How to retrieve desired records? - sql

Question

Input Table : Regions
+---------------+---------------+---------- +-----------+
| Child | Parent | Level | levelname|
+---------------+---------------+---------- +-----------+
| All Region | All Region | 1 | national |
| Africa Region | All Region | 2 | region |
| America | All Region | 2 | region |
| Asia | All Region | 2 | region |
| Europe Region | All Region | 2 | region |
| Africa | Africa Region | 3 | Subregion |
| Asia Pacific | Asia | 3 | Subregion |
| Europe | Europe Region | 3 | Subregion |
| North America | America | 3 | Subregion |
| South America | America | 3 | Subregion |
| Argentina | South America | 4 | Country |
| Australia | Asia Pacific | 4 | Country |
| Pakistan | Asia Pacific | 4 | Country |
| South Africa | Africa | 4 | Country |
| Tunisia | Africa | 4 | Country |
| Uruguay | South America | 4 | Country |
+-------------------------------------------------------+
Here , regions are of 4 levels
All region
Region
Sub Region
Country
they have 0,1,2 and 3 ancestors,such as a country has subregion,region and allregion as ancestors ,suppose we give "Uruguay" ,then output will be South America, America , All Region.
Now, I need a query for this table,which will retrieve all ancestors for a given "child"

Answer 1

Your best bet is a recursive CTE:
With recRegions AS
(
/*Recursive Seed*/
SELECT
Child,
Parent,
Level,
0 as Depth,
CAST(Child as VARCHAR(5000)) as Path
FROM
Regions
WHERE
Child=<WhateverChildYouAreWanting>
UNION ALL
/*Recursive Term*/
SELECT
Regions.Child,
Regions.Parent,
Region.Level,
recRegions.Depth + 1,
recRegions.Path || '>' || Region.Child
FROM
recRegions
INNER JOIN Regions on
recRegions.parent = Regions.Child
Where recRegions.Depth < 10
)
Select Parent as Ancestors FROM recRegions;
Recursive queries can be a little tricky to wrap your head around at first, but if you break up the pieces of it, it makes sense:
Recursive Seed - This is the part where we get the first term we are after. In your case, we just want the record where the Child is the country you are wanting to query.
Recursive Term - This is the part where the query refers back to itself. It joins the recursive CTE recRegions to your Region table, connecting the child to the parent. The DB will hit this recursive term until no more records come back, which means we've climbed all the way up your hierarchy.
The final select statement just pulls back the records from your recursive query. You wanted all the ancestors, so that would be all of the Parent field records.
Generally when you see a table with a layout child | parent | attributes | of | that | relationship you can turn to the super powerful recursive CTE to make quick sense out of it all.
As #dnoeth mentioned in your Q's comments, you could also join Regions table to itself 4 times since your hierarchy seems to be only 4 deep. A recursive query doesn't care about depth though, so if you add more depth to your hierarchy, you won't have to edit your SQL to pull the ancestors.
Updated to add a "Depth" field to track recursions and stop after 10. Also added a "Path" field to track the hierarchy as it's built up from the Child. If you have an issue with your hierarchy cycling (a child reporting to a parent that reports to the child causing an endless loop) then you can use the following SQL statement instead of the SELECT parent FROM version above:
SELECT * FROM recRegions;
Now you will see Path and Depth of each node of your hierarchy so you can fix your data, or the recursive CTE to avoid the cycling.