多对一连接，但显示为一个结果

Question

我必须在 SQLite 中加入我想要加入的表。一张表有练习的 ID 和标题。另一个表有该练习的图像。每个练习可以有许多图像。

我尝试使用 JOIN 编写 SQL，但每次练习它给我 2 行（因为每个练习至少有图像）。但是我想要它在一行，因为我将在 Android Java 适配器中使用结果。

我将在下面发布我的表以及我拥有的 SQL。我还将发布 Java 方法，以防万一这可以通过使用 JOIN 以外的其他方法解决。

exercise_index：

╔════════════╦═══════════════╗
║ exercise_id║ exercise_title║
╠════════════╬═══════════════╣
║  1         ║ Bench press   ║
║  2         ║ Sit ups       ║
║  3         ║ Push ups      ║
╚════════════╩═══════════════╩

exercise_index_images：

╔═══════════════════╦═══════════════════════════╦════════════════════╗
║ exercise_image_id ║ exercise_image_exercise_id║ exercise_image_file║
╠═══════════════════╬═══════════════════════════╬════════════════════╣
║  1                ║ 1                         ║ bench_press_1.png  ║
║  2                ║ 1                         ║ bench_press_2.png  ║
║  3                ║ 2                         ║ sit_ups_1.png      ║
║  4                ║ 2                         ║ sit_ups_2.png      ║
║  5                ║ 3                         ║ push_ups_1.png     ║
║  6                ║ 3                         ║ push_ups_2.png     ║
╚═══════════════════╩═══════════════════════════╩════════════════════╩

我现在的查询：

String query = "SELECT exercise_index.exercise_id, exercise_index.exercise_title, " +
        "exercise_index_images.exercise_image_id, exercise_index_images.exercise_image_file " +
        "FROM exercise_index " +
        "JOIN exercise_index_images ON exercise_index.exercise_id=exercise_index_images.exercise_image_exercise_id";

这给了我：

╔════════════╦═══════════════╗╔═══════════════════╦═══════════════════════════╦════════════════════╗
║ exercise_id║ exercise_title║║ exercise_image_id ║ exercise_image_exercise_id║ exercise_image_file║
╠════════════╬═══════════════╣╠═══════════════════╬═══════════════════════════╬════════════════════╣
║  1         ║ Bench press   ║║  1                ║ 1                         ║ bench_press_1.png  ║
║  1         ║ Bench press   ║║  2                ║ 1                         ║ bench_press_2.png  ║
║  2         ║ Sit ups       ║║  3                ║ 2                         ║ sit_ups_1.png      ║
║  2         ║ Sit ups       ║║  4                ║ 2                         ║ sit_ups_2.png      ║
║  3         ║ Push ups      ║║  5                ║ 3                         ║ push_ups_1.png     ║
║  3         ║ Push ups      ║║  6                ║ 3                         ║ push_ups_2.png     ║
╚════════════╩═══════════════╩╚═══════════════════╩═══════════════════════════╩════════════════════╩

我想要的是这样的：

╔════════════╦═══════════════╗╔═════════════════════╦═════════════════════════════╦══════════════════════╗╔═════════════════════╦═════════════════════════════╦══════════════════════╗
║ exercise_id║ exercise_title║║ exercise_image_id_1 ║ exercise_image_exercise_id_1║ exercise_image_file_1║║ exercise_image_id_2 ║ exercise_image_exercise_id_2║ exercise_image_file_2║
╠════════════╬═══════════════╣╠═════════════════════╬═════════════════════════════╬══════════════════════╣╠═════════════════════╬═════════════════════════════╬══════════════════════╣
║  1         ║ Bench press   ║║  1                  ║ 1                           ║ bench_press_1.png    ║║  2                  ║ 1                           ║ bench_press_2.png    ║
║  2         ║ Sit ups       ║║  3                  ║ 2                           ║ sit_ups_1.png        ║║  4                  ║ 2                           ║ sit_ups_2.png        ║
║  3         ║ Push ups      ║║  5                  ║ 3                           ║ push_ups_1.png       ║║  6                  ║ 3                           ║ push_ups_2.png       ║
╚════════════╩═══════════════╩╚═════════════════════╩═════════════════════════════╩══════════════════════╩╚═════════════════════╩═════════════════════════════╩══════════════════════╩

Android/Java 方法：

public void populateExercises(){

        /* Database */
        DBAdapter db = new DBAdapter(this);
        db.open();

        // Get all food for that category
        String currentLanguageSQL = db.quoteSmart(currentLanguage);
        String query = "SELECT exercise_index._id, exercise_index.exercise_id, exercise_index.exercise_title, exercise_index.exercise_type_id " +
                "FROM exercise_index " +
                "WHERE exercise_language=" + currentLanguageSQL + " AND exercise_muscle_group_id_main=" + currentMuscleGroupMainId;
        query = query + " ORDER BY exercise_title ASC";

        listCursor = db.rawQuery(query);


        // Find ListView to populate
        ListView lvItems = findViewById(R.id.listViewExercises);

        // Setup cursor adapter using cursor from last step
        ExercisesCMuscleGroupsOpenMainCursorAdapter exercisesAdapter = new ExercisesCMuscleGroupsOpenMainCursorAdapter(this, listCursor);

        // Attach cursor adapter to the ListView
        try {
            lvItems.setAdapter(exercisesAdapter); // uses ContinensCursorAdapter
        } catch (Exception e) {
            Toast.makeText(this, e.toString(), Toast.LENGTH_LONG).show();
        }
        // Close db
        db.close();


        // OnClick
        lvItems.setOnItemClickListener(new AdapterView.OnItemClickListener() {
            @Override
            public void onItemClick(AdapterView<?> arg0, View arg1, int arg2, long arg3) {
                exerciseListItemClicked(arg2);
            }
        });


    } // populateExercises

Answer 1

如果你只有两张图片，那么你可以进行条件聚合：

select exercise_id, exercise_title, 
       max(case when seq = 1 then exercise_image_id end) as exercise_image_id_1,
       max(case when seq = 1 then exercise_image_file end) as exercise_image_file_1,
       max(case when seq = 2 then exercise_image_id end) as exercise_image_id_2,
       max(case when seq = 2 then exercise_image_file end) as exercise_image_file_2
from (select *, row_number() over (partition by ei.exercise_id order by eimg.exercise_image_id) seq
      from exercise_index ei inner join
           exercise_index_images eimg
           on eimg.exercise_image_exercise_id = ei.exercise_id
     ) t
group by exercise_id, exercise_title;