如何在不同的列中选择第一个、第二个和第三个值 - Ms Access

Question

我在表格中有这些数据：

 ID     ItemID    ItemSupplier
 1        1           E
 2        2           E
 3        2           B
 4        3           B
 5        4           B
 6        4           E
 7        4           C
 8        5         'NULL'
 9        6           C

我想写一个查询来选择它：

ItemID    Supplier1    Supplier2    Supplier3 
  1           E
  2           E            B
  3           B
  4           B            E            C
  5           
  6           C

但我只能得到第一列：

SELECT ItemID, FIRST(ItemSupplier) AS Supplier1
FROM myTable GROUP BY ItemID

谢谢

Answer 1

MS Access 并不是最好的工具。

一种方法使用相关子查询来枚举值，然后进行条件聚合：

select itemid,
       max(iif(seqnum = 1, itemsupplier, null)) as supplier_1,
       max(iif(seqnum = 2, itemsupplier, null)) as supplier_2,
       max(iif(seqnum = 3, itemsupplier, null)) as supplier_3
from (select t.*,
             (select count(*)
              from t as t2
              where t2.itemid = t.itemid and t2.id <= t.id
             ) as seqnum
      from t
     ) as t
group by itemid;

几乎所有其他数据库都支持窗口函数，这将提高效率。