访问组中的前n个

Question

我有一张桌子,我需要为每个类别获得前n个最高金额的项目.

Category Item  InventoryCount
-------  ----- ------------- 
Beverage  milk    3  
Beverage  water   2 
Beverage  beer    9 
Utensil   fork    7 
Utensil   spoon   2 
Utensil   knife   1 
Utensil   spork   4 

我想要的输出是最顶层2类别的最高库存.

Category Item  InventoryCount
-------  ----- ------------- 
Beverage  beer   9 
Beverage  milk   3 
Utensil   fork   7 
Utensil  spork   4 

Answer 1

这应该适合你.如果它不符合您的要求,请回复您需要的内容.你最初的愿望是拥有25,所以你只需要修改最后一个条款HAVING COUNT(*) <= 25

SELECT  a.item, 
        a.category, 
        a.inventorycount, 
        COUNT(*) AS ranknumber
FROM inv AS a 
INNER JOIN inv AS b 
     ON (a.category = b.category) 
     AND (a.inventorycount <= b.inventorycount)
GROUP BY  a.category, 
          a.item, 
          a.inventorycount
HAVING COUNT(*) <= 2
ORDER BY a.category, COUNT(*) DESC

如果要从表中选择更多列,只需将它们添加到SELECT和"GROUP BY"子句中.

只有当你想扩展" 每个类别的顶部n,foo,bar "时,你才会将这些列添加到该INNER JOIN子句中.

--show the top 2 items for each category and year.
SELECT  a.item, 
        a.category, 
        a.year,
        a.inventorycount, 
        COUNT(*) AS ranknumber
FROM inv AS a 
INNER JOIN inv AS b 
     ON (a.category = b.category) 
     AND (a.year = b.year) 
     AND (a.inventorycount <= b.inventorycount)
GROUP BY  a.category, a.item, a.year, a.inventorycount
HAVING COUNT(*) <= 2
ORDER BY a.year, a.category, COUNT(*) DESC