Window PostgreSQL 中的函数

Question

我有以下 table:

id username
1   Jack
2   Will
3   Grace
4   Niv

我想编写一个 3 列查询，它根据字典顺序显示用户名、之前的用户名和之后的用户名。

含义：

before username  after
       Grace     Jack
Grace  Jack      Niv
Jack   Niv       Will
Nive   Will

我写了这个查询：

select lag(username,1) over (partition by username order by username ) as before,
    username,
    lead(username,1) over (partition by username order by username ) as after
from users
order by username

但是没用。它仅在用户名列中显示数据。我做错了什么？

Answer 1

您应该删除 PARTITION BY:

SELECT
    LAG(username, 1) OVER (ORDER BY username) AS before,
    username,
    LEAD(username, 1) OVER (ORDER BY username) AS after
FROM users
ORDER BY username;

DBFiddle Demo

如果 users(username) 中有重复项，您可以添加 DISTINCT:

SELECT ...
FROM (SELECT DISTINCT username FROM users) AS sub
ORDER BY ...;

编辑：

But it doesnt work. It shows me data only in the username column. What am I doing wrong?

当您使用分区依据时，您基本上将行分成组，这些组共享 PARTITION BY 表达式的相同值。所以你的 window 只包含一个值。这就是你得到 NULL 的原因（没有 prior/next 值）。

DBFidde Demo2

DBFiddle Demo3 with duplicates per window

Answer 2

不使用 window 函数的另一种方法

select a.username, 
                  (select min(username) from users b
                    where b.username > a.username) as after,
                   (select max(username) from users c
                   where c.username < a.username) as before
 from users a
order by username;

Window PostgreSQL 中的函数

Window functions in PostgreSQL

sql

postgresql

window-functions