SQL

Question

你好所以我的问题基本上是，我有一个 table files 像这样：

foreign_key	string	created_at
1	"a"	2021-11-10
2	"b"	2021-11-15
2	"c"	2021-11-18

现在我需要的是 select 每个不同的 foreign_key 一个记录（字符串）并且它总是需要是 最新的 （created_at)

我不能在select秒内使用子查询因为速度。

结果应如下所示：

foreign_key	string	created_at
1	"a"	2021-11-10
2	"c"	2021-11-18

Answer 1

使用ROW_NUMBER:

WITH cte AS (
    SELECT t.*, ROW_NUMBER() OVER (PARTITION BY foreign_key
                                   ORDER BY created_at DESC) rn
    FROM yourTable t
)

SELECT foreign_key, string, created_at
FROM cte
WHERE rn = 1;

另一种方法，使用连接：

SELECT t1.*
FROM yourTable t1
INNER JOIN
(
    SELECT foreign_key, MAX(created_at) AS max_created_at
    FROM yourTable
    GROUP BY foreign_key
) t2
    ON t2.foreign_key = t1.foreign_key AND
       t2.max_created_at = t1.created_at;

SQL - select 每个外键只有最新记录

SQL - select only latest record for each foreign key

greatest-n-per-group