sql server - SQL - Select statement inside case -

i want select particular columns table when field in other table not null.

in other words, have 2 tables, have common fields when field named "salesordrkey" in table number 1 not null need common fields table number 2 otherwise common fields table number 1

here's trying do

select slsordr.salesordrkey,        whissue.warehissuekey,         issuppk.issueprodpackkey,        (case when  whissue.salesordrkey not null            (select slsordr.busipartnerkey,                    slsordr.contractkey,                    slsordr.salesmankey,                    slsordr.customerkey)         else            (select whissue.busipartnerkey,                    whissue.contractkey,                    whissue.salesmankey,                    slsordr.customerkey)         end)  warehissues whissue       inner join issueprodpacks issuppk on whissue.warehissuekey = issuppk.warehissuekey      left join slssalesordrs slsordr on whissue.salesordrkey = slsordr.salesordrkey         whissue.partitionkey = @prtnkey ,        issuppk.prodpackkey = @prodpackkey 

but didn't work .. there other way achieve that?

i assuming logic backwards. instead of:

case when  whissue.salesordrkey not null . . . 

i think intend:

case when  slsordr.salesordrkey not null 

the first doesn't check see if left join working.

so, can use coalesce():

select slsordr.salesordrkey, whissue.warehissuekey, issuppk.issueprodpackkey,        coalesce(slsordr.busipartnerkey, whissue.busipartnerkey) busipartnerkey,        coalesce(slsordr.contractkey, whissue.contractkey) contractkey,        coalesce(slsordr.salesmankey, whissue.salesmankey),        coalesce(slsordr.customerkey, whissue.customerkey) warehissues whissue inner join      issueprodpacks issuppk      on whissue.warehissuekey = issuppk.warehissuekey left join       slssalesordrs slsordr      on whissue.salesordrkey = slsordr.salesordrkey whissue.partitionkey = @prtnkey ,        issuppk.prodpackkey = @prodpackkey 

i should note not 100% same logic. possible records match have null values in slsordr table. using coalesce() bring in values other table, though there match. however, logic.

otherwise, need use multiple case statements:

       (case when slsorderkey not null slsordr.busipartnerkey              else whissue.busipartnerkey         end) busipartnerkey, 

case expression. returns 1 value, not multiple values.