mysql - SQL クエリに Null 値を含める

Question

特定のタイプの不動産を購入する各人の平均年齢を見つける SQL クエリがあります。クエリは各プロパティ タイプの平均年齢を返しますが、「平均年齢」フィールドが null であるプロパティ タイプは含まれません。どうすればこれを行うことができますか？

SELECT round(avg(round((datediff(now(),dateofbirth)/365),0))) as 'Average Age', propertyname 
from apartment, buyer
WHERE buyer.propertyID = apartment.propID 
group by propertyname
UNION
SELECT round(avg(round((datediff(now(),dateofbirth)/365),0))) as 'Average Age', propertyname 
from house, buyer
WHERE buyer.propertyID = house.propID
group by propertyname

Answer 1

試す：

SELECT round(avg(round((datediff(now(),dateofbirth)/365),0))) as 'Average Age', 
       propertyname 
from apartment
LEFT JOIN buyer ON buyer.propertyID = apartment.propID 
group by propertyname
UNION ALL
SELECT round(avg(round((datediff(now(),dateofbirth)/365),0))) as 'Average Age', 
       propertyname 
from house
LEFT JOIN buyer ON buyer.propertyID = house.propID
group by propertyname