左外联接与isull

Question

这是一个非常简单的LEFT OUTER JOIN，但返回的行为零：

SELECT All_varieties.*,variety_join.*
FROM All_varieties LEFT JOIN 
     variety_join 
     ON All_varieties.varietyID = variety_join.varietyID
WHERE (variety_join.sourceID = Null);

但是，我正在使用查询 All_varieties

SELECT plant.plantName, plant.plantGenus, plant.plantSpecies, variety.varietyLatin, variety.varietyEnglish, variety.varietyID
FROM plant LEFT JOIN 
     variety 
     ON plant.plantID = variety.plantID;

查询 All_varieties 可能是问题所在吗？

表格：

Answer 1

应为variety_join.sourceID is null而不是variety_join.sourceID = Null

SELECT All_varieties.*,variety_join.*
FROM All_varieties LEFT JOIN 
     variety_join 
     ON All_varieties.varietyID = variety_join.varietyID
WHERE variety_join.sourceID is Null