SQL 覆盖分组依据中列中的值

Question

假设我有关注 sql-table

fruit | amount | date
-------------------------
apple |    2   | 2019
apple |    3   | 2018
apple |    2   | 
peach |    2   | 
peach |    3   | 2017

我想用每个水果的最早日期填充日期列中的空值，以便日期列不包含空值。结果应如下所示：

fruit | amount | date
-------------------------
apple |    2   | 2019
apple |    3   | 2018
apple |    2   | 2018
peach |    2   | 2017
peach |    3   | 2017

知道如何在 sql 中执行此操作吗？

Answer 1

您可以使用 window 函数：

select t.*,
       coalesce(date, min(date) over (partition by fruit)) as imputed_date
from t;

Answer 2

执行此无分析函数的一种方法是使用相关子查询：

SELECT
    fruit,
    amount,
    COALESCE(date, (SELECT MIN(date) FROM yourTable t2 WHERE t2.fruit = t1.fruit)) date
FROM yourTable t1;

我们也可以使用聚合连接方法：

SELECT
    t1.fruit,
    t1.amount,
    COALESCE(t1.date, t2.min_date) AS date
FROM yourTable t1
INNER JOIN
(
    SELECT fruit, MIN(date) AS min_date
    FROM yourTable
    GROUP BY fruit
) t2
    ON t2.fruit = t1.fruit;

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