配置单元应自动忽略 NULL 此处报告的骨料值。
为了可读性，我建议使用 COALESCE 而不是 IF IS NULL 陈述如下： COALESCE(Math,0) as Math

每个学生的平均成绩：

select school,SL_No,Name,Math,Phy,Chem,(if(Math is NULL,0,Math)+if(Phy is NULL,0,Phy)+if(Chem is NULL,0,Chem))/3 as avg_marks from my_table

各校平均分

select school,avg(avg_marks) from (select school,SL_No,Name,Math,Phy,Chem,(if(Math is NULL,0,Math)+if(Phy is NULL,0,Phy)+if(Chem is NULL,0,Chem))/3 as avg_marks from my_table
) temp group by school