的BigQuery：獲取行+信息有關最近的前面列，其中列x具有前一行一些價值

Question

假設我有一個表與下面的架構

name  | type 
---------------------- 
id   | STRING 
timestamp | TIMESTAMP 
event_type | STRING 
some_value | STRING 
... 

我想'x'類型的所有事件。但是，我還希望爲每個返回的行添加一個附加參數。如果最近的事件WHERE event_type='y'具有some_value='necessary value'，此參數是一個布爾值，應該是TRUE。

例如，假設下面的行按時間戳升序排序：

event_type | some_value 
------------------------ 
y   | 'true value' 
x   | 'not relevant' 
y   | 'false value' 
x   | 'not relevant 2' 
y   | 'true value' 
y   | 'false value' 
x   | 'not relevant3' 
x   | 'not relevant4' 

我會得到下面的行回從我的查詢：

event_type | some_value  | previous_true 
------------------------------------- 
x   | 'not relevant' | TRUE 
x   | 'not relevant2' | FALSE 
x   | 'not relevant3' | FALSE 
x   | 'not relevant4' | FALSE 

我以爲加入可能做的伎倆但我無法弄清楚這將如何工作。起初，LAG也似乎是一個好主意，但後來我意識到LAG將採取前一行，不管它是什麼，我不知道我將如何使用它。

Answer 1

與BigQuery標準SQL - 試試下面
（確保取消Use Legacy SQL複選框下顯示選項）

WITH YourTable AS (
    SELECT 1 AS ts, 'y' AS event_type, 'true value' AS some_value UNION ALL 
    SELECT 2 AS ts, 'x' AS event_type, 'not relevant' AS some_value UNION ALL 
    SELECT 3 AS ts, 'y' AS event_type, 'false value' AS some_value UNION ALL 
    SELECT 4 AS ts, 'x' AS event_type, 'not relevant2' AS some_value UNION ALL 
    SELECT 5 AS ts, 'y' AS event_type, 'true value' AS some_value UNION ALL 
    SELECT 6 AS ts, 'y' AS event_type, 'false value' AS some_value UNION ALL 
    SELECT 7 AS ts, 'x' AS event_type, 'not relevant3' AS some_value UNION ALL 
    SELECT 8 AS ts, 'x' AS event_type, 'not relevant4' AS some_value 
) 
SELECT 
    event_type, 
    some_value, 
    (SELECT some_value = 'true value' FROM YourTable 
    WHERE event_type = 'y' AND ts < a.ts 
    ORDER BY ts DESC LIMIT 1 
    ) AS previous_true 
FROM YourTable AS a 
WHERE event_type = 'x' 
ORDER BY ts 

結果是：

event_type some_value  previous_true  
x   not relevant true  
x   not relevant2 false  
x   not relevant3 false  
x   not relevant4 false  

對於傳統的BigQuery SQL - 嘗試

SELECT 
    event_type, some_value, 
    previous_true = 'true value' AS previous_true 
FROM (
    SELECT 
    ts, event_type, some_value, 
    FIRST_VALUE(some_value) OVER(PARTITION BY grp ORDER BY ts) AS previous_true 
    FROM (
    SELECT 
     ts, event_type, some_value, 
     SUM(step) OVER(ORDER BY ts) AS grp 
    FROM (
     SELECT 
     ts, event_type, some_value, 
     IF(event_type = 'x' , 0, 1) AS step 
     FROM 
     (SELECT 1 AS ts, 'y' AS event_type, 'true value' AS some_value), 
     (SELECT 2 AS ts, 'x' AS event_type, 'not relevant' AS some_value), 
     (SELECT 3 AS ts, 'y' AS event_type, 'false value' AS some_value), 
     (SELECT 4 AS ts, 'x' AS event_type, 'not relevant2' AS some_value), 
     (SELECT 5 AS ts, 'y' AS event_type, 'true value' AS some_value), 
     (SELECT 6 AS ts, 'y' AS event_type, 'false value' AS some_value), 
     (SELECT 7 AS ts, 'x' AS event_type, 'not relevant3' AS some_value), 
     (SELECT 8 AS ts, 'x' AS event_type, 'not relevant4' AS some_value) 
    ) 
) 
) 
WHERE event_type = 'x' 
ORDER BY ts 

Answer 2

以下是一種方法：您可以對「y」使用最大掃描來獲取每個「x」的最近y的id。然後用join進行計算：

select t.*, 
     (case when some_value = 'necessary value' then 1 else 0 end) as previous_true 
from (select t.*, 
      max(case when event_type = 'y' then id end) over (order by timestamp) as yid 
     from t 
    ) t join 
    t ty 
    on ty.id = t.yid 
where t.event_type = 'x'; 

我不知道有關的id和timestamp的確切作用。此版本假定id相對於timestamp一律增加。或者，您可以使用timestamp - 但不清楚這是否足以滿足join。