从 Postgres 表中选择列的最新值

Question

我在 Postgres 表中有一些数据，如下所示：

Name | Date      | Balance
--------------------------
A    |2020-01-01 |    1
B    |2020-01-01 |    0
B    |2020-01-02 |    2
A    |2020-01-03 |    5

（请注意，A 缺少 2020-01-02 和 B 的值 2020-01-03）

我想用该名称的最新值填写缺失的日期。换句话说，我想

Name | Date      | Balance
--------------------------
A    |2020-01-01 |    1
B    |2020-01-01 |    0
A    |2020-01-02 |    1 <--- filled in with previous balance
B    |2020-01-02 |    2
A    |2020-01-03 |    5
B    |2020-01-03 |    2 <--- filled in with previous balance

请注意，实际上，可能会连续缺少多个日期，在这种情况下，应始终选择该名称的最新日期。

Answer 1

我在想generate_series() 和窗口函数：

select 
    n.name, 
    s.date, 
    coalesce(t.balance, lag(balance) over(partition by n.name order by s.date) balance
from (select generate_series(min(date), max(date), interval '1 day') date from mytable) s
cross join (select distinct name from mytable) n
left join mytable t on t.name = n.name and t.date = s.date
order by n.name, s.date

如果您可能连续丢失多个日期，则需要更多逻辑 - 这基本上模拟了 lag() 与 ignore nulls 选项：

select
    name,
    date,
    coalesce(balance, first_value(balance) over(partition by name, grp)) balance
from (
    select 
        n.name, 
        s.date, 
        t.balance,
        sum( (t.balance is not null)::int ) over(partition by n.name order by s.date) grp
    from (select generate_series(min(date), max(date), interval '1 day') date from mytable) s
    cross join (select distinct name from mytable) n
    left join mytable t on t.name = n.name and t.date = s.date
) t
order by name, date