SQL连接多行

Question

我正在使用Teradata,我有一张这样的桌子

ID       String
123      Jim
123      John
123      Jane
321      Jill
321      Janine
321      Johan

我想查询表,所以我得到了

ID       String
123      Jim, John, Jane
321      Jill, Janine, Johan

我试过分区,但可以有很多名字.我如何得到这个结果.甚至,指出我正确的方向将是伟大的.

Answer 1

不幸的是Teradata没有PIVOT(14.10只有TD_UNPIVOT).

如果运气好的话,你的站点会有一个聚合UDF进行组连接(可能性很小).

否则有两种选择:递归或聚合.

如果已知每个id的最大行数,则聚合通常更快.这是很多代码,但大多数是基于剪切和粘贴.

SELECT
  id,
     MAX(CASE WHEN rn = 1 THEN string END)
  || MAX(CASE WHEN rn = 2 THEN ',' || string ELSE '' END)
  || MAX(CASE WHEN rn = 3 THEN ',' || string ELSE '' END)
  || MAX(CASE WHEN rn = 4 THEN ',' || string ELSE '' END)
  || ... -- repeat up to the known maximum
FROM
 (
   SELECT
      id, string, 
      ROW_NUMBER() 
      OVER (PARTITION BY id
            ORDER BY string) AS rn
   FROM t
 ) AS dt
GROUP BY 1;

对于大型表,当您首先使用GROUP BY列作为PI实现Volatile Table中的派生表的结果时,它会更有效.

对于递归,您也应该使用易失性表,因为递归部分中不允许OLAP函数.使用视图将重复计算OLAP功能,从而导致性能不佳.

CREATE VOLATILE TABLE vt AS
 (
   SELECT
      id
      ,string
      ,ROW_NUMBER()
       OVER (PARTITION BY id
             ORDER BY string DESC) AS rn -- reverse order!
      ,COUNT(*)
       OVER (PARTITION BY id) AS cnt
   FROM t
 ) WITH DATA 
UNIQUE PRIMARY INDEX(id, rn)
ON COMMIT PRESERVE ROWS;

WITH RECURSIVE cte
(id, list, rn) AS
 (
   SELECT
      id
      ,CAST(string AS VARCHAR(1000)) -- define maximum size based on maximum number of rows 
      ,rn
   FROM vt
   WHERE rn = cnt

   UNION ALL

   SELECT
      vt.id
      ,cte.list || ',' || vt.string
      ,vt.rn
   FROM vt
   JOIN cte
   ON vt.id = cte.id
   AND vt.rn = cte.rn - 1
)
SELECT id, list
FROM cte
WHERE rn = 1;

这种方法存在一个问题,它可能需要大量的线轴,当您省略时很容易看到WHERE rn = 1.

Answer 2

SELECT ID,  
TRIM(TRAILING ',' FROM (XMLAGG(TRIM(String)|| ',' ORDER BY String) (VARCHAR(10000)))) as Strings
FROM db.table
GROUP BY 1