是否应该为我的数据库中的每个表创建一个继承SQLiteOpenHelper的类? [英] Should I create a class that inherits SQLiteOpenHelper for each table in my database?

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本文介绍了是否应该为我的数据库中的每个表创建一个继承SQLiteOpenHelper的类?的处理方法，对大家解决问题具有一定的参考价值，需要的朋友们下面随着小编来一起学习吧！

问题描述

我需要创建一个Android应用程序数据库，但我想知道是否应该为我数据库中存在的每个表创建一个继承SQLiteOpenHelper的类?还是我应该创建一个类:例如从SQLiteOpenHelper继承并在onCreate方法中创建所有表以及更新和删除函数的MyAppDatabase?

I need to create an android app database , but I was wondering if I should create one class that inherits SQLiteOpenHelper for each table that exists in my database ? Or should I create One class : for example MyAppDatabase that inherits from SQLiteOpenHelper and create all my tables in onCreate method as well as update and delete functions ?

有没有建议?

推荐答案

对于数据库中的所有表，您只需要一个数据库助手，因此，您将使用一个 onCreate 方法来创建数据库.桌子.请注意，如果您有多个数据库助手，则只有打开数据库的第一个助手会调用一次 onCreate (和 onUpdate 方法)，因此除了拥有多个帮助程序效率低下，拥有多个数据库帮助程序可能会更加复杂.

You only need one database helper for all tables in a database and thus you'd use the one onCreate method for the creation of the tables. Noting that if you had multiple database helpers that the onCreate (and onUpdate method) would only be called once by the first helper that opened the database and thus in addition to having multiple helpers being inefficient it could be more complicated to have multiple database helpers.

更具体地说，只有在数据库不存在时才自动调用 onCreate .到 onCreate 调用时，数据库本身已经创建.

More specifically onCreate is only automatically called when the database doesn't exist. By the time onCreate is called the database itself has been created.

onUpdate 仅在打开数据库时被调用(通过超级调用)传递给调用的版本号大于存储在数据库文件中的版本号时被调用.然后，此时将更新文件中存储的版本号，以反映较新的版本.因此，随后的调用将不会再调用 onUpgrade 方法.

onUpdate is called only when upon opening the database, the version number passed to the call (via the super call) is greater than the version number stored in the database file. The version number stored in the file, at this time, is then updated to reflect the newer version. Therefore subsequent calls will not then invoke the onUpgrade method.

您是否可以选择是否拆分方法和标识符，例如单个表的列/表名.有些人可能认为拆分很尴尬，其他人可能认为这更清楚.

Whether or not you split the methods and identifiers, such as the column/table names for individual tables is a choice you could make. Some may consider it awkward to split others may think it is clearer.

以下代码是3个排列(以及数据库)的示例，它们全部利用2个表，即 table001 (列 _id 和 mydata >)和 table001 (列名 _id 和 myotherdata ).

The following code is an example of 3 permutations (and also databases) that all utilises 2 tables, namely table001 (columns _id and mydata) and table001 (column names _id and myotherdata).

  1. 使用单个数据库助手(DBHelper001)，并将所有内容嵌入在助手中.该数据库为 mydb001

使用单个databasehelper(DBHelper002)以及面向表的特定类(面向Table001的类和面向Table002的类)中的特定于Table的方法和常量.

uses a single databasehelper (DBHelper002) with the Table specific methods and constants in specific table orientated classes (class Table001 and class Table002).

使用两个单独的数据库助手(DBHelperTable001和DBHelperTable002)，为了简化代码，使用Table001和Table002类.

uses two separate databaseshelpers (DBHelperTable001 and DBHelperTable002) and for simplicity of code utilise the Table001 and Table002 classes.

  • 请注意，要克服仅在 onOpen 方法一旦被调用时调用 onCreate 的情况，该尝试也会尝试创建相应的表(在这种情况下，CREATE TABLE IF NOT EXISTS ......对于避免失败而言很重要该表确实存在).
  • 请注意，这仅仅是拥有多个助手的一种低效率.
  • Note that to overcome onCreate being called only once the onOpen method also attempts to create the respective table (CREATE TABLE IF NOT EXISTS ...... being important in this case to avert a failure when the table does in fact exist).
  • Note this is just one inefficiency of having multiple helpers.

首先是表特定的类(第一个排列未使用)

Table001.java 

public class Table001 {

    public static final String TBL_TABLE001 = "table001";
    public static final String COL_TABLE001_ID = BaseColumns._ID;
    public static final String COL_TABLE001_MYDATA = "mydata";

    public static String getCrtSQL() {
        return "CREATE TABLE IF NOT EXISTS " + TBL_TABLE001 + "(" +
                COL_TABLE001_ID + " INTEGER PRIMARY KEY, " +
                COL_TABLE001_MYDATA + " TEXT" +
                ")";
    }

    public static long insert(SQLiteDatabase db, String mydata) {
        ContentValues cv = new ContentValues();
        cv.put(COL_TABLE001_MYDATA,mydata);
        return db.insert(TBL_TABLE001,null,cv);
    }

    public static Cursor getAll(SQLiteDatabase db) {
        return db.query(TBL_TABLE001,null,null,null,null,null,null);
    }
}

Table002.java 

public class Table002 {

    public static final String TBL_TABLE002 = "table002";
    public static final String COL_TABLE002_ID = BaseColumns._ID;
    public static final String COL_TABLE002_MYOTHERDATA = "myotherdata";

    public static String getCrtSQL() {
        return "CREATE TABLE IF NOT EXISTS " + TBL_TABLE002 + "(" +
                COL_TABLE002_ID + " INTEGER PRIMARY KEY, " +
                COL_TABLE002_MYOTHERDATA + " TEXT" +
                ")";
    }

    public static long insert(SQLiteDatabase db, String mydata) {
        ContentValues cv = new ContentValues();
        cv.put(COL_TABLE002_MYOTHERDATA,mydata);
        return db.insert(TBL_TABLE002,null,cv);
    }

    public static Cursor getAll(SQLiteDatabase db) {
        return db.query(TBL_TABLE002,null,null,null,null,null,null);
    }
}

四个数据库帮助程序类

DBHelper001.java-(自包含)

The four database helper classes

DBHelper001.java - (self-contained)

public class DBHelper001 extends SQLiteOpenHelper {

    public static final String DBNAME = "mydb001";
    public static final int DBVERSION = 1;

    public static final String TBL_TABLE001 = "table001";
    public static final String TBL_TABLE002 = "table002";
    public static final String COL_TABLE001_ID = BaseColumns._ID;
    public static final String COL_TABLE001_MYDATA = "mydata";
    public static final String COL_TABLE002_ID = BaseColumns._ID;
    public static final String COL_TABLE002_MYOTHERDATA = "myotherdata";

    public DBHelper001(Context context) {
        super(context, DBNAME, null, DBVERSION);
    }

    @Override
    public void onCreate(SQLiteDatabase db) {
        String crt_table001_sql = "CREATE TABLE IF NOT EXISTS " + TBL_TABLE001 + "(" +
                COL_TABLE001_ID + " INTEGER PRIMARY KEY," +
                COL_TABLE001_MYDATA + " TEXT" +
                ")";
        String crt_table002_sql = "CREATE TABLE IF NOT EXISTS " + TBL_TABLE002 + "(" +
                COL_TABLE002_ID + " INTEGER PRIMARY KEY," +
                COL_TABLE002_MYOTHERDATA + " TEXT" +
                ")";
        db.execSQL(crt_table001_sql);
        db.execSQL(crt_table002_sql);
    }

    @Override
    public void onUpgrade(SQLiteDatabase db, int oldVersion, int newVersion) {

    }

    public long insertIntoTable001(String mydata) {
        SQLiteDatabase db = this.getWritableDatabase();
        ContentValues cv = new ContentValues();
        cv.put(COL_TABLE001_MYDATA,mydata);
        return db.insert(TBL_TABLE001,null,cv);
    }

    public long insertIntoTable002(String myotherdata) {
        SQLiteDatabase db = this.getWritableDatabase();
        ContentValues cv = new ContentValues();
        cv.put(COL_TABLE002_MYOTHERDATA,myotherdata);
        return db.insert(TBL_TABLE002,null,cv);
    }

    public Cursor getAllFromTable001() {
        SQLiteDatabase db = this.getWritableDatabase();
        return db.query(TBL_TABLE001,null,null,null,null,null,null);
    }

    public Cursor getAllFromTable002() {
        SQLiteDatabase db = this.getWritableDatabase();
        return db.query(TBL_TABLE002,null,null,null,null,null,null);
    }
}

DBHelper002.java (其他地方的表特定代码) 

public class DBHelper002 extends SQLiteOpenHelper {

    public static final String DBNAME = "mydb002";
    public static final int DBVERSION = 1;

    public DBHelper002(Context context) {
        super(context, DBNAME, null, DBVERSION);
    }

    @Override
    public void onCreate(SQLiteDatabase db) {
        db.execSQL(Table001.getCrtSQL());
        db.execSQL(Table002.getCrtSQL());
    }

    @Override
    public void onUpgrade(SQLiteDatabase db, int oldVersion, int newVersion) {

    }
}

DBHelperTable001.java(特定于table001的帮助器)

public class DBHelperTable001 extends SQLiteOpenHelper {

    public static final String DBNAME = "mydb003";
    public static final int DBVERSION = 1;

    public DBHelperTable001(Context context) {
        super(context, DBNAME, null, DBVERSION);
    }

    @Override
    public void onCreate(SQLiteDatabase db) {
        db.execSQL(Table001.getCrtSQL());
        //NOTE Table002 won't get created as onCreate is only called once
    }

    @Override
    public void onUpgrade(SQLiteDatabase db, int oldVersion, int newVersion) {

    }

    @Override
    public void onOpen(SQLiteDatabase db) {
        super.onOpen(db);
        db.execSQL(Table001.getCrtSQL());
    }
}

  • 注意: onOpen 方法用于绕过onCreate，在数据库的整个生命周期中仅被调用一次.
    • 调用execSQL是这种方法效率低下的一个例子.

      • Note the onOpen method is utilised to circumvent onCreate only being invoked once for the lifetime of the database.
        • invoking the execSQL is an example of an inefficiency of this methodology.

          • public class DBHelperTable002 extends SQLiteOpenHelper {

    public static final String DBNAME = "mydb003";
    public static final int DBVERSION = 1;

    public DBHelperTable002(Context context) {
        super(context, DBNAME, null, DBVERSION);
    }

    @Override
    public void onCreate(SQLiteDatabase db) {
        db.execSQL(Table002.getCrtSQL());
        //NOTE Table001 won't get created as onCreate is only called once
    }

    @Override
    public void onUpgrade(SQLiteDatabase db, int oldVersion, int newVersion) {

    }

    @Override
    public void onOpen(SQLiteDatabase db) {
        super.onOpen(db);
        db.execSQL(Table002.getCrtSQL());
    }
}

          把他们都绑在一起

          以下活动利用( MainActivity.java )利用所有3个排列.对于每个表，每行都添加一行，然后将每个表中的所有数据提取到游标中，然后将其转储(输出到日志).

          Tying them all together

          The following activity utilises (MainActivity.java) utilises all 3 permutations. For each a row is added to each table and then all data from each table is extracted into a cursor which is then dumped (output to the log).

          请注意，对于特定于表的助手，每个助手都用于提取行(显示冗余方面).

          Note that for the table specific helpers each helper is used to extract rows (showing the redundancy aspect).

          public class MainActivity extends AppCompatActivity {

    DBHelper001 mDBHlpr1;
    DBHelper002 mDBHlpr2;
    DBHelperTable001 mTblDBHlpr1;
    DBHelperTable002 mTblDBHlpr2;
    Cursor mCsr;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        mDBHlpr1 = new DBHelper001(this);
        mDBHlpr1.insertIntoTable001("my data for table001 in mydb001");
        mDBHlpr1.insertIntoTable002("my other data for table002 in mydb001");
        mCsr = mDBHlpr1.getAllFromTable001();
        DatabaseUtils.dumpCursor(mCsr);
        mCsr = mDBHlpr1.getAllFromTable002();
        DatabaseUtils.dumpCursor(mCsr);


        mDBHlpr2 = new DBHelper002(this);
        Table001.insert(mDBHlpr2.getWritableDatabase(),"my data for table001 in mydb002");
        Table002.insert(mDBHlpr2.getWritableDatabase(),"my other data for table002 in mydb002");
        mCsr = Table001.getAll(mDBHlpr2.getWritableDatabase());
        DatabaseUtils.dumpCursor(mCsr);
        mCsr = Table002.getAll(mDBHlpr2.getWritableDatabase());
        DatabaseUtils.dumpCursor(mCsr);

        //Oooops???? wouldn't normally do this
        mCsr = Table001.getAll(mDBHlpr1.getWritableDatabase()); //?????????? from other database!!!
        DatabaseUtils.dumpCursor(mCsr);

        mTblDBHlpr1 = new DBHelperTable001(this);
        Table001.insert(mTblDBHlpr1.getWritableDatabase(),"my data for table001 in mydb003");
        mTblDBHlpr2 = new DBHelperTable002(this);
        Table002.insert(mTblDBHlpr2.getWritableDatabase(),"my data for table002 in mydb003");
        mCsr = Table001.getAll(mTblDBHlpr1.getWritableDatabase());
        DatabaseUtils.dumpCursor(mCsr);
        mCsr = Table002.getAll(mTblDBHlpr1.getWritableDatabase()); //???????????? but OK
        DatabaseUtils.dumpCursor(mCsr);
        mCsr = Table001.getAll(mTblDBHlpr2.getWritableDatabase());
        DatabaseUtils.dumpCursor(mCsr);
        mCsr = Table002.getAll(mTblDBHlpr2.getWritableDatabase()); //??????????? but OK
        DatabaseUtils.dumpCursor(mCsr);
    }
}

          结果

          以下是第一次运行的结果(请注意，多次运行而不卸载该应用程序将导致添加2个新行):-

          Result

          The following is the result from the first run (note running multiple times without uninstalling the App will result in 2 new rows being added) :-

          03-06 11:27:18.453 11093-11093/? I/System.out: >>>>> Dumping cursor android.database.sqlite.SQLiteCursor@12c4306e
03-06 11:27:18.453 11093-11093/? I/System.out: 0 {
03-06 11:27:18.453 11093-11093/? I/System.out:    _id=1
03-06 11:27:18.453 11093-11093/? I/System.out:    mydata=my data for table001 in mydb001
03-06 11:27:18.453 11093-11093/? I/System.out: }
03-06 11:27:18.453 11093-11093/? I/System.out: <<<<<
03-06 11:27:18.453 11093-11093/? I/System.out: >>>>> Dumping cursor android.database.sqlite.SQLiteCursor@275e530f
03-06 11:27:18.453 11093-11093/? I/System.out: 0 {
03-06 11:27:18.453 11093-11093/? I/System.out:    _id=1
03-06 11:27:18.453 11093-11093/? I/System.out:    myotherdata=my other data for table002 in mydb001
03-06 11:27:18.453 11093-11093/? I/System.out: }
03-06 11:27:18.453 11093-11093/? I/System.out: <<<<<


03-06 11:27:18.472 11093-11093/? I/System.out: >>>>> Dumping cursor android.database.sqlite.SQLiteCursor@1006e57a
03-06 11:27:18.472 11093-11093/? I/System.out: 0 {
03-06 11:27:18.472 11093-11093/? I/System.out:    _id=1
03-06 11:27:18.472 11093-11093/? I/System.out:    mydata=my data for table001 in mydb002
03-06 11:27:18.472 11093-11093/? I/System.out: }
03-06 11:27:18.472 11093-11093/? I/System.out: <<<<<
03-06 11:27:18.472 11093-11093/? I/System.out: >>>>> Dumping cursor android.database.sqlite.SQLiteCursor@2337112b
03-06 11:27:18.473 11093-11093/? I/System.out: 0 {
03-06 11:27:18.473 11093-11093/? I/System.out:    _id=1
03-06 11:27:18.473 11093-11093/? I/System.out:    myotherdata=my other data for table002 in mydb002
03-06 11:27:18.473 11093-11093/? I/System.out: }
03-06 11:27:18.473 11093-11093/? I/System.out: <<<<<


03-06 11:27:18.473 11093-11093/? I/System.out: >>>>> Dumping cursor android.database.sqlite.SQLiteCursor@37ef9988
03-06 11:27:18.473 11093-11093/? I/System.out: 0 {
03-06 11:27:18.473 11093-11093/? I/System.out:    _id=1
03-06 11:27:18.473 11093-11093/? I/System.out:    mydata=my data for table001 in mydb001
03-06 11:27:18.473 11093-11093/? I/System.out: }
03-06 11:27:18.473 11093-11093/? I/System.out: <<<<<


03-06 11:27:18.499 11093-11093/? I/System.out: >>>>> Dumping cursor android.database.sqlite.SQLiteCursor@2b786c5d
03-06 11:27:18.500 11093-11093/? I/System.out: 0 {
03-06 11:27:18.500 11093-11093/? I/System.out:    _id=1
03-06 11:27:18.500 11093-11093/? I/System.out:    mydata=my data for table001 in mydb003
03-06 11:27:18.500 11093-11093/? I/System.out: }
03-06 11:27:18.500 11093-11093/? I/System.out: <<<<<
03-06 11:27:18.500 11093-11093/? I/System.out: >>>>> Dumping cursor android.database.sqlite.SQLiteCursor@2038a9d2
03-06 11:27:18.500 11093-11093/? I/System.out: 0 {
03-06 11:27:18.501 11093-11093/? I/System.out:    _id=1
03-06 11:27:18.501 11093-11093/? I/System.out:    myotherdata=my data for table002 in mydb003
03-06 11:27:18.501 11093-11093/? I/System.out: }
03-06 11:27:18.501 11093-11093/? I/System.out: <<<<<
03-06 11:27:18.501 11093-11093/? I/System.out: >>>>> Dumping cursor android.database.sqlite.SQLiteCursor@264a7aa3
03-06 11:27:18.501 11093-11093/? I/System.out: 0 {
03-06 11:27:18.501 11093-11093/? I/System.out:    _id=1
03-06 11:27:18.501 11093-11093/? I/System.out:    mydata=my data for table001 in mydb003
03-06 11:27:18.501 11093-11093/? I/System.out: }
03-06 11:27:18.502 11093-11093/? I/System.out: <<<<<
03-06 11:27:18.502 11093-11093/? I/System.out: >>>>> Dumping cursor android.database.sqlite.SQLiteCursor@3c2611a0
03-06 11:27:18.502 11093-11093/? I/System.out: 0 {
03-06 11:27:18.502 11093-11093/? I/System.out:    _id=1
03-06 11:27:18.502 11093-11093/? I/System.out:    myotherdata=my data for table002 in mydb003
03-06 11:27:18.503 11093-11093/? I/System.out: }
03-06 11:27:18.503 11093-11093/? I/System.out: <<<<<

          这篇关于是否应该为我的数据库中的每个表创建一个继承SQLiteOpenHelper的类?的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！

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