需要一个SQL查询，该查询将与另一个表保持连接，该表将返回基于时间的最新值，并分组到一行中

我需要一个SELECT SQL查询的帮助，该查询将与id列上的另一个表保持连接，该表将根据时间返回最新的值，并分组为一行

基本上，连接两个表的方式是，对于时间序列表中存在的users表中的每条记录，根据分组到一行中的时间返回最新的值

用户表：

id  |  name
----+--------
 1  | "Joe" 
 2  | "Ron" 

时间序列表：

time| id  |  a  |  b  | c  | d
----+-----+-----+-----+----+----
 1  |  1  |  a1 |     |    |
 2  |  1  |     |  b1 |    |
 3  |  1  |     |     | c0 |
 4  |  1  |  a3 |     | c3 |
 5  |  1  |  a0 |     |    |
 6  |  2  |  a3 |     | c3 | d1
 7  |  2  |  a2 |     |    | d3

结果应该如下所示：

id  |  a  |  b  | c  | d  | name
----+-----+-----+----+----+------
 1  |  a0 |  b1 | c3 |    | "Joe"
 2  |  a2 |     |    | d3 | "Ron"

---

一个选项使用两个子查询：

select u.*,
    (select ts.a from time_series ts where ts.id = u.id and ts.a is not null order by ts.time desc limit 1) a,
    (select ts.b from time_series ts where ts.id = u.id and ts.b is not null order by ts.time desc limit 1) b,
    (select ts.c from time_series ts where ts.id = u.id and ts.c is not null order by ts.time desc limit 1) c,
    (select ts.d from time_series ts where ts.id = u.id and ts.d is not null order by ts.time desc limit 1) d
from users u

另一种解决方案是使用

row\u number（）

对每列的行进行排序，优先考虑非

null

值，然后进行条件聚合：

select u.id,
    max(case when ts.rn_a = 1 then ts.a end) a,
    max(case when ts.rn_b = 1 then ts.b end) a,
    max(case when ts.rn_c = 1 then ts.c end) a,
    max(case when ts.rn_d = 1 then ts.d end) d
from users u
inner join (
    select ts.*,
        row_number() over(order by (case when a is null then 1 else 0 end), time desc) rn_a,
        row_number() over(order by (case when b is null then 1 else 0 end), time desc) rn_b,
        row_number() over(order by (case when c is null then 1 else 0 end), time desc) rn_c,
        row_number() over(order by (case when d is null then 1 else 0 end), time desc) rn_d
    from time_series ts
) ts on ts.id = u.id
group by u.id

---

我认为您正在寻找的函数是

coalesce

SELECT u.id
    , coalesce(a)
    , coalesce(b)
    , coalesce(c)
    , coalesce(d)
    , name
FROM users u 
LEFT JOIN time_series ts
    ON u.id = ts.id
GROUP BY 1, 6
ORDER BY ts.time DESC;