SQL 查询获取列中最常见的值

Question

我有两个表如下 -

销售记录:

    Date    |   Customer   |    ItemSold 
-----------------------------------------
11/01/2013  |     Alex     |     Pen
12/01/2013  |     Rony     |     Paper
13/01/2013  |     Alex     |     Eraser
14/01/2013  |     Marty    |     Eraser
15/01/2013  |     Alex     |     Pen
16/01/2013  |     Rob      |     Paper
17/01/2013  |     Alex     |     Pencil
18/01/2013  |     Alex     |     Pen
19/01/2013  |     Ned      |     Pen
20/01/2013  |     Alex     |     Paper
21/01/2013  |     Alex     |     Pencil
22/01/2013  |     Ned      |     Pen
23/01/2013  |     Alex     |     Eraser
24/01/2013  |     Alex     |     Pen
25/01/2013  |     Alex     |     Pen
26/01/2013  |     Alex     |     Paper
27/01/2013  |     Ned      |     Paper
28/01/2013  |     Alex     |     Pen
29/01/2013  |     Alex     |     Eraser
30/01/2013  |     Alex     |     Pen
31/01/2013  |     Rony     |     Pencil
01/02/2013  |     Alex     |     Eraser
02/02/2013  |     Ned      |     Paper
03/02/2013  |     Alex     |     Pen

优先级:

ItemName    |    Priority
--------------------------
Pen         |       1
Paper       |       2
Pencil      |       3
Eraser      |       4

我想获得一个列表，以了解哪些客户可能会购买以下商品 -

Name   |   Item
----------------
Alex   |   Pen
Rob    |   Paper
Ned    |   Pen
Marty  |   Eraser
Rony   |   Paper

如果项目相同，则应选择优先级最高的项目。 Ned 分别购买了两次笔和纸，但应选择笔，因为它比纸具有更高的优先级。

这个的sql查询是什么？

Answer 1

从统计上看，您正在寻找的术语是模式。这是使用窗口/分析函数计算它的一种方法:

select customer, ItemSold
from (select customer, ItemSold, count(*),
             row_number() over (partition by customer order by count(*) desc, p.priority
                               ) as seqnum
      from sales s left outer join
           priority p
           on s.ItemSold = p.ItemName
      group by customer, ItemSold
     ) ci
where seqnum = 1;