bex report

hi, i am creating one report having below format:

contract | date | sub-contract | date | sales order | date | invoice no | date |

im reading from sales datasources (2LIS_11_VAHDR and 2LIS_11_VAITM). it comprises data as per below relation :

doc_no      | reference doc

--------------------------------------

contrct1       -

sub-contr1    contract1

sales ordr1    sub-contrc1

invoice         sales order1

but in some cases, sub-contract is not created, means it is optional to create as per requirement. so the data is also coming in this format :

doc_no      | reference doc

--------------------------------------

contrct1       -

sales ordr1    contrc1

invoice         sales order1