Question

这是 DatabaseHelper类，没有编写任何编译错误数据库，但是没有创建任何表，因此已写入所有内容，而我在sqlite3命令行中也遇到了麻烦。我不知道为什么未在 android studio 3.5

中创建它

import android.content.ContentValues;
import android.content.Context;
import android.database.sqlite.SQLiteDatabase;
import android.database.sqlite.SQLiteOpenHelper;



public class DatabaseHelper extends SQLiteOpenHelper {

    public static final String DATABASE_NAME = "Student.db";
    public static final int DATABASE_VERSION = 1;
    public static final String TABLE_NAME = "Student";
    public static final String  COL_1 = "ID";
    public static final String COL_2 = "Name";
    public static final String COL_3 = "Surname";
    public static final String COL_4 = "Marks";


    //constructor
    public DatabaseHelper( Context context) {
        super(context, DATABASE_NAME,  null, 1);

            SQLiteDatabase db = this.getWritableDatabase();
    }



    @Override
    public void onCreate(SQLiteDatabase db) {
        db.execSQL("CREATE TABLE "+TABLE_NAME+" (ID INTEGER PRIMARY KEY AUTOINCREMENT, NAME TEXT, SURNAME TEXT, MARKS TEXT )" );

    }

    @Override
    public void onUpgrade(SQLiteDatabase db, int oldVersion, int newVersion) {
        // below method check wheather tha same name database is exist or not
        //if exists it will delete and create new database
        db.execSQL("DROP TABLE IF EXISTS "+ TABLE_NAME);
        onCreate(db);
    }

主要方法

public class MainActivity extends AppCompatActivity {
    DatabaseHelper mydb;
    EditText  mName, mSurname, mMarks;
    Button btn;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        mydb = new DatabaseHelper(this);
      }

}

Answer 1

好吧，看来您的做法不正确。请参考以下代码，并尝试了解它们之间的区别。

public class DatabaseHelper extends SQLiteOpenHelper {

    public static final String DATABASE_NAME = "Student.db";
    public static final int DATABASE_VERSION = 1;
    public static final String TABLE_NAME = "Student";
    public static final String  COL_1 = "ID";
    public static final String COL_2 = "Name";
    public static final String COL_3 = "Surname";
    public static final String COL_4 = "Marks";


    //constructor
    public DatabaseHelper( Context context) {
        super(context, DATABASE_NAME,  null, 1);
    }

    @Override
    public void onCreate(SQLiteDatabase db) {
        db.execSQL("CREATE TABLE "+TABLE_NAME+" (ID INTEGER PRIMARY KEY AUTOINCREMENT, NAME TEXT, SURNAME TEXT, MARKS TEXT );" );

    }

    @Override
    public void onUpgrade(SQLiteDatabase db, int oldVersion, int newVersion) {
        // below method check wheather tha same name database is exist or not
        //if exists it will delete and create new database
        db.execSQL("DROP TABLE IF EXISTS "+ TABLE_NAME);
        onCreate(db);
    }
}

主要方法

这与您的不同。原因是，即使您在 DatabaseHelper 中定义了 SQLiteDatabase ，也不会将其称为main方法。因此，您必须在main方法本身中定义它。

public class MainActivity extends AppCompatActivity {
    DatabaseHelper dbHelper;
    SQLiteDatabase database;
    EditText  mName, mSurname, mMarks;
    Button btn;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        dbHelper = new DatabaseHelper(this);
        database = dbHelper.getWritableDatabase();
      }
}

创建数据库但未创建表是sqlite android

1 个答案: