How to handle recursion in a flat table? - sql

Question

I have two tables that keep track of permissions for groups of users. The first table is just two columns, an identifier and a name, used solely for the names of the permissions. The second table is where the permissions are applied and parent permissions are assigned to create an hierarchy. My problem is that I'm using joins to create a permission hierarchy "string" based on parent permissions and, without knowing how deep that parent recursion might go, I have no way of knowing how many joins to make. My questions is, is there a more correct way to solve this problem?
I've included a complete working script, but I stripped unnecessary columns:
CREATE TABLE #TempPermissions
(
Permission_ID INT IDENTITY,
Permission VARCHAR(50)
)
CREATE TABLE #TempAppPermissions
(
AppPermission_ID INT IDENTITY,
Permission_ID INT,
Parent_ID INT
)
INSERT INTO #TempPermissions VALUES ('Users')
INSERT INTO #TempPermissions VALUES ('Add')
INSERT INTO #TempPermissions VALUES ('Edit')
INSERT INTO #TempPermissions VALUES ('Remove')
INSERT INTO #TempPermissions VALUES ('Permissions')
INSERT INTO #TempPermissions VALUES ('Configure')
INSERT INTO #TempAppPermissions VALUES (1, -1)
INSERT INTO #TempAppPermissions VALUES (2, 1)
INSERT INTO #TempAppPermissions VALUES (3, 1)
INSERT INTO #TempAppPermissions VALUES (4, 1)
INSERT INTO #TempAppPermissions VALUES (5, 1)
INSERT INTO #TempAppPermissions VALUES (6, 5)
SELECT app.AppPermission_ID,
(CASE WHEN NOT child3.Permission IS NULL THEN '/' + child3.Permission ELSE '' END)+
(CASE WHEN NOT child2.Permission IS NULL THEN '/' + child2.Permission ELSE '' END)+
'/' + child1.Permission AS PermissionString
FROM #TempAppPermissions app
INNER JOIN #TempPermissions child1
ON child1.Permission_ID = app.Permission_ID
LEFT JOIN #TempAppPermissions parent1
ON parent1.AppPermission_ID = app.Parent_ID
LEFT JOIN #TempPermissions child2
ON child2.Permission_ID = parent1.Permission_ID
LEFT JOIN #TempAppPermissions parent2
ON parent2.AppPermission_ID = parent1.Parent_ID
LEFT JOIN #TempPermissions child3
ON child3.Permission_ID = parent2.Permission_ID
DROP TABLE #TempPermissions, #TempAppPermissions
This provides me with the results:
AppPermission_ID PermissionString
1 /Users
2 /Users/Add
3 /Users/Edit
4 /Users/Remove
5 /Users/Permissions
6 /Users/Permissions/Configure
This works fine as is, but if I were to go another parent deep with:
INSERT INTO #TempPermissions VALUES ('Reports')
INSERT INTO #TempAppPermissions VALUES (7, 6)
I would have to compensate for it with another set of joins and another case expression in the select statement:
(CASE WHEN NOT child4.Permission IS NULL THEN '/' + child4.Permission ELSE '' END)+
...
LEFT JOIN #TempAppPermissions parent3
ON parent3.AppPermission_ID = parent2.Parent_ID
LEFT JOIN #TempPermissions child4
ON child4.Permission_ID = parent3.Permission_ID
If I do not, I will end up losing the topmost parent on the last result:
1 /Users
2 /Users/Add
3 /Users/Edit
4 /Users/Remove
5 /Users/Permissions
6 /Users/Permissions/Configure
7 /Permissions/Configure/Reports
Technically, I could repeat this any number of times to compensate for how deep that structure may go, but I have the feeling there is probably a better approach this problem. Thanks in advance.

Answer 1

I would use CTE (Common Table Expressions).
;WITH t AS (
SELECT 1 AS iteration, p.Permission_ID AS PermissionID, p.Permission_ID, CAST(N'/' + p.Permission AS NVARCHAR(MAX)) AS Permission
FROM #TempPermissions AS p
UNION ALL
SELECT iteration + 1, t.PermissionID, p.Parent_ID, COALESCE(N'/' + (SELECT s.Permission FROM #TempPermissions AS s WHERE s.Permission_ID = p.Parent_ID), N'') + t.Permission
FROM t INNER JOIN #TempAppPermissions AS p ON t.Permission_ID = p.Permission_ID
)
SELECT PermissionID, Permission FROM t
WHERE Permission_ID = -1
ORDER BY PermissionID, Iteration
Let me know if this helps!

Answer 2

Supplementing the top answer here since I can't post the code in a comment. After playing with JoeFletch's code a bit, I realized the actual recursion should only be happening with the table #TempAppPermissions. With JoeFletch's code, I would hit the maximum recursion of 100 on a larger table. Also the iteration is unimportant.
One thing to note is "SELECT r.AppPermission_ID," on line 13 in the recursion, because I need that child's ID (not the parent's) from #TempAppPermissions to reference back to the user to see if they have that permission. The "Permission_ID" from #TempPermissions is omitted from the select because it is only necessary to get the actual permission name and "Parent_ID" is only used to filter out single instances of dependent permissions.
Thanks again, JoeFletch.
;WITH r AS
(
SELECT p.AppPermission_ID,
p.Parent_ID,
CAST('/' + (SELECT s.Permission
FROM #TempPermissions AS s
WHERE s.Permission_ID = p.Permission_ID)
AS NVARCHAR(MAX)) AS Permission
FROM #TempAppPermissions p
UNION ALL
SELECT r.AppPermission_ID,
p.Parent_ID,
COALESCE(N'/' + (SELECT s.Permission
FROM #TempPermissions AS s
WHERE s.Permission_ID = p.Permission_ID), N'')
+ r.Permission
FROM r
INNER JOIN #TempAppPermissions p
ON p.AppPermission_ID = r.Parent_ID
)
SELECT r.AppPermission_ID, r.Permission
FROM r
WHERE r.Parent_ID = -1
ORDER BY r.AppPermission_ID ASC