SQLite 聚合查询答案

【问题标题】：SQLite aggregation querySQLite 聚合查询
【发布时间】：2019-05-07 22:10:00
【问题描述】：

我有一张带列的手机表

身份证
品牌名称
型号名称
条件

示例表：

id    brand_name model_name condition sell_status
1       Apple        6s       New      unsold
2       Apple        6s       Used     unsold
3       Apple        6s       New      unsold
4       Apple        5s       New      sold

我希望结果像

     brand_name model_name Quantity New Used
       Apple        6s       3       2   1

我试过了，但没有得到“新”和“用过”的商品结果：

public List<Mobile> getBrandMobileModel(String brand_name) {

    List<Mobile> mobileData = new ArrayList<Mobile>();
    String selectAllQuery = "SELECT brand_name, model_name, count(*), count(condition = ?), count(condition = ?) FROM " + Table_Mobile_Details + " WHERE brand_name = ? AND sell_status = ? GROUP BY model_name";

    SQLiteDatabase db = this.getReadableDatabase();
    Cursor cursor = db.rawQuery(selectAllQuery, new String[]{"New", "Used", brand_name, "unsold"});

    if (cursor.moveToFirst()) {
        do {

            Mobile mobile = new Mobile();

            //mobile.setId(cursor.getInt(0));
            mobile.setBrand_name(cursor.getString(1));
            mobile.setModel_name(cursor.getString(2));
            mobile.setQuantity(cursor.getString(3));
            mobile.setCondition_new(cursor.getString(4));
            mobile.setCondition_used(cursor.getString(5));

            mobileData.add(mobile);

        } while (cursor.moveToNext());
    }
    return mobileData;
}

【问题讨论】：

您的数据库未规范化。这可能是一个严重的问题。

标签： java android sql sqlite android-sqlite

【解决方案1】：

您需要同时使用GROUP BYbrand_name、model_name 并使用SUM 和CASE 来获取值为'New' 和'Used' 的行数

String selectAllQuery = 
    "SELECT brand_name, model_name, " +
            "count(*) as Quantity, " + 
        "SUM(CASE WHEN condition = 'New' THEN 1 ELSE 0 END) as New, " +
        "SUM(CASE WHEN condition = 'Used' THEN 1 ELSE 0 END) as Used " +
    "FROM " + Table_Mobile_Details + " WHERE brand_name = ? AND sell_status = ? GROUP BY brand_name, model_name";

你还需要2个参数，所以：

Cursor cursor = db.rawQuery(selectAllQuery, new String[]{brand_name, "unsold"});

【讨论】：