android怎么查询并显示数据库

在Android中，可以使用SQLite数据库来存储和查询数据。以下是一个简单的示例，演示如何查询并显示数据库中的数据：

1. 首先，在Android项目的app/build.gradle文件中添加SQLite依赖项：

dependencies {     implementation 'androidx.sqlite:sqlite:2.1.0'     implementation 'androidx.sqlite:sqlite-framework:2.1.0' } 

2. 创建一个DatabaseHelper类来管理数据库的创建和升级：

import android.content.Context; import android.database.Cursor; import android.database.sqlite.SQLiteDatabase; import android.database.sqlite.SQLiteOpenHelper;  public class DatabaseHelper extends SQLiteOpenHelper {      private static final String DATABASE_NAME = "mydatabase.db";     private static final int DATABASE_VERSION = 1;      private static final String TABLE_NAME = "mytable";     private static final String COLUMN_ID = "id";     private static final String COLUMN_NAME = "name";     private static final String COLUMN_AGE = "age";      public DatabaseHelper(Context context) {         super(context, DATABASE_NAME, null, DATABASE_VERSION);     }      @Override     public void onCreate(SQLiteDatabase db) {         String createTableQuery = "CREATE TABLE " + TABLE_NAME + "("                 + COLUMN_ID + " INTEGER PRIMARY KEY AUTOINCREMENT,"                 + COLUMN_NAME + " TEXT,"                 + COLUMN_AGE + " INTEGER)";         db.execSQL(createTableQuery);     }      @Override     public void onUpgrade(SQLiteDatabase db, int oldVersion, int newVersion) {         String dropTableQuery = "DROP TABLE IF EXISTS " + TABLE_NAME;         db.execSQL(dropTableQuery);         onCreate(db);     }      public Cursor getAllData() {         SQLiteDatabase db = this.getReadableDatabase();         return db.query(TABLE_NAME, null, null, null, null, null, null);     } } 

3. 在需要显示数据库的地方，实例化DatabaseHelper类并查询数据：

import androidx.appcompat.app.AppCompatActivity; import android.database.Cursor; import android.os.Bundle; import android.widget.TextView;  public class MainActivity extends AppCompatActivity {      private TextView textViewData;     private DatabaseHelper databaseHelper;      @Override     protected void onCreate(Bundle savedInstanceState) {         super.onCreate(savedInstanceState);         setContentView(R.layout.activity_main);                  textViewData = findViewById(R.id.text_view_data);         databaseHelper = new DatabaseHelper(this);          Cursor cursor = databaseHelper.getAllData();         StringBuilder stringBuilder = new StringBuilder();          if (cursor.moveToFirst()) {             do {                 String name = cursor.getString(cursor.getColumnIndex(DatabaseHelper.COLUMN_NAME));                 int age = cursor.getInt(cursor.getColumnIndex(DatabaseHelper.COLUMN_AGE));                 stringBuilder.append("Name: ").append(name).append(", Age: ").append(age).append("\n");             } while (cursor.moveToNext());         }          textViewData.setText(stringBuilder.toString());          cursor.close();         databaseHelper.close();     }  } 

4. 在布局文件activity_main.xml中添加一个TextView控件用于显示数据：

<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"     xmlns:tools="http://schemas.android.com/tools"     android:layout_width="match_parent"     android:layout_height="match_parent"     android:orientation="vertical"     android:padding="16dp">      <TextView         android:id="@+id/text_view_data"         android:layout_width="match_parent"         android:layout_height="wrap_content"         android:textAppearance="?android:attr/textAppearanceMedium" />  </LinearLayout> 

这样，当应用程序启动时，它将查询数据库并在TextView中显示数据。请确保在查询数据库后关闭游标和数据库连接，以避免资源泄漏。