Select statments in SysComputedColumn in AX2012

Dear community hope you are doing well ,

I am having issue while select statment from diffrent datasources and apply operations between them .

Example : select LINEVALUE,CONTRACTLINENUM,* from PSACONTRACTLINEITEMS where CONTRACTLINENUM = 'PJB-0001'  -
select INVOICEDAMOUNT,* from AXPCONTRACTLINEINVOICEDAMOUNT where CONTRACTLINENUM = 'PJB-0001'

I want to subtract both query outputs and show in my computedColumn field .

I am working on AX2012