sql - 如何查找只有一个值的列 - Postgresql

Question

我有 2 个表，person(email, first_name, last_name, postcode, place_name) 和 location(postcode, place_name)。我试图找到住在只有一个人居住的地方的人。我尝试使用 SELECT COUNT() 但失败了，因为我不知道在这种情况下要计算什么。

SELECT DISTINCT email,
                first_name,
                last_name
FROM person
INNER JOIN location USING(postcode,
                          place_name)
WHERE 1 <=
    (SELECT COUNT(?))

Answer 1

聚合函数总是伴随着having:

SELECT DISTINCT first_value(email) over (partition by place_name),
                first_value(first_name) over (partition by place_name),
                first_value(last_name) over (partition by place_name),
                count(*)
FROM person
INNER JOIN location USING(postcode,
                          place_name)
GROUP BY place_name
HAVING count(*) = 1

有关窗口函数(如 first_value)的更多信息，请查看 this tutorial .