在postgres中轉發（或向後填充）

問題是在表中填充缺失值。在pandas，可以使用向前（或向後）填充這樣做，如下圖所示：在postgres中轉發（或向後填充）

$> import pandas as pd 
$> df = pd.DataFrame({'x': [None, 1, None, None, 2, None, 3, None]}) 
$> df['y'] = df['x'].fillna(method='ffill') 
$> df 
    x y 
0 NaN NaN 
1 1 1 
2 NaN 1 
3 NaN 1 
4 2 2 
5 NaN 2 
6 3 3 
7 NaN 3

有沒有辦法做到這一點的SQL和更精確的Postgres裏？我想window functions可以幫助，但我無法弄清楚如何。

Postgres裏，這將是這樣的：

sandbox=# SELECT x, ?? 
FROM 
    (SELECT NULL AS x 
    UNION ALL SELECT 1 AS x 
    UNION ALL SELECT NULL AS x 
    UNION ALL SELECT NULL AS x 
    UNION ALL SELECT 2 AS x 
    UNION ALL SELECT NULL AS x 
    UNION ALL SELECT 3 AS x 
    UNION ALL SELECT NULL AS x) a; 
x 
--- 

1 


2 

3 

(8 rows)

來源

2016-06-18 Guillaume Thomas

這裏是表？你想填寫什麼樣的缺失值？ –

我編輯它，我忘記了一個重要的熊貓行，它使用fillna向前填充，我想在postgres –

上重現，你將需要在這裏的功能 - 如果你只是'滯後'窗口功能 - 你會得到前一個x，螺母不在旁邊。換句話說，你需要某種類型的RECURSIVE LAG，重複以前的值，但是之前定義的值 –

window functions here

這麼多的別名，因爲你的查詢非常sencitive訂購。我增加了更多的空x線，以證明它是修剪幾個空行...

select x,y from (
select r,x, case when y is not null then y else min(y) over (partition by x order by r) end y from (
SELECT row_number() over() r,x, case when x is not null then x else lag(x) over() end y 
FROM 
    (SELECT NULL AS x 
    UNION ALL SELECT 1 AS x 
    UNION ALL SELECT NULL AS x 
    UNION ALL SELECT NULL AS x 
    UNION ALL SELECT NULL AS x 
    UNION ALL SELECT NULL AS x 
    UNION ALL SELECT 2 AS x 
    UNION ALL SELECT NULL AS x 
    UNION ALL SELECT 3 AS x 
    UNION ALL SELECT NULL AS x 
    ) a 
    ) b 
order by r 
    ) c 
    ;

來源

2016-06-18 15:35:56

在postgres中轉發（或向後填充）

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