Stop Recursion Once Condition Satisfied - sql

Question

I am using a CTE to explode out a Bill of Materials and need to locate all those materials that have recursive components.
What I was attempting, was to limit the number of cycles (levels) deep, by setting BOM_Level in the child node to a maximum bound:
exec pr_sys_drop_object '#BOMExploded'
;with BOM
as
(
select
Prod_Plant_CD
, Demand_Plant_CD
, material_cd
, component_cd
, component_quantity
, component_quantity AS Calculated_Component_Quantity
, BOM_Level
, Demand_Quantity
, CONVERT(float,1) AS Produced_Ratio
, Material_CD AS Demand_Material_CD
from #firstLevel a
UNION ALL
SELECT
b.Plant_CD as 'Prod_Plant_CD'
, a.Demand_Plant_CD
, b.Material_CD
, b.Component_CD
, b.component_quantity
, b.component_quantity
, a.BOM_Level + 1
, a.Demand_Quantity
, a.Produced_Ratio * a.Component_Quantity -- Produced Quantity for the current level = Produced Quantity (level -1) * Component_Quantity (level -1)
, a.Demand_Material_CD
FROM BOM a
inner join #BOM_ProdVersion_Priority b
on a.component_cd = b.material_cd
inner join #base_routes c
on a.Demand_Plant_CD = c.Recipient_Plant_CD
and b.Plant_CD = c.Source_Plant_CD
and c.Material_CD = b.Material_CD -- Need to have material_cd to link
where b.Material_CD != b.Component_CD
and b.Component_Quantity > 0
and BOM_Level < 5 -- Give the max number of levels deep we are allowed to cyncle to
)
select *
into #BOMExploded
from BOM
OPTION (MAXRECURSION 20)
Using this method however, would require a post-process to locate when the cycling on the recursive component level started, then back trace.
How can a CTE recursive query be stopped given a certain condition?
ie. when top-level material_cd = component_cd for a deeper BOM_Level

Answer 1

If I understand you correctly, you don't need to stop at a certain depth/level, or rather you want to stop at a certain level, but you also need to stop in case you start cycling through materials repeatedly.
In the case of the following recursive path: mat_1->mat_2->mat_3->mat_1, you would want to stop before that last mat_1 starts cycling again to mat_2 and so on.
If that's correct, then your best bet is to add a Path field to your recursive query that tracks each term that it finds as it moves down the recursive path:
exec pr_sys_drop_object '#BOMExploded'
;with BOM
as
(
select
Prod_Plant_CD
, Demand_Plant_CD
, material_cd
, component_cd
, component_quantity
, component_quantity AS Calculated_Component_Quantity
, BOM_Level
, Demand_Quantity
, CONVERT(float,1) AS Produced_Ratio
, Material_CD AS Demand_Material_CD
, CAST(material_cd AS VARCHAR(100)) AS Path
from #firstLevel a
UNION ALL
SELECT
b.Plant_CD as 'Prod_Plant_CD'
, a.Demand_Plant_CD
, b.Material_CD
, b.Component_CD
, b.component_quantity
, b.component_quantity
, a.BOM_Level + 1
, a.Demand_Quantity
, a.Produced_Ratio * a.Component_Quantity -- Produced Quantity for the current level = Produced Quantity (level -1) * Component_Quantity (level -1)
, a.Demand_Material_CD
, a.Path + '|' + b.material_cd
FROM BOM a
inner join #BOM_ProdVersion_Priority b
on a.component_cd = b.material_cd
inner join #base_routes c
on a.Demand_Plant_CD = c.Recipient_Plant_CD
and b.Plant_CD = c.Source_Plant_CD
and c.Material_CD = b.Material_CD -- Need to have material_cd to link
where b.Material_CD != b.Component_CD
and b.Component_Quantity > 0
and BOM_Level < 5 -- Give the max number of levels deep we are allowed to cyncle to
and a.path NOT LIKE '%' + b.material_cd + '%'
)
select *
into #BOMExploded
from BOM
OPTION (MAXRECURSION 20)
Now you have a path that is pipe delimited and you can test your current material_cd to see if it's already in the path. If it is, then you end that leg of the recursion to prevent cycling.
Updating to include a version where we capture material_cd cycles and only report those at the end of the recursion:
exec pr_sys_drop_object '#BOMExploded'
;with BOM
as
(
select
Prod_Plant_CD
, Demand_Plant_CD
, material_cd
, component_cd
, component_quantity
, component_quantity AS Calculated_Component_Quantity
, BOM_Level
, Demand_Quantity
, CONVERT(float,1) AS Produced_Ratio
, Material_CD AS Demand_Material_CD
, CAST(material_cd AS VARCHAR(100)) AS Path
, CAST(NULL AS CHAR(5)) as Cycle_Flag
, 0 as Cycle_Depth
from #firstLevel a
UNION ALL
SELECT
b.Plant_CD as 'Prod_Plant_CD'
, a.Demand_Plant_CD
, b.Material_CD
, b.Component_CD
, b.component_quantity
, b.component_quantity
, a.BOM_Level + 1
, a.Demand_Quantity
, a.Produced_Ratio * a.Component_Quantity -- Produced Quantity for the current level = Produced Quantity (level -1) * Component_Quantity (level -1)
, a.Demand_Material_CD
, a.Path + '|' + b.material_cd
, CASE WHEN a.path NOT LIKE '%' + b.material_cd + '%' then 'Cycle' END AS Cycle_Flag,
, CASE WHEN a.path NOT LIKE '%' + b.material_cd + '%' then a.Cycle_Depth + 1 END as Cycle_Depth
FROM BOM a
inner join #BOM_ProdVersion_Priority b
on a.component_cd = b.material_cd
inner join #base_routes c
on a.Demand_Plant_CD = c.Recipient_Plant_CD
and b.Plant_CD = c.Source_Plant_CD
and c.Material_CD = b.Material_CD -- Need to have material_cd to link
where b.Material_CD != b.Component_CD
and b.Component_Quantity > 0
and a.cycle_depth < 2 --stop the query if we start cycling, but only after we capture at least one full cycle
)
select *
into #BOMExploded
from BOM
WHERE cycle_flag IS NOT NULL
OPTION (MAXRECURSION 20)
This will capture cycle_depth which is a counter that measures how deep into a cycle we get. We stop the recursion after we get to cycle_depth of 1 so the cycle can be captures in the final select.