Android/Room/Sqlite : 获取 Room 中每列的最大值/最小值列表

Question

我的 android 应用程序中有一个类似于下面给出的表格。

Demo Table

我希望查询表格以获取表格中每一列的最小值/最大值的列表。

例如，对于给定的样本值：

Sample Values

我希望输出是

最大值：{ "Value 1":76.1, "Value 2":1000.0, "Value 3":4897.847, "Value 4":99.0 }

最小值：{ "Value 1":2.3, "Value 2":0.0, "Value 3":9.1, "Value 4":99.0 }

在 Android 中使用 Room 实现这一目标的最佳方法是什么？

Answer 1

下面是一个实现你想要的结果的例子。

Demo实体（表格）：-

@Entity
class Demo {
    @PrimaryKey
    Long id;
    Double value1;
    Double value2;
    Double value3;
    Double value4;

    Demo(){}

    Demo(Double value1, Double value2, Double value3, Double value4) {
        this.value1 = value1;
        this.value2 = value2;
        this.value3 = value3;
        this.value4 = value4;
    }

    public Long getId() {
        return id;
    }

    public void setId(Long id) {
        this.id = id;
    }

    public Double getValue1() {
        return value1;
    }

    public void setValue1(Double value1) {
        this.value1 = value1;
    }

    public Double getValue2() {
        return value2;
    }

    public void setValue2(Double value2) {
        this.value2 = value2;
    }

    public Double getValue3() {
        return value3;
    }

    public void setValue3(Double value3) {
        this.value3 = value3;
    }

    public Double getValue4() {
        return value4;
    }

    public void setValue4(Double value4) {
        this.value4 = value4;
    }
}

DemoDao 道：-

@Dao
interface DemoDao {

    @Insert
    Long insertDemoRow(Demo demo);

    @Query("SELECT '{\"Value 1\":'||max(value1)||', \"Value 2\":'||max(value2)||', \"Value 3\":'||max(value3)||' \"Value 4\":'||max(value4) AS Maximum FROM demo")
    String getMaxvaluesFromDemo();
    @Query("SELECT '{\"Value 1\":'||min(value1)||', \"Value 2\":'||min(value2)||', \"Value 3\":'||min(value3)||' \"Value 4\":'||min(value4) AS Minimum FROM demo")
    String getMinvaluesFromDemo();

}

• 单独的查询更容易，但它们可以合并。

DemoDatabase抽象类

@Database(entities = {Demo.class},version = 1)
abstract class DemoDatabase extends RoomDatabase {
    abstract DemoDao getDemoDao();
}

最后是一个调用活动（为了方便和简洁，使用了主线程）：-

public class MainActivity extends AppCompatActivity {

    DemoDatabase db;
    DemoDao dao;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        db = Room.databaseBuilder(this,DemoDatabase.class,"demo.db")
                .allowMainThreadQueries()
                .build();
        dao = db.getDemoDao();
        dao.insertDemoRow(new Demo(76.1,null,4897.847,null));
        dao.insertDemoRow(new Demo(44.2,87.1,47.0,null));
        dao.insertDemoRow(new Demo(null,1000.0,12.345,null));
        dao.insertDemoRow(new Demo(2.3,0.0,9.1,99.0));

        Log.d("DEMOINFO","Maximum " + dao.getMaxvaluesFromDemo());
        Log.d("DEMOIBFO","Minimums " + dao.getMinvaluesFromDemo());
    }
}

运行时（仅设计为运行一次）日志中的结果是：-

D/DEMOINFO: Maximum {"Value 1":76.1, "Value 2":1000.0, "Value 3":4897.847 "Value 4":99.0
D/DEMOIBFO: Minimums{"Value 1":2.3, "Value 2":0.0, "Value 3":9.1 "Value 4":99.0