select
    Table1.*,
    case
        when Table1.KSCHL = 'Y'
        then Table1.KBETR
        else
            Table1.KBETR * t2.KBETR
    end as Expected
from Table1
left join Table1 as t2
    on Table1.matnr = t2.matnr and
       Table1.werk  = t2.werk and
       t2.kschl = 'Y';