使用CASE合并成一列，再加上其他列？

如何解决使用CASE合并成一列，再加上其他列？

例如，我有一张桌子，称为“就业”：

df.groupby('variety').transform(lambda x: x.fillna(x.mode()))

我想做的是建立一个包含以下列的报告：

ID        NAME         Employer           StartDate          EndDate
-----------------------------------------------------------------------
20        John         Company A          2-5-2015           5-5-2018
20        John         Company B          5-10-2018          NULL
100       Bill         Company C          10-10-2017         10-15-2018
100       Bill         Company D          11-20-2018         3-5-2019
100       Bill         Company E          4-5-2019           NULL

例如，基于上表，我想显示：

ID         NAME        Employer1      Employer2         Employer3

我希望Employer1列始终显示其最近/当前雇主，并从那里降级到Employer2和Employer3列。如果某人只有1个雇主，则Employer2和Employer3应该为NULL，而如果他们只有2个雇主，则Employer3应该为NULL。

解决方法

具有ROW_NUMBER()窗口功能：

select id,name,max(case when rn = 1 then employer end) Employer1,max(case when rn = 2 then employer end) Employer2,max(case when rn = 3 then employer end) Employer3
from (
  select *,row_number() over (partition by id,name order by startdate desc) rn
  from employment
) e
group by id,name
order by id,name

请参见demo。
结果：

>  id | name | Employer1 | Employer2 | Employer3
> --: | :--- | :-------- | :-------- | :--------
>  20 | John | Company B | Company A | null     
> 100 | Bill | Company E | Company D | Company C

,

我认为这样会起作用。条件聚合

File unzip(File zip) throws IOException
    {
        ZipInputStream zis = new ZipInputStream(new FileInputStream(zip));
        
        //helper directory to store files into while program is running!
        File helperDir = Files.createDirectories(Paths.get("zipFile")).toFile();  
        //helperDir.deleteOnExit();
        
        byte[] buffer = new byte[1024];
        for (ZipEntry entry; (entry = zis.getNextEntry()) != null; )
        {
            if (!entry.isDirectory()) //true if file in zip is not a folder
            {
                File newFile = new File(helperDir,entry.getName());
                //newFile.deleteOnExit();
                
                FileOutputStream fos = new FileOutputStream(newFile);
                for (int len = zis.read(buffer); len > 0; len = zis.read(buffer))
                    fos.write(buffer,len);
                fos.close();
            }
            else
            {
                //What to do if there are folders in zip...
            }
        }
        
        zis.close();
        return helperDir;
    }