sql - 单独放置 WHERE 语句以使 UNION 函数起作用

Question

我正在进行一项黑客排名练习，其中我必须找到名称最短和最长的城市，如果名称中有许多相同数量的字符，则按字母顺序取第一个

这是提供的表格

经过多次尝试，我成功创建了这段有效的代码

SELECT * FROM (
SELECT CITY, LENGTH(CITY) AS LENGTH_CITY FROM STATION ORDER BY LENGTH_CITY ASC, CITY) WHERE ROWNUM = 1
UNION
SELECT * FROM (
SELECT CITY, LENGTH(CITY) AS LENGTH_CITY FROM STATION ORDER BY LENGTH_CITY DESC, CITY) WHERE ROWNUM = 1;

所以我的问题是，最初我编写了另一段不起作用的代码，我想知道为什么我必须将“WHERE ROWNUM = 1”单独放在 SELECT* 中才能使 UNION 工作？

SELECT CITY, LENGTH(CITY) AS LENGTH_CITY FROM STATION WHERE ROWNUM = 1 ORDER BY LENGTH_CITY ASC, CITY
UNION
SELECT CITY, LENGTH(CITY) AS LENGTH_CITY FROM STATION WHERE ROWNUM = 1 ORDER BY LENGTH_CITY DESC, CITY;

Answer 1

I was wondering why do I have to place the "WHERE ROWNUM = 1" separately in a SELECT* for the UNION to work?

它与UNION无关，但与ORDER BY有关。

您发布的第二个查询肯定不会执行您想要的操作，因为 WHERE 子句在 ORDER BY 之前执行，因此:

select city
from station
where rownum = 1
order by city

说:给我表中的第一个随机行，然后按城市对该行进行排序。

因此:首先对行进行排序，然后取第一个:

select *
from (select city
      from station
      order by city
     )
where rownum = 1

将其应用于您的情况，长度等等。