show only categories that have products in them

问题

excuse the bad title but I couldn't find a good way to express what I want in abstract terms.

Anyway I have 3 tables

tbl_product:

PID | productname
1   | product 1
2   | product 2
3   | product 3
4   | product 4
..

tbl_categories, motherCategory allows me to nest categories:

CID | categoriename    | motherCategory
1   | electronics      | NULL
2   | clothing         | NULL
3   | Arduino          | 1
4   | Casings, extra's | 3
..

tbl_productInCategory PID and CID are foreign keys to PID and CID in tbl_product and tbl_categories respectively. A product can have multiple categories assigned to it so PID can occur more than once in this table.

PID | CID
1   | 1
2   | 1
3   | 3
4   | 4

Now I have a query that returns all categories if I give the mothercategory. What I want to do is show ONLY the categories that have products in them recursively.

for instance on the example data above I show all categories(motherCategory is null), I want it to return only electronics since there are no products category 2, clothing.

However the problem I am having is that I also want this to work recursively. Consider this tbl_productInCategory:

PID | CID
1   | 2
2   | 2
3   | 2
4   | 4

Now it should return both clothing and electronics even though there are no products in electronics, because there are products in the nested category arduino->Casings, extra's. If I show all categories with motherCategory, electronics it should also return arduino.

I can't figure out how to do this and any help or pointers are appreciated.

回答1:

First you should select all categories where products exist. On the next steps select mother categories.

WITH CTE AS
(
SELECT tbl_categories.*
FROM
tbl_categories
JOIN tbl_productInCategory on tbl_productInCategory.CID = tbl_categories.CID

UNION ALL 

SELECT tbl_categories.*
FROM tbl_categories 
JOIN CTE on tbl_categories.CID = CTE.motherCategory 
)
SELECT DISTINCT * FROM CTE

回答2:

Use a recursive CTE to get a derived table of your category tree, and then INNER JOIN it to your ProductCategory table.

回答3:

It's not something I've done before, but some googling indicates it is possible.

https://technet.microsoft.com/en-us/library/ms186243(v=sql.105).aspx

The semantics of the recursive execution is as follows: Split the CTE expression into anchor and recursive members. Run the anchor member(s) creating the first invocation or base result set (T0). Run the recursive member(s) with Ti as an input and Ti+1 as an output. Repeat step 3 until an empty set is returned. Return the result set. This is a UNION ALL of T0 to Tn.

USE AdventureWorks2008R2;
GO
WITH DirectReports (ManagerID, EmployeeID, Title, DeptID, Level)
AS
(
-- Anchor member definition
    SELECT e.ManagerID, e.EmployeeID, e.Title, edh.DepartmentID, 
        0 AS Level
    FROM dbo.MyEmployees AS e
    INNER JOIN HumanResources.EmployeeDepartmentHistory AS edh
        ON e.EmployeeID = edh.BusinessEntityID AND edh.EndDate IS NULL
    WHERE ManagerID IS NULL
    UNION ALL
-- Recursive member definition
    SELECT e.ManagerID, e.EmployeeID, e.Title, edh.DepartmentID,
        Level + 1
    FROM dbo.MyEmployees AS e
    INNER JOIN HumanResources.EmployeeDepartmentHistory AS edh
        ON e.EmployeeID = edh.BusinessEntityID AND edh.EndDate IS NULL
    INNER JOIN DirectReports AS d
        ON e.ManagerID = d.EmployeeID
)
-- Statement that executes the CTE
SELECT ManagerID, EmployeeID, Title, DeptID, Level
FROM DirectReports
INNER JOIN HumanResources.Department AS dp
    ON DirectReports.DeptID = dp.DepartmentID
WHERE dp.GroupName = N'Sales and Marketing' OR Level = 0;
GO

来源：https://stackoverflow.com/questions/37682384/show-only-categories-that-have-products-in-them