Error: Cannot make a static reference to the non-static field db in Android database.

package com.example.databases;

import android.content.ContentValues;
import android.content.Context;
import android.database.Cursor;
import android.database.SQLException;
import android.database.sqlite.SQLiteDatabase;
import android.database.sqlite.SQLiteOpenHelper;
import android.util.Log;

public class DBAdapter
{
	public static final String KEY_ROWID="_id";
	public static final String KEY_NAME="name";
	public static final String KEY_EMAIL="email";
	private static final String TAG="DBAdapter";
	private static final String DATABASE_NAME="MyDB";
	private static final String DATABASE_TABLE="contacts";
	private static final int DATABASE_VERSION=1;
	private static final String DATABASE_CREATE="create table contacts(_id integer primary key autoincrement,"+
												"name text not null,email text not null);";
	
	private final Context context;
	private DatabaseHelper DBHelper;
	private SQLiteDatabase db;
	
	public DBAdapter(Context ctx)
	{
		this.context=ctx;
		DBHelper=new DatabaseHelper(context);
	}
	
	private static class DatabaseHelper extends SQLiteOpenHelper
	{
		DatabaseHelper(Context context)
		{
			super(context,DATABASE_NAME,null,DATABASE_VERSION);
		}
		
		@Override
		public void onCreate(SQLiteDatabase db)
		{
			try
			{
				db.execSQL(DATABASE_CREATE);
			}
			catch(SQLException e)
			{
				e.printStackTrace();
			}
		}
		
		@Override
		public void onUpgrade(SQLiteDatabase db,int oldVersion,int newVersion)
		{
			Log.w(TAG,"Updating database from version"+oldVersion+"to"+newVersion+" which will destroy all old data");
			db.execSQL("DROP TABLE IF EXISTS contacts");
			onCreate(db);
		}
		
		//opens the database
		public DBAdapter open() throws SQLException
		{
			db=DBHelper.getWritableDatabase();
			return this;
		}
		
		//closes the database
		public void close()
		{
			DBHelper.close();
		}
		
		//insert a contact into the database
		public long insertContact(String name,String email)
		{
			ContentValues initialValues=new ContentValues();
			initialValues.put(KEY_NAME,name);
			initialValues.put(KEY_EMAIL,email);
			return db.insert(DATABASE_TABLE,null,initialValues);
		}
		
		//deletes a particular contact
		public boolean deleteContact(long rowId)
		{
			return db.delete(DATABASE_TABLE,KEY_ROWID+"="+rowId,null)>0;
		}
		
		//retrieves all contacts
		public Cursor getAllContacts()
		{
			return db.query(DATABASE_TABLE,new String[]{KEY_ROWID,KEY_NAME,KEY_EMAIL},null,null,null,null,null);
		}
		
		//retrieves a particular contact
		public Cursor getContact(long rowId) throws SQLException
		{
			Cursor mCursor=db.query(true,DATABASE_TABLE,new String[] {KEY_ROWID,KEY_NAME,KEY_EMAIL},KEY_ROWID+"="+rowId,
					null,null,null,null,null);
			if(mCursor!=null)
			{
				mCursor.moveToFirst();
			}
			return mCursor;
		}
		
		//updates a contact
		public boolean updateContact(long rowId,String name,String email)
		{
			ContentValues args=new ContentValues();
			args.put(KEY_NAME,name);
			args.put(KEY_EMAIL,email);
			return db.update(DATABASE_TABLE,args,KEY_ROWID+"="+rowId,null);
		}
	}
}