case when

select case parent_status 
         when 1 then  
              case child_status
        when 1 then 'aa'
        when 2 then 'bb'
        when 3 then 'cc'
          else ''
              end 
        when 2 then  
              case child_status
        when 1 then 'dd'
        when 2 then 'ee'
        when 3 then 'ff'
          else ''
              end 
        when 3 then  
              case child_status
        when 1 then 'gg'
        when 2 then 'hh'
        when 3 then 'll'
          else ''
              end 
    else ''            
    end  
  from table; 
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原文链接：https://blog.csdn.net/qq_28933803/article/details/83417356