case when 嵌套使用
SELECT CASE WHEN (CASE WHEN 2>1 THEN 1 ELSE 0 END)=1 THEN (CASE WHEN (CASE WHEN 2>1 THEN 1 ELSE 0 END)<1 THEN 1 ELSE 0 END)ELSE (CASE WHEN 2>1 THEN 1 ELSE 0 END)END