个晚宴保留的记录。

请注意，我也将JOIN删除为kindgom，但如果需要，您可以将其添加回来：

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，则可以使用ROW（）结构：

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

SQL：子查询有太多列

2 个答案: