2 case statements which are linked

Question

I am relatively new in SQL coding and have a question regarding case statements.

What I want to achieve: I want to create a query being used for calculating correct obso

Answer 1

You don't have access to the aliases in the select clause in the same select clause. There is a simple fix and that is using a derived table or a common table expression:

;with cte as
(
    Select 
        [Material]
        ,[Plnt]
        ,case
            when [calculate 5-year demand] = 0
                then 9.01
            when  [BAAS SO GI (601) 36] = 0
                then 9.01
            when [MS] <> 'BI' or [MS] <> 'BO' 
                then ([Stock all SP WH]/([calculate 5-year demand]/5))
            when [MS] = 'BO'
                then ([Stock all SP WH]/[BAAS SO GI (601) 36])
            when [MS] ='BI'
                then 0
            else 9.01
        end as [Inventory Reach]
    from [BAAS_PowerBI].[dbo].[Obs]
)

select [Material]
        ,[Plnt]
        ,[Inventory Reach]
        ,case
            when [Inventory Reach] > 9
                then 1
            else 0.9
        end as [Devaluation Class]
from cte

The above code is using a common table expression which is (in this schenario) basically the same as using a derived table, only more readable. Here's the derived table equivalent:

select [Material]
        ,[Plnt]
        ,[Inventory Reach]
        ,case
            when [Inventory Reach] > 9
                then 1
            else 0.9
        end as [Devaluation Class]
from 
(
    Select 
        [Material]
        ,[Plnt]
        ,case
            when [calculate 5-year demand] = 0
                then 9.01
            when  [BAAS SO GI (601) 36] = 0
                then 9.01
            when [MS] <> 'BI' or [MS] <> 'BO' 
                then ([Stock all SP WH]/([calculate 5-year demand]/5))
            when [MS] = 'BO'
                then ([Stock all SP WH]/[BAAS SO GI (601) 36])
            when [MS] ='BI'
                then 0
            else 9.01
        end as [Inventory Reach]
    from [BAAS_PowerBI].[dbo].[Obs]
) derived

Answer 2

A column alias is not permitted in the FROM, WHERE, GROUP BY, or HAVING clauses. You can use a subquery or CTE, but SQL Server supports lateral joins via the APPLY keyword. This can be quite useful for introducing a column alias:

select o.Material, o.Plnt, v.[Inventory Reach],
       (case when v.[Inventory Reach] > 9
             then 1
             else 0.9
        end) as [Devaluation Class]
from [BAAS_PowerBI].[dbo].[Obs] o cross apply
     (values (case when o.[calculate 5-year demand] = 0
                   then 9.01
                   when o.[BAAS SO GI (601) 36] = 0
                   then 9.01
                   when o.[MS] in ('BI', 'BO')
                   then (o.[Stock all SP WH] / (o.[calculate 5-year demand] / 5))
                   when o.[MS] = 'BO'
                   then (o.[Stock all SP WH] / o.[BAAS SO GI (601) 36])
                   when o.[MS] ='BI'
                   then 0
                  else 9.01
              end
             )
     ) values([Inventory Reach])