SQL Server：使用CTE作为子查询将行压缩为串联字符串

Question

我正在重写一个写得不好的应用程序的几页，我试图将此逻辑作为子查询合并到主查询中，以便我可以一口气获取所有数据，而不必像当前代码那样连接50K次。

我开始与

DECLARE @combinedString NVARCHAR(MAX),@mds   NVARCHAR(MAX)

SELECT 
    @combinedString = COALESCE(@combinedString + ', ', '') + serial,
    @mds = MDS  
FROM 
    (aircraftserials 
LEFT JOIN  
    serialnums ON serialnums.ID = aircraftserials.Serialnum_ID) 
WHERE 
    (85 = aircraftserials.Aircraft_ID AND serial IS NOT NULL)

我想消除这些变量并使它成为子查询，

我认为我需要CTE； 最终结果必须是

MDS, concatenated serials

我已经试过这篇文章，但不能完全理解简化CTE字符串串联吗？

这是数据样本

CREATE TABLE TestBed 
(
     Aircraft_ID INT, 
     Serial VARCHAR(50),
     MDS VARCHAR(50)
)

INSERT INTO TestBed (Aircraft_ID, Serial, MDS) 
VALUES (85, '56-1965', 'T-37B'),
       (85, '56-1967', 'T-37B'),
       (85, '56-3547', 'T-37B'),
       (85, '56-3577', 'T-37B'),
       (85, '57-2265', 'T-37B'),
       (85, '57-2272', 'T-37B'),
       (85, '58-1915', 'T-37B'),
       (85, '58-1925', 'T-37B'),
       (85, '59-0249', 'T-37B'),
       (85, '59-0273', 'T-37B'),
       (85, '59-0299', 'T-37B')

select * from TestBed

Answer 1

我认为您不是在寻找CTE，而是在寻找带有字符串连接的聚合。 在SQL Server中，可以使用子查询FOR XML来完成。

SELECT tb1.aircraft_id,
       stuff((SELECT concat(', ', tb2.serial)
              FROM testbed tb2
              WHERE tb2.aircraft_id = tb1.aircraft_id
                    AND tb2.mds = tb1.mds
              FOR XML PATH ('')), 1, 2, '') serial,
       tb1.mds
       FROM testbed tb1
       GROUP BY tb1.aircraft_id,
                tb1.mds;

分贝<>小提琴

顺便说一句：如果每架飞机的mds始终相同，则它不应在testbed而应在飞机表中。

Answer 2

我根据您的测试数据提出了一个递归CTE

;with ForceId as 
    (SELECT ROW_NUMBER() OVER(partition by MDS order by serial) as RN,* FROM TestBed),
    RECURS as
    (SELECT f.MDS,f.RN,f.Serial, cast(f.serial as nvarchar(max)) as x  FROM ForceId F WHERE F.RN = 1
     UNION ALL
     SELECT f2.mds,f2.rn, r.Serial,r.x  + ', ' + f2.Serial  FROM ForceId F2 
                    JOIN RECURS R
                    ON F2.mds = R.mds 
                    AND R.RN + 1 = F2.rn        
    )
    SELECT MDS, x  FROM 
            recurs r where r.rn = 
            (select max(r2.rn) from recurs r2 where r.MDS = r2.mds)