如何在Android的SQLite数据库中删除数据？

在之前的文章中，我们在Android应用程序中看到了CRUD操作的三种操作，例如创建，读取和更新操作。在本文中，我们将介绍添加删除操作以删除存储在SQLite数据库中的项目。

我们将在本文中构建什么?

我们将构建一个简单的应用程序，在该应用程序中，我们将从Android应用程序中的SQLite数据库中删除该课程。下面提供了一个示例视频，以使您对我们在本文中将要做的事情有个大概的了解。

分步实施

步骤1：更新我们的DBHandler类

由于我们必须从SQLite数据库中删除数据。为此，我们必须创建一种从SQLite数据库中删除数据的方法。导航到应用程序> Java >应用程序的程序包名称> DBHandler，然后将以下代码添加到其中。

Java

// below is the method for deleting our course.
public void deleteCourse(String courseName) {
    
        // on below line we are creating
        // a variable to write our database.
        SQLiteDatabase db = this.getWritableDatabase();
  
        // on below line we are calling a method to delete our
        // course and we are comparing it with our course name.
        db.delete(TABLE_NAME, "name=?", new String[]{courseName});
        db.close();
}

下面是DBHandler的更新代码。 Java文件 添加以上代码段后。

Java

import android.content.ContentValues;
import android.content.Context;
import android.database.Cursor;
import android.database.sqlite.SQLiteDatabase;
import android.database.sqlite.SQLiteOpenHelper;
  
import java.util.ArrayList;
  
public class DBHandler extends SQLiteOpenHelper {
  
    // creating a constant variables for our database.
    // below variable is for our database name.
    private static final String DB_NAME = "coursedb";
  
    // below int is our database version
    private static final int DB_VERSION = 1;
  
    // below variable is for our table name.
    private static final String TABLE_NAME = "mycourses";
  
    // below variable is for our id column.
    private static final String ID_COL = "id";
  
    // below variable is for our course name column
    private static final String NAME_COL = "name";
  
    // below variable id for our course duration column.
    private static final String DURATION_COL = "duration";
  
    // below variable for our course description column.
    private static final String DESCRIPTION_COL = "description";
  
    // below variable is for our course tracks column.
    private static final String TRACKS_COL = "tracks";
  
    // creating a constructor for our database handler.
    public DBHandler(Context context) {
        super(context, DB_NAME, null, DB_VERSION);
    }
  
    // below method is for creating a database by running a sqlite query
    @Override
    public void onCreate(SQLiteDatabase db) {
        // on below line we are creating
        // an sqlite query and we are
        // setting our column names
        // along with their data types.
        String query = "CREATE TABLE " + TABLE_NAME + " ("
                + ID_COL + " INTEGER PRIMARY KEY AUTOINCREMENT, "
                + NAME_COL + " TEXT,"
                + DURATION_COL + " TEXT,"
                + DESCRIPTION_COL + " TEXT,"
                + TRACKS_COL + " TEXT)";
  
        // at last we are calling a exec sql
        // method to execute above sql query
        db.execSQL(query);
    }
  
    // this method is use to add new course to our sqlite database.
    public void addNewCourse(String courseName, String courseDuration, String courseDescription, String courseTracks) {
  
        // on below line we are creating a variable for
        // our sqlite database and calling writable method
        // as we are writing data in our database.
        SQLiteDatabase db = this.getWritableDatabase();
  
        // on below line we are creating a
        // variable for content values.
        ContentValues values = new ContentValues();
  
        // on below line we are passing all values
        // along with its key and value pair.
        values.put(NAME_COL, courseName);
        values.put(DURATION_COL, courseDuration);
        values.put(DESCRIPTION_COL, courseDescription);
        values.put(TRACKS_COL, courseTracks);
  
        // after adding all values we are passing
        // content values to our table.
        db.insert(TABLE_NAME, null, values);
  
        // at last we are closing our
        // database after adding database.
        db.close();
    }
  
    // we have created a new method for reading all the courses.
    public ArrayList readCourses() {
        // on below line we are creating a
        // database for reading our database.
        SQLiteDatabase db = this.getReadableDatabase();
  
        // on below line we are creating a cursor with query to read data from database.
        Cursor cursorCourses = db.rawQuery("SELECT * FROM " + TABLE_NAME, null);
  
        // on below line we are creating a new array list.
        ArrayList courseModalArrayList = new ArrayList<>();
  
        // moving our cursor to first position.
        if (cursorCourses.moveToFirst()) {
            do {
                // on below line we are adding the data from cursor to our array list.
                courseModalArrayList.add(new CourseModal(cursorCourses.getString(1),
                        cursorCourses.getString(4),
                        cursorCourses.getString(2),
                        cursorCourses.getString(3)));
            } while (cursorCourses.moveToNext());
            // moving our cursor to next.
        }
        // at last closing our cursor
        // and returning our array list.
        cursorCourses.close();
        return courseModalArrayList;
    }
  
    // below is the method for updating our courses
    public void updateCourse(String originalCourseName, String courseName, String courseDescription,
                             String courseTracks, String courseDuration) {
  
        // calling a method to get writable database.
        SQLiteDatabase db = this.getWritableDatabase();
        ContentValues values = new ContentValues();
  
        // on below line we are passing all values
        // along with its key and value pair.
        values.put(NAME_COL, courseName);
        values.put(DURATION_COL, courseDuration);
        values.put(DESCRIPTION_COL, courseDescription);
        values.put(TRACKS_COL, courseTracks);
  
        // on below line we are calling a update method to update our database and passing our values.
        // and we are comparing it with name of our course which is stored in original name variable.
        db.update(TABLE_NAME, values, "name=?", new String[]{originalCourseName});
        db.close();
    }
  
    // below is the method for deleting our course.
    public void deleteCourse(String courseName) {
          
        // on below line we are creating
        // a variable to write our database.
        SQLiteDatabase db = this.getWritableDatabase();
  
        // on below line we are calling a method to delete our
        // course and we are comparing it with our course name.
        db.delete(TABLE_NAME, "name=?", new String[]{courseName});
        db.close();
    }
  
    @Override
    public void onUpgrade(SQLiteDatabase db, int oldVersion, int newVersion) {
        // this method is called to check if the table exists already.
        db.execSQL("DROP TABLE IF EXISTS " + TABLE_NAME);
        onCreate(db);
    }
}

第2步：添加按钮以删除我们的课程

导航至应用程序> res>布局> activity_update_course.xml文件，然后在此布局内添加一个按钮以删除课程。以下是该文件的代码。

以下是activity_update_course.xml文件的更新代码 添加以上代码段后。

步骤3：初始化按钮以删除我们的课程

导航到应用程序> Java >您的应用程序包名称> UpdateCourseActivity。 Java文件，并向其中添加以下代码。

Java

// adding on click listener for delete button to delete our course.
deleteCourseBtn.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                // calling a method to delete our course.
                dbHandler.deleteCourse(courseName);
                Toast.makeText(UpdateCourseActivity.this, "Deleted the course", Toast.LENGTH_SHORT).show();
                Intent i = new Intent(UpdateCourseActivity.this, MainActivity.class);
                startActivity(i);
       }
});

下面是UpdateCourseActivity的更新代码。 Java文件 添加以上代码段后。

Java

import android.content.Intent;
import android.os.Bundle;
import android.view.View;
import android.widget.Button;
import android.widget.EditText;
import android.widget.Toast;
  
import androidx.appcompat.app.AppCompatActivity;
  
public class UpdateCourseActivity extends AppCompatActivity {
  
    // variables for our edit text, button, strings and dbhandler class.
    private EditText courseNameEdt, courseTracksEdt, courseDurationEdt, courseDescriptionEdt;
    private Button updateCourseBtn, deleteCourseBtn;
    private DBHandler dbHandler;
    String courseName, courseDesc, courseDuration, courseTracks;
  
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_update_course);
  
        // initializing all our variables.
        courseNameEdt = findViewById(R.id.idEdtCourseName);
        courseTracksEdt = findViewById(R.id.idEdtCourseTracks);
        courseDurationEdt = findViewById(R.id.idEdtCourseDuration);
        courseDescriptionEdt = findViewById(R.id.idEdtCourseDescription);
        updateCourseBtn = findViewById(R.id.idBtnUpdateCourse);
        deleteCourseBtn = findViewById(R.id.idBtnDelete);
  
        // on below line we are initialing our dbhandler class.
        dbHandler = new DBHandler(UpdateCourseActivity.this);
  
        // on below lines we are getting data which
        // we passed in our adapter class.
        courseName = getIntent().getStringExtra("name");
        courseDesc = getIntent().getStringExtra("description");
        courseDuration = getIntent().getStringExtra("duration");
        courseTracks = getIntent().getStringExtra("tracks");
  
        // setting data to edit text
        // of our update activity.
        courseNameEdt.setText(courseName);
        courseDescriptionEdt.setText(courseDesc);
        courseTracksEdt.setText(courseTracks);
        courseDurationEdt.setText(courseDuration);
  
        // adding on click listener to our update course button.
        updateCourseBtn.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
  
                // inside this method we are calling an update course
                // method and passing all our edit text values.
                dbHandler.updateCourse(courseName, courseNameEdt.getText().toString(), courseDescriptionEdt.getText().toString(), courseTracksEdt.getText().toString(), courseDurationEdt.getText().toString());
  
                // displaying a toast message that our course has been updated.
                Toast.makeText(UpdateCourseActivity.this, "Course Updated..", Toast.LENGTH_SHORT).show();
  
                // launching our main activity.
                Intent i = new Intent(UpdateCourseActivity.this, MainActivity.class);
                startActivity(i);
            }
        });
  
        // adding on click listener for delete button to delete our course.
        deleteCourseBtn.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                // calling a method to delete our course.
                dbHandler.deleteCourse(courseName);
                Toast.makeText(UpdateCourseActivity.this, "Deleted the course", Toast.LENGTH_SHORT).show();
                Intent i = new Intent(UpdateCourseActivity.this, MainActivity.class);
                startActivity(i);
            }
        });
    }
}

现在运行您的应用程序，并查看该应用程序的输出。在删除数据之前，请确保将数据添加到我们的数据库中。

输出：

下面是执行删除操作后的完整项目文件结构：