安卓系统如何删除 SQLite 数据库中的数据?
原文:https://www . geesforgeks . org/如何删除数据-in-SQLite-database-in-Android/
在之前的文章中,我们已经看到了 CRUD 操作的三个操作,比如创建、读取、更新操作在我们的安卓 app 中。在本文中,我们将看一下添加删除操作,用于删除存储在 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,在这个应用程序中,我们将从我们的安卓应用程序中的 SQLite 数据库中删除课程。下面给出了一个示例视频,让我们了解一下在本文中要做什么。
[https://media.geeksforgeeks.org/wp-content/uploads/20210218093245/Screenrecorder-2021-02-18-09-31-40-7.mp4](https://media.geeksforgeeks.org/wp-content/uploads/20210218093245/Screenrecorder-2021-02-18-09-31-40-7.mp4)分步实施
第一步:更新我们的 DBHandler 类
因为我们必须从 SQLite 数据库中删除数据。为此,我们必须创建一个方法来从 SQLite 数据库中删除我们的数据。导航到 app > java >你的 app 的包名> DBHandler 并添加下面的代码。
Java 语言(一种计算机语言,尤用于创建网站)
// 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();
}
以下是添加上述代码片段后文件 的更新代码。
Java 语言(一种计算机语言,尤用于创建网站)
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<CourseModal> 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<CourseModal> 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);
}
}
*第二步:添加按钮删除我们的课程*
导航到应用程序> res >布局>activity _ update _ course . XML文件,并在该布局中添加一个用于删除课程的按钮。下面是该文件的代码。
可扩展标记语言
<!--button for deleting our course-->
<Button
android:id="@+id/idBtnDelete"
android:layout_width="match_parent"
android:layout_height="wrap_content"
android:layout_margin="10dp"
android:text="Delete Course"
android:textAllCaps="false" />
以下是添加上述代码片段后 活动 _update_course.xml 文件 的更新代码。
可扩展标记语言
<?xml version="1.0" encoding="utf-8"?>
<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"
tools:context=".UpdateCourseActivity">
<!--Edit text to enter course name-->
<EditText
android:id="@+id/idEdtCourseName"
android:layout_width="match_parent"
android:layout_height="wrap_content"
android:layout_margin="10dp"
android:hint="Enter course Name" />
<!--edit text to enter course duration-->
<EditText
android:id="@+id/idEdtCourseDuration"
android:layout_width="match_parent"
android:layout_height="wrap_content"
android:layout_margin="10dp"
android:hint="Enter Course Duration" />
<!--edit text to display course tracks-->
<EditText
android:id="@+id/idEdtCourseTracks"
android:layout_width="match_parent"
android:layout_height="wrap_content"
android:layout_margin="10dp"
android:hint="Enter Course Tracks" />
<!--edit text for course description-->
<EditText
android:id="@+id/idEdtCourseDescription"
android:layout_width="match_parent"
android:layout_height="wrap_content"
android:layout_margin="10dp"
android:hint="Enter Course Description" />
<!--button for updating our course-->
<Button
android:id="@+id/idBtnUpdateCourse"
android:layout_width="match_parent"
android:layout_height="wrap_content"
android:layout_margin="10dp"
android:text="Update Course"
android:textAllCaps="false" />
<!--button for deleting our course-->
<Button
android:id="@+id/idBtnDelete"
android:layout_width="match_parent"
android:layout_height="wrap_content"
android:layout_margin="10dp"
android:text="Delete Course"
android:textAllCaps="false" />
</LinearLayout>
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