查找分区中的第一个非空值

Question

我正在使用 SQL 服务器。我有一个数据库，其中有一个人和一年（组合创建唯一性），其中另一列（我们称之为已婚状态）具有空值。我想归因于这些空值。我认为由于此列通常不会经常更改，所以我将为该人取下一个非空值，或者如果它位于数据末尾，则取前一个非空值。例如：

人	年	婚姻状况
萌	2001	空
萌	2002	空
萌	2003	已婚
拉里	2001	单身
拉里	2002	空
拉里	2003	空
卷发	2001	单身
卷发	2002	空
卷发	2003	已婚

Moe 的 null 应更改为已婚，Larry 的 null 应更改为单身，Curly 的 null 应更改为已婚。

我的想法是像这样使用 coalesce with over（使用类似的逻辑来选择前面的空值）：

select
    Person,
    Year,
    coalesce(MaritalStatus) over (partition by Person order by Year rows between current row and unbounded following)
from mytable

它似乎不适用于 coalesce。有没有一些简单的方法可以在没有 CTE 或子查询的情况下执行此操作（如果可能，我会尽量避免这种情况，因为这会让下一个人更难理解）。

编辑：根据蒂姆的回答，我想我有一些东西：

cte AS (
    SELECT 
        *, 
        ROW_NUMBER() OVER 
            (PARTITION BY Person,
            CASE WHEN MaritalStatus IS NULL THEN 0 ELSE 1 END
            ORDER BY Year DESC) rn
    FROM mytable
),

cte2 as (
SELECT 
    t1.Person,
    t1.Year,
    max(t2.rn) as maxrn,
    min(t3.rn) as minrn
FROM mytable t1
LEFT JOIN cte t2
    ON t2.Person = t1.Person AND
       t2.MaritalStatus IS NOT NULL and
       t1.year<t2.year
LEFT JOIN cte t3
    ON t3.Person = t1.Person AND
       t3.MaritalStatus IS NOT NULL and
       t1.year>t3.year
group by t1.Person,t1.Year
),

cte3 as(
    select
        t1.person,
        t1.year,
        coalesce(t1.maritalstatus,t4.maritalstatus,t3.maritalstatus) as maritalstatus
    from mytable t1
        left join cte2 t2
            on t1.person=t2.person and
            t1.year=t2.year
        left join cte t3
            on t1.person=t3.person and
            t3.maritalstatus is not null and
            t2.maxrn=t3.rn
        left join cte t4
            on t1.person=t4.person and
            t4.maritalstatus is not null and
            t2.minrn=t4.rn
            
)
select * from cte3

Answer 1

我们可以试试下面的方法。在这里，我们应用 ROW_NUMBER 对人进行分区，也对婚姻状况值是否为 NULL 进行分区。然后，我们使用每人最近的非 NULL 婚姻状况值来填写任何 NULL 缺失的婚姻状况值。

WITH cte AS (
    SELECT *, ROW_NUMBER() OVER (
                  PARTITION BY Person,
                               CASE WHEN MaritalStatus IS NULL THEN 0 ELSE 1 END
                  ORDER BY Year DESC) rn
    FROM mytable
)

SELECT t1.Person, t1.Year, COALESCE(t1.MaritalStatus, t2.MaritalStatus) AS MaritalStatus
FROM mytable t1
LEFT JOIN cte t2
    ON t2.Person = t1.Person AND
       t2.MaritalStatus IS NOT NULL AND
       t2.rn = 1;

Demo

Answer 2

您只能使用 window 函数来完成此操作。关键想法是获得有婚姻状况的第一年。然后将当年的婚姻状况散布在所有行上：

SELECT t.*,
       MAX(CASE WHEN year = first_year_ms THEN MaritalStatus END) OVER (PARTITION BY person) as first_marital_status
FROM (SELECT t.*,
             MIN(CASE WHEN MaritalStatus IS NOT NULL THEN year END) OVER (PARTITION BY person) as first_year_ms
      FROM t
     ) t
ORDER BY person, year;

一个更简单的方法可能是使用横向连接：

select *
from t outer apply
     (select top (1) t2.maritalstatus
      from t t2
      where t2.person = t.person and t2.maritalstatus is not null
      order by t2.year asc
     ) t2;

使用 (person, maritalstatus, year) 上的索引，这可能是最快的方法。

Here 是一个 db<>fiddle.

编辑：

您仍然可以使用 window 函数执行此操作：

SELECT t.*,
       COALESCE(MAX(MaritalStatus) OVER (PARTITION BY person, grp_after),
                MAX(MaritalStatus) OVER (PARTITION BY person, grp_before)
               ) as next_marital_status
FROM (SELECT t.*,
             COUNT(MaritalStatus) OVER (PARTITION BY person ORDER BY year DESC) as grp_after,
             COUNT(MaritalStatus) OVER (PARTITION BY person ORDER BY year ASC) as grp_before
      FROM t
     ) t
ORDER BY person, year;

或使用apply:

select *
from t outer apply
     (select top (1) t2.maritalstatus
      from t t2
      where t2.person = t.person and t2.maritalstatus is not null
      order by (case when t2.year >= t.year then 1 else 2 end),
               (case when t2.year >= t.year then t2.year end) asc,
               t2.year desc
     ) t2

Here 是 SQL Fiddle

查找分区中的第一个非空值

Finding the first non-null in a partition

sql

sql-server

tsql

coalesce

Demo