我可以在 SQL SELECT 查询中使用 WHILE 循环吗

Question

我需要从两个不具有相同元素数的表的 SQL 查询中创建包含偶数列的格式化文本 (CSV) 输出。我似乎无法将数据填充到所需的列数。

样本数据

table Students(id, name)
values
    (1, Alex),
    (2, Bob),
    (3, Charlie),
    (4, David)

table Hobbies (studentId, hobby)
values
    (2,'skating')
    (2,'sailing')
    (3,'reading')
    (4,'video games')
    (4,'paintball')
    (4,'nascar')
    (4,'baseball')

期望的输出

[ID][OutputString]
[0][Student,Hobby1,Hobby2,Hobby3,Hobby4]
[1][Alex,,,,]
[2][Bob,skating,sailing,]
[3][Charlie,reading,,,]
[4][David,video games,paintball,nascar,baseball]

损坏的伪代码

DECLARE @maxHobbies INT = 4;
DECLARE @hobbyCount INT = 1;

DECLARE headerString varchar(max) = 'Student';
WHILE @hobbyCount <= @maxHobbies
BEGIN
    set @headerString = @headerString + ',Hobby' + @hobbyCount
    @hobbyCount = @hobbyCount + 1
END

DECLARE @outputString varchar(max)
SET @outputString = concat(@headerSting, char(13)) +
SELECT 0 AS ID, @headerString AS OutputString
UNION 
SELECT
    id,
    name + (WHILE @hobbyCount <= @maxHobbies BEGIN
        (CASE
            WHEN EXISTS SELECT hobby FROM Hobbies where studentId = Students.id
            THEN ',' + SELECT hobby FROM Hobbies where studentId = Students.id
            ELSE ','
        END)
    END)
FROM Students

Answer 1

您最多需要 4 个爱好。您可以使用条件聚合来做到这一点：

select s.id,
       (s.name + ',' +
        max(case when seqnum = 1 then h.hobby else '' end) + ',' +
        max(case when seqnum = 2 then h.hobby else '' end) + ',' +
        max(case when seqnum = 3 then h.hobby else '' end) + ',' +
        max(case when seqnum = 4 then h.hobby else '' end)
       ) as outputstring
from students s left join
     (select h.*,
             row_number() over (partition by h.studentid order by (select null)) as seqnum
      from hobbies h
     ) h
     on h.studentid = s.id
group by s.id, s.name;