在linq查询中嵌入if语句

Question

我正在尝试将以下代码压缩为一个select语句。这可能吗？

if (atcCode.HasValue)
{
    primaryOcc = (from f in db.aopFactor
                  where f.atcCode == atcCode
                  where f.endingDate == null
                  select f.primaryOccupancyID).FirstOrDefault();
}
var factorTable = (from f in db.aopFactor
                    where f.factorTypeID == factorType
                    where f.endingDate == null
                    where f.primaryOccupancyID == primaryOcc
                    select new { f.factorID, f.factorDescription }).ToDictionary(x => x.factorID, y => y.factorDescription);