如果id只有一条记录，则选择一条为null的记录，当id有多条记录时，选择not null值

Question

如果id只有一条记录，我希望得到一条为null的记录，当id有多条记录时，选择not null值

以下是示例示例。

Id Field1 Field2
1  Null    34
1  Yes     52
2  Null    56
3  No      46

和输出

Id Field1 Field2
1  Yes    52
2  Null   56
3  No     46

如何使用sql查询完成？

Answer 1

对2008+版本的sql server使用以下查询。

 ;with cte_1
  As
   ( select *, count(1) over (partition by id order by id) Cnt
     From YourTable)
   Select Id,Field1,Field2
   From Cte_1
   Where Field1 is null and Cnt=1
   UNION
   Select Id,Field1,Field2
   From YourTable 
   Where field1 is not null

示例输出：

使用以下查询2005版。

SELECT t.Id,Field1,Field2
FROM #T t
   JOIN   (select ID, count(ID) CntId
           From #t
           GROUP BY ID
           HAVING COUNT(ID)=1)t1 on t.ID=t1.ID
 WHERE t.Field1 is  null

    UNION 

 SELECT Id,Field1,Field2
 FROM #T 
 WHERE Field1 is NOT NULL
 ORDER BY ID

示例输出：

Answer 2

听起来每个组只能有一行或两行，其中一行必须为null。使用这些假设，您可以通过简单的查询逃脱。

select
    Id,
    min(Field1) as Field1,
    coalesce(min(case when Field1 is not null then Field2 end), min(Field2)) as Field2
from T
group by Id

它还假设Field2不可为空。实际上它比这更微妙，但是如果你需要的话，还有一个解决方法。

使用exists和子查询的解决方案是另一种选择：

select * from T t1
where Field is not null or not exists (
    select 1 from T t2
    where t2.Id = t1.Id and t2.Field is not null
)

Answer 3

使用此代码：

Select Distinct ID,
(Select Max(Field1) From Table1 Where ID=Tbl1.ID) as Field1,
(Select Max(Field1) From Table1 Where ID=Tbl1.ID) as Field2
From Table1 as Tbl1

结果：

ID          Field1     Field2
----------- ---------- -----------
1           Yes        52
2           NULL       56
3           No         46

(3 row(s) affected)

同样在代码下面得到相同的结果：

Select Distinct ID,
(Select Top 1 Field1 From Table1 Where ID=Tbl1.ID  Order By Field1 Desc) as Field1,
(Select Top 1 Field2 From Table1 Where ID=Tbl1.ID Order BY field1 Desc) as Field2
From Table1 as Tbl1