问题
I have a table where I need to get the top n highest amount items for each Category.
Category Item InventoryCount ------- ----- ------------- Beverage milk 3 Beverage water 2 Beverage beer 9 Utensil fork 7 Utensil spoon 2 Utensil knife 1 Utensil spork 4
My desired output is the highest Inventory of the topmost 2 Categories.
Category Item InventoryCount ------- ----- ------------- Beverage beer 9 Beverage milk 3 Utensil fork 7 Utensil spork 4
回答1:
This should work for you. If it doesn't satisfy your requirements, post back what you need.
Your original desire was to have 25, so you'd simply modify the last clause to be 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
If you wanted to select more columns from the table, simply add them to the SELECT and `GROUP BY' clauses.
Only when you want to expand the "TOP n for each Category, foo, bar", then you would add those columns to the INNER JOIN clause as well.
--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
回答2:
If the order is not important:
SELECT * FROM inv
ORDER BY InventoryCount DESC LIMIT 5
来源：https://stackoverflow.com/questions/3481916/access-top-n-in-group