select CASE WHEN a.contractPosition.contract.mainContract is not null THEN b.contractPosition.contract.mainContract.number ELSE a.contractPosition.contract.number END from rtneo$Accrual a left outer join rtneo$Accrual b on a.id = '18781b72-c499-1d1e-5e9f-50216d447438'

Работает

select CASE WHEN not a.contractPosition.contract.mainContract is null THEN b.number ELSE a.contractPosition.contract.number END from rtneo$Accrual a left join a.contractPosition.contract.mainContract b where a.id = '18781b72-c499-1d1e-5e9f-50216d447438'

select CASE WHEN not a.contractPosition.contract.mainContract is null THEN b.number ELSE a.contractPosition.contract.number END, CASE WHEN not a.contractPosition.contract.mainContract is null THEN b.date ELSE a.contractPosition.contract.date END from rtneo$Accrual a left outer join a.contractPosition.contract.mainContract b where a.id = '18781b72-c499-1d1e-5e9f-50216d447438'