mysql - 在 SQL 查询中包含 Null 值

Question

我有一个 SQL 查询，可以找到购买特定房产类型的每个人的平均年龄。查询返回每个属性类型的平均年龄，但不包括“平均年龄”字段为空的属性类型。我该怎么做呢？

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

尝试：

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