SQL Server 2008 R2：从列中获取第一个非空值答案

【问题标题】：SQL Server 2008 R2: Get first not null values from columnSQL Server 2008 R2：从列中获取第一个非空值
【发布时间】：2017-12-04 05:49:23
【问题描述】：

我有以下表格来显示预期结果。

表格：

CREATE TABLE Family
(
    ID int,
    Name varchar(20),
    Gender char(1)
);

INSERT INTO Family VALUES
(1,'Ram','M'),
(2,'Suraj','M'),
(3,'Sunitha','F'),
(4,'Deepika','F'),
(5,'Minakshi','F'),
(6,'Somu','M');

CREATE TABLE Child_parent
(
    Child_ID int,
    Parent_ID int
);

INSERT INTO Child_parent VALUES
(1,2),
(1,3),
(4,5),
(4,6);

预期结果：

Child_ID    ChildName   FatherName  MotherName
-----------------------------------------------
1           Ram         Suraj       Sunitha
4           Deepika     Somu        Minakshi

我的尝试：

SELECT  cp.Child_ID,
        f1.Name as ChildName,
        CASE WHEN f2.Gender = 'M' THEN f2.Name END FatherName,
        CASE WHEN f2.Gender = 'F' THEN f2.Name END MotherName
FROM Child_parent cp
LEFT JOIN Family f1
ON cp.Child_ID = f1.ID
LEFT JOIN Family f2
ON cp.Parent_ID = f2.ID

但是得到的结果显示：

Child_ID    ChildName   FatherName  MotherName
----------------------------------------------
1           Ram         Suraj       NULL
1           Ram         NULL        Sunitha
4           Deepika     NULL        Minakshi
4           Deepika     Somu        NULL

【问题讨论】：

标签： sql sql-server sql-server-2008-r2

【解决方案1】：

试试这个

SELECT  cp.Child_ID,
        f1.Name as ChildName,
        max(CASE WHEN f2.Gender = 'M' THEN f2.Name END) FatherName,
        max(CASE WHEN f2.Gender = 'F' THEN f2.Name END) MotherName
FROM Child_parent cp
LEFT JOIN Family f1
ON cp.Child_ID = f1.ID
LEFT JOIN Family f2
ON cp.Parent_ID = f2.ID
group by cp.Child_ID,
        f1.Name 
 order by cp.Child_ID

【讨论】：

【解决方案2】：

这是使用单个 JOIN 的替代解决方案。

SELECT CP.Child_ID, 
    MAX(CASE WHEN CP.Parent_ID <> F.ID THEN F.Name END) ChildName,
    MAX(CASE WHEN CP.Parent_ID = F.ID AND F.Gender ='M' THEN F.Name END) FatherName,
    MAX(CASE WHEN CP.Parent_ID = F.ID AND F.Gender ='F' THEN F.Name END) MotherName
FROM 
    Child_parent CP
    LEFT JOIN Family F ON CP.Parent_ID = F.ID OR CP.Child_ID = F.ID
GROUP BY 
    Child_ID

【讨论】：