Postgres：如何根据操作结果排序时获取行号？

Question

让我们假设有一个表'items'有三列：'id'，'pos'和'neg'，选择的结果应该按操作结果pos - neg排序。

所以，以下是这样运作的：

SELECT 
    id, 
    pos - neg AS diff 
FROM items 
ORDER BY diff DESC 

现在我需要获取特定的行（表）的位置排序的“差异”的结果。我试过这个：

WITH summary AS (
    SELECT 
     i.id, 
     i.pos - i.neg AS diff, 
     ROW_NUMBER() OVER(ORDER BY diff) AS position 
    FROM items i) 
SELECT s.* FROM summary s WHERE s.id = 351435254 

但是执行返回ERROR：column“diff”不存在。

那么，有可能获得这个位置，或者将差异保存在一个单独的列中会更好吗？

Answer 1

尝试：

WITH summary AS (
    SELECT 
     i.id, 
     i.pos - i.neg AS diff, 
     ROW_NUMBER() OVER(ORDER BY (i.pos - i.neg)) AS position 
    FROM items i) 
SELECT s.* FROM summary s WHERE s.id = 351435254 

Answer 2

为了避免重复计算，只需移动ROW_NUMBER()到外部查询...

WITH summary AS (
    SELECT 
     i.id, 
     i.pos - i.neg AS diff  
    FROM 
     items i 
) 
SELECT 
    s.*, 
    ROW_NUMBER() OVER(ORDER BY s.diff) AS position 
FROM 
    summary s 
WHERE 
    s.id = 351435254 

在内部查询，以diff引用将范围限制于SELECT之前存在，否则您可以获得循环引用。

通过外部查询新字段diff存在，并且因此可以作为参数来其他计算，函数被供给的时间等