如果产品是较早订购的

【问题标题】：If a product was ordered earlier如果产品是较早订购的
【发布时间】：2021-10-25 22:00:22
【问题描述】：

表：

order_date.     order_id.    customer_id.    product id
2020-01-01      O1.          C1.             P1
2020-01-01      O1.          C1.             P2
2020-01-01      O1.          C1.             P3
2020-01-10      O2.          C1.             P2
2020-01-10      O2.          C1.             P3
2020-01-10      O2.          C1.             P4
2020-01-10      O2.          C1.             P5

我使用了如下代码，但是对于大量数据（比如 10M 行），它会超时。有没有更好的方法来提高此查询的性能？

select a.order_date, a.customer_id, a.order_id, a.product_id, case when b.product_id = a.product_id then 'Y' else 'N' end as prev_purchase_sts
from table a
left join table b on a.customer_id = b.customer_id and a.order_date > b.order_date

【问题讨论】：

标签： sql amazon-redshift window-functions exists self-join

【解决方案1】：

如果您想检查客户之前是否订购过相同的产品，请使用EXISTS：

SELECT a.*, 
       CASE 
         WHEN EXISTS (
                SELECT 1 
                FROM table b 
                WHERE a.customer_id = b.customer_id AND a.product_id = b.product_id AND a.order_date > b.order_date
              ) THEN 'Y' 
         ELSE 'N' 
       END AS prev_purchase_sts
FROM table a;

或者，LAG()窗口函数：

SELECT *, 
       CASE 
         WHEN LAG(order_date) OVER (PARTITION BY customer_id, product_id ORDER BY order_date) IS NOT NULL THEN 'Y' 
         ELSE 'N' 
       END AS prev_purchase_sts
FROM table;

或者，ROW_NUMBER()窗口函数：

SELECT *, 
       CASE 
         WHEN ROW_NUMBER() OVER (PARTITION BY customer_id, product_id ORDER BY order_date) > 1 THEN 'Y' 
         ELSE 'N' 
       END AS prev_purchase_sts
FROM table;

【讨论】：