Question

注意：我已经在GAE相关帖子中看到了JOIN ，但在我的网站上看到了GROUP BY 要求让我做了一个新的帖子，我已经找到了我自己的解决方案，我想检查它有多好

数据存储区实体属性和类型（数据库）

实体：PartyEntity 允许将其称为
- String partyName
- String partyId
- BlobKey图像
实体：InsertEntity 允许将其称为b
- String partyIdentifier
- 字符串名称
- String constituency

目标

SELECT * FROM a JOIN b on a.partyId = b.partyIdentifier GROUP BY b.constituency;

下面我将回答我的问题，代码工作精细，建议我做任何改变或更好的方式（我只是一名学生）

Answer 1

我制作的解决方案如下，建议任何更改或更好的方法

// SELECT * FROM a（PartyEntity拥有唯一的partyId）每个partyId SELECT * FROM b
的和

private List<ListDisplay> initData() {

        DatastoreService datastore = DatastoreServiceFactory
                .getDatastoreService();

        List<ListDisplay> data = new ArrayList<ListDisplay>();

        Query query = new Query("PartyEntity");
        PreparedQuery preparedQuery = datastore.prepare(query);
        List<Entity> entities = preparedQuery.asList(FetchOptions.Builder
                .withDefaults());

        for (Entity entity : entities) {

            String PARTY_ID = (String) entity.getProperty("partyId");

            // fetch from another
            Query query2 = new Query("InsertEntity");
            Filter filterByPartyId = new Query.FilterPredicate(
                    "partyIdentifier", FilterOperator.EQUAL, PARTY_ID);
            query2.setFilter(filterByPartyId);
            PreparedQuery preparedQuery2 = datastore.prepare(query2);

            List<Entity> entities2 = preparedQuery2.asList(FetchOptions.Builder
                    .withDefaults());

            for (Entity entity2 : entities2) {
                ListDisplay display = new ListDisplay();
                display.setPartyId((String) entity.getProperty("partyId"));
                display.setPartyName((String) entity.getProperty("partyName"));
                display.setImage((BlobKey) entity.getProperty("image"));
                display.setName((String) entity2.getProperty("name"));
                display.setConstituency((String) entity2
                        .getProperty("constituency"));
                data.add(display);
            } // end loop entity2

        } // end loop entity

        return data;
    }

// GROUP BY选区子句

public HashMap<String, List<ListDisplay>> getData() {
        HashMap<String, List<ListDisplay>> data = new HashMap<>();
        List<ListDisplay> list = initData();

        for (ListDisplay d : list) {
            // if null add
            if (data.get(d.getConstituency()) == null) {
                List<ListDisplay> internal = new ArrayList<>();
                internal.add(d);
                data.put(d.getConstituency(), internal);
            }
            // else modify
            else {
                List<ListDisplay> hashlist = data.get(d.getConstituency());
                hashlist.add(d);
                data.remove(d.getConstituency());
                data.put(d.getConstituency(), hashlist);
            }
        }

        return data;
    }

GAE数据存储区JOIN + GROUP BY

1 个答案: