Question

我正在尝试使用窗口函数来获取ColC变为True的第一个日期（按ID分组）并创建一个新列。

id         date           ColC
1        2017-04-20        t
1        2017-04-19        t
1        2017-04-18        t
2        2017-04-20        t
2        2017-04-19        f

因此，我希望得到以下结果：

id       first_date           
1        2017-04-18 
2        2017-04-20

到目前为止，我已经知道使用FIRST_VALUE（日期）（按ID排序按日期分区）或min（）。但是，如何结合条件ColC = t

仍然是一个问题

是否可以用一行获得结果？

提前谢谢！

Answer 1

我很想做：

select min(case when colc = 't' then date end) over (partition by id)

当然，对于您的特定结果，group by更简单：

select id, min(date)
from t
where colc = 't'
group by id;

Answer 2

下面是BigQuery Standard SQL并处理更多通用案例，当ColC可以将其状态从f更改为t并返回f然后再次返回t多次

#standardSQL
WITH yourTable AS (
  SELECT 1 AS id, DATE '2017-04-20' AS date, 't' AS ColC UNION ALL
  SELECT 1, DATE '2017-04-19', 't' UNION ALL
  SELECT 1, DATE '2017-04-18', 't' UNION ALL
  SELECT 2, DATE '2017-04-21', 't' UNION ALL
  SELECT 2, DATE '2017-04-20', 't' UNION ALL
  SELECT 2, DATE '2017-04-19', 'f' UNION ALL
  SELECT 2, DATE '2017-04-18', 't' UNION ALL
  SELECT 2, DATE '2017-04-17', 'f' 
)
SELECT id, date AS switch_date
FROM (
  SELECT 
    id, date, ColC, 
    LAG(ColC) OVER(PARTITION BY id ORDER BY date) AS prevColC
  FROM yourTable
)
WHERE ColC = 't' AND IFNULL(prevColC, 'f') != 't'
-- ORDER BY id, date DESC

Answer 3

怎么样：

select id, min(date) as first_date from tablename where ColC='t' group by id

使用窗口函数的条件最小日期

3 个答案: