case when语句第一种方式：  case      when 表达式1 then 结果1     when 表达式2 then 结果2     else 结果n end 举例1： select sum(         case             when rental_rate=0.99 then 1             else 0         end     ) as "aa",          sum(      case         when rental_rate=2.99 then 1         else 0      end     ) as "bb",          sum(         case             when rental_rate=4.99 then 1             else 0         end     ) as "cc"          from film; 结果： aa     bb     cc 341    323    336   【注】：as后接的别名需要带双引号，否则报语法错误   case when语句第二种方式：  case 表达式     when 匹配1 then 结果1     when 匹配2 then 结果2     else 结果n end 举例2：     select sum(         case rental_rate             when 0.99 then 1             else 0         end     ) as "aa",          sum(         case rental_rate             when 2.99 then 1             else 0         end     ) as "bb",          sum(         case rental_rate             when 4.99 then 1             else 0         end     ) as "cc"     from film; 结果： aa     bb     cc 341    323    336

上面是在网上搜到的介绍

下面是自己实际用到的情况，供自己学习记录。

SELECT  c.unit, 		case when d.money ::DECIMAL = 0.00 		then '0%' 		   else concat(round(c.number ::DECIMAL/d.money ::DECIMAL*100,2),'%')  end as rate   FROM ( SELECT p.unit,sum(p.number::DECIMAL) as number FROM table p  WHERE  p.code IN (SELECT  code FROM code_table )  GROUP BY  p.unit ) c left join ( SELECT  unit,sum(money::DECIMAL) as money FROM code_table  GROUP BY unit ) d on  c.unit = d.unit 

使用时case when then 的数据类型要相同，否则会报错