sql语句中如何将查询没有的记录也显示在查询结果中，显示为0

Question

用如下代码查询表中数据
select defectClass, count(*)
from qua_particle_iis_his
where defectClass in (1, 2, 3, 4, 5, 6)
and virtualCode = '122009200013'
Group by defectClass

显示结果如下：

其中2和4 因为没有记录。 但是我需要显示出来。

想要的显示出的结果是

Answer 1

select defectClass, sum(case when defectClass in (1, 2, 3, 4, 5, 6) and virtualCode = '122009200013' then 1 else 0 end) as cnt
from qua_particle_iis_his
Group by defectClass