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：

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'
}

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);
    }
}

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();
    }

}

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。

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