理解case表达式 dodo

理解case表达式
1基本形式：
    case
 when then
 when then
 else
    end

2如有下数据
id     name               department
3      william silverman  1
4      walt welinski      1
1      carry grant        2
2      michael lancer     2
5      sally fields       3
6      walt frazier       3
7      melissa gomez      4

如果想把部门号转为实际的字符串，可使用以下语句
select id,name,case department
   when 1 then 'marketing'
   when 2 then 'customer service'
   when 3 then 'collictions'
   when 4 then 'customer relations'
  end as dept_name
from employees  order by dept_name

用搜索的case表达式为：
select id,name,case when department=1 then 'marketing'
  when department=2 then 'customer service'
  when department=3 then 'collictions'
  when department=4 then 'customer relations'
  end as dept_name
from employees  order by dept_name

3下面来学习使用case表达式更新列值
如: update employees
    set bonus_level =
  case when total_sales< 1000 then 'rookie'
       when (total_sales >=1000) and (total_sales< 10000) then 'associate'
       when total_sales>10000 then 'vice president'
  end

除了使用实际的字符串外，可将列名和数学表达式用于计算由case表达式返回的值中
如：update employees
 set commission =
  case bonus_level
   when 'rookie' then total_sales * 0.01
   when 'associate' then total_sales * 0.05
   when 'vice president' then total_sales * 0.15
  end

4 使用case表达式避免错误条件
如： select id ,name case when sales_count > 0 then sales_calls/sales_count * 100.0
     else 0
       end as closing_percentage
     from employees where department = 1
     这样就避免了如果sales_count值为null或0时，产生错误。