SQL Server 2019

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-- https://stackoverflow.com/questions/65818438/possible-to-store-value-of-one-select-column-and-use-it-for-the-next-one/65818648#65818648

create table dbo.tst_CrossApply
(
  GivenDay varchar(32) null,
  OtherData  varchar(32) null,
  CODRPL varchar(32) null,
  COD varchar (32) null,
  BODRPL varchar(32) null,
  BOD varchar (32) null,
)


insert into dbo.tst_CrossApply values ( 'Day1','OtherData1','<', '5','', '10')
insert into dbo.tst_CrossApply values ( 'Day2','OtherData2', '', '20','<', '30')

SELECT * FROM dbo.tst_CrossApply

SELECT t.[GivenDay],t.[OtherData],v.[RPL],v.[Result]
FROM [dbo].[tst_CrossApply] t
CROSS APPLY (VALUES ([CODRPL], [COD]),([BODRPL], [BOD])) v ([RPL],[Result])



GivenDay
OtherData
CODRPL
COD
BODRPL
BOD


Day1
OtherData1
<
5

10

Day2
OtherData2

20
<
30




GivenDay
OtherData
RPL
Result


Day1
OtherData1
<
5

Day1
OtherData1

10

Day2
OtherData2

20

Day2
OtherData2
<
30


SELECT t.[GivenDay]
       , t.[OtherData]
       , v.[Param]
       , v.[RPL]
       , v.[Result]
FROM    [dbo].[tst_CrossApply] t
CROSS APPLY (
      VALUES ('COD', [CODRPL], [COD])
           , ('BOD', [BODRPL], [BOD])
    ) v ([Param], [RPL],[Result])


GivenDay
OtherData
Param
RPL
Result


Day1
OtherData1
COD
<
5

Day1
OtherData1
BOD

10

Day2
OtherData2
COD

20

Day2
OtherData2
BOD
<
30


SELECT cod.GivenDay, cod.OtherData, cod.Param, cod.RPL, cod.Result
FROM   (
          SELECT  GivenDay, OtherData, COD, CODRPL AS RPL
          FROM    [dbo].[tst_CrossApply] t
       ) pvt
       UNPIVOT  
       (
          Result FOR Param IN (COD)  
       ) AS cod
UNION ALL 
SELECT bod.GivenDay, bod.OtherData, bod.Param, bod.RPL, bod.Result
FROM   (
          SELECT  GivenDay, OtherData, BOD, BODRPL AS RPL
          FROM    [dbo].[tst_CrossApply] t
       ) pvt
       UNPIVOT  
       (
          Result FOR Param IN (BOD)  
       ) AS bod


GivenDay
OtherData
Param
RPL
Result


Day1
OtherData1
COD
<
5

Day2
OtherData2
COD

20

Day1
OtherData1
BOD

10

Day2
OtherData2
BOD
<
30
GivenDay	OtherData	RPL	Result
Day1	OtherData1	<	5
Day1	OtherData1		10
Day2	OtherData2		20
Day2	OtherData2	<	30