case .. when ...then .. else .. end

条件判断函数
举例1：

select name , 
sum( case when result = 'win' then 1 else 0 end ) as 'win',
sum(case when result = 'lose' then 1 else 0 end) as 'lose',
from score
group by name

从胜负记录表中，查询出每个人胜场个数和败场个数。

举例2:

select name,subject,score, 
case
when score>80 and score < 100 then '优秀'  
else  
  case  when score > 60 then '合格' else '不合格' end 
end 
from test_score

case内部在包含一个case，输出成绩结果的判断，80-100为优秀，60-80合格，60以下的不合格