Question

我有一个包含宽字符串表的txt文件。我想将其转换为基于ID列的广泛列表。数据看起来像这样

    Long format has 3 columns: country, key, value
  - M*N rows.
  e.g. 
  'USA', 'President', 'Obama'
   ...
  'USA', 'Currency', 'Dollar'

Wide format has N=16 columns: county, key1, ..., keyN
  - M rows
example: 
   country, President, ... , Currency
   'USA', 'Obama', ... , 'Dollar'

我想知道

    SELECT country, 
       MAX( IF( key='President', value, NULL ) ) AS President,
       MAX( IF( key='Currency', value, NULL ) ) AS Currency,
       ...

FROM table 
GROUP BY country;

在Java中！

Answer 1

我认为您可以使用一些Collectors.groupingBy（）使其更容易一些，但更简单的版本是这样：

    List<String[]> list = new ArrayList<>();

    list.add(new String[] { "USA", "President", "Obama" });
    list.add(new String[] { "USA", "Currency", "Dollar" });
    list.add(new String[] { "Germany", "President", "Steinmeier" });
    list.add(new String[] { "Germany", "Currency", "Euro" });
    list.add(new String[] { "United Kingdom", "President", "Queen Elisabeth" });
    list.add(new String[] { "United Kingdom", "Currency", "Pound" });

    Map<String, Map<String, String>> map = new HashMap<>();
    list.forEach(s -> {
        map.putIfAbsent(s[0], new HashMap<>());
        map.get(s[0]).put(s[1], s[2]);
    });

    List<String[]> wideList = map.entrySet().stream()
            .map(m -> new String[] { m.getKey(), m.getValue().get("President"), m.getValue().get("Currency") })//
            .collect(Collectors.toList());

    System.out.println("country, President, Currency");
    wideList.forEach(s -> System.out.println(s[0] + ", " + s[1] + ", " + s[2]));

在Java中将窄表转换为宽表

1 个答案: