好用的ORM library for Android : greenDAO (一) 簡介

本篇想要介紹greenDAO這個平台，對於簡單的SQL來說非常好用，也可以直行一些grouy by跟巢狀查詢的指令，只要不太複雜的功能都可以簡單的實作出來。本篇預設你已經有在Android中撰寫SQL相關程式的經驗。
這系列的文章總共分三篇

• 簡介
• greenDAO的使用
• 進階使用與心得

Android平台有提供一個library SQLiteOpenHelper可以讓developer簡單的(?)呼叫資料庫，根據developer.android.com當中提供的例子，用中文快速回復一下記憶

首先，要先定義你的table schema

public final class FeedReaderContract {
    public FeedReaderContract() {}

    /* Inner class that defines the table contents */
    public static abstract class FeedEntry implements BaseColumns {
        public static final String TABLE_NAME = "entry";
        public static final String COLUMN_NAME_ENTRY_ID = "entryid";
        public static final String COLUMN_NAME_TITLE = "title";
        public static final String COLUMN_NAME_SUBTITLE = "subtitle";
        ...
    }
}

定義資料庫的創建(生)與刪除(死)的script

private static final String TEXT_TYPE = " TEXT";
private static final String COMMA_SEP = ",";
private static final String SQL_CREATE_ENTRIES =
    "CREATE TABLE " + FeedEntry.TABLE_NAME + " (" +
    FeedEntry._ID + " INTEGER PRIMARY KEY," +
    FeedEntry.COLUMN_NAME_ENTRY_ID + TEXT_TYPE + COMMA_SEP +
    FeedEntry.COLUMN_NAME_TITLE + TEXT_TYPE + COMMA_SEP +
    ... // Any other options for the CREATE command
    " )";
private static final String SQL_DELETE_ENTRIES =
    "DROP TABLE IF EXISTS " + FeedEntry.TABLE_NAME;

然後，要建立一個DBHelper，來幫助管理資料庫的創建、刪除與升級(復活)

public class FeedReaderDbHelper extends SQLiteOpenHelper {

    public static final int DATABASE_VERSION = 1;
    public static final String DATABASE_NAME = "FeedReader.db";

    public FeedReaderDbHelper(Context context) {
        super(context, DATABASE_NAME, null, DATABASE_VERSION);
    }
    public void onCreate(SQLiteDatabase db) {
        db.execSQL(SQL_CREATE_ENTRIES);
    }
    public void onUpgrade(SQLiteDatabase db, int oldVersion, int newVersion) {
        db.execSQL(SQL_DELETE_ENTRIES);
        onCreate(db);
    }
    public void onDowngrade(SQLiteDatabase db, int oldVersion,int newVersion){
        onUpgrade(db, oldVersion, newVersion);
    }
}

接著，要新增資料的時後code長這樣

SQLiteDatabase db = mDbHelper.getWritableDatabase();

ContentValues values = new ContentValues();
values.put(FeedEntry.COLUMN_NAME_ENTRY_ID, id);
values.put(FeedEntry.COLUMN_NAME_TITLE, title);
values.put(FeedEntry.COLUMN_NAME_CONTENT, content);

long newRowId;
newRowId = db.insert(
         FeedEntry.TABLE_NAME,
         FeedEntry.COLUMN_NAME_NULLABLE,
         values);

要查詢資料的時候，code要寫這樣

SQLiteDatabase db = mDbHelper.getReadableDatabase();

String[] projection = {
    FeedEntry._ID,
    FeedEntry.COLUMN_NAME_TITLE,
    FeedEntry.COLUMN_NAME_UPDATED,
    ...
    };

String sortOrder =
    FeedEntry.COLUMN_NAME_UPDATED + " DESC";
Cursor c = db.query(
    FeedEntry.TABLE_NAME,  // The table to query
    projection,            // The columns to return
    selection,             // The columns for the WHERE clause
    selectionArgs,         // The values for the WHERE clause
    null,                  // don't group the rows
    null,                  // don't filter by row groups
    sortOrder              // The sort order
    );

刪除資料要寫這樣

// Define 'where' part of query.
String selection = FeedEntry.COLUMN_NAME_ENTRY_ID + " LIKE ?";
// Specify arguments in placeholder order.
String[] selectionArgs = { String.valueOf(rowId) };
// Issue SQL statement.
db.delete(table_name, selection, selectionArgs);

更新資料要寫這樣

SQLiteDatabase db = mDbHelper.getReadableDatabase();
// New value for one column
ContentValues values = new ContentValues();
values.put(FeedEntry.COLUMN_NAME_TITLE, title);
// Which row to update, based on the ID
String selection = FeedEntry.COLUMN_NAME_ENTRY_ID + " LIKE ?";
String[] selectionArgs = { String.valueOf(rowId) };
int count = db.update(
    FeedReaderDbHelper.FeedEntry.TABLE_NAME,
    values,
    selection,
    selectionArgs);

而上面的每一步，如果忘記更新到一個欄位，或是多/少一個欄位要的時候，就要在project裡面到處找了。

如果今天developer要寫的code加起來只有這樣，那生活不是簡單多了嗎?
至於要怎麼開始使用呢? 讓我們繼續看下去.......或上官網自己查文件XD