Android SQLite 使用外键约束

外键约束是很常用的一个数据约束方式。但是使用起来并不是那么简单。定义外键约束的方法，使用FOREIGN KEY，可以用于单列或者多列。FOREIGN KEY和REFERNCES经常联合使用。References表示该列引用哪个表的哪个列作为外键。单列时，FOREIGN KEY可以省略。

在Android2.1以前的版本使用的SQLite版本是3.5.9，而SQLite在3.6.19版本才开始支持外键约束。在Android的2.2版本中使用了版本号3.6.22的SQLite，但是为了兼容以前的程序，默认并没有启用该功能。如果要启用该功能，就在SQLiteOpenHelper的onOpen方法里加入如下方法：

@Override
public void onOpen(SQLiteDatabase db) {
// TODO Auto-generated method stub
super.onOpen(db);
if(!db.isReadOnly()) { // Enable foreign key constraints
db.execSQL("PRAGMA foreign_keys=ON;");
}
}

然后就可以使用外键约束了。demo代码如下：

private static final String TABLE_THEME = "themeTable";
private static final String THEME_TABLE_ID = "_id";
private static final String THEME_TABLE_THEME_ID = "tid";
private static final String THEME_TABLE_NAME = "name";
private static final String THEME_TABLE_WATERMARK_ID = "wid";
private static final String THEME_TABLE_MUSIC_ID = "mid";
private static final String THEME_TABLE_ICON_PATH = "icon_path";//icon 路径
private static final String THEME_TABLE_ICON_URL = "icon_url";//icon url

private static final String TABLE_WATERMARK = "watermarkTable";
private static final String WATERMARK_TABLE_ID = "_id";
private static final String WATERMARK_TABLE_WATERMARK_ID = "wid";

private static final String TABLE_MUSIC = "watermarkTable";
private static final String MUSIC_TABLE_ID = "_id";
private static final String MUSIC_TABLE_MUSIC_ID = "mid";

@Override
public void onCreate(SQLiteDatabase db) {
// TODO Auto-generated method stub

//db.execSQL(CREATE_TABLE_CONTACT);

db.execSQL( "CREATE TABLE IF NOT EXISTS "+
TABLE_WATERMARK+
"("+
WATERMARK_TABLE_ID + " INTEGER PRIMARY KEY AUTOINCREMENT," + // rowID
WATERMARK_TABLE_WATERMARK_ID + "TEXT ,"+ //水印id

")"
);
db.execSQL( "CREATE TABLE IF NOT EXISTS "+
TABLE_MUSIC+
"("+
MUSIC_TABLE_ID + " INTEGER PRIMARY KEY AUTOINCREMENT," + // rowID
MUSIC_TABLE_MUSIC_ID + "TEXT ,"+ //音乐id

")"
);
db.execSQL("CREATE TABLE IF NOT EXISTS "+
TABLE_THEME+
"("+
THEME_TABLE_ID + " INTEGER PRIMARY KEY AUTOINCREMENT," + // rowID
THEME_TABLE_THEME_ID + "TEXT ,"+ //主题id
THEME_TABLE_NAME + " TEXT ," + // 主题名称
THEME_TABLE_WATERMARK_ID + " TEXT," + // 主题包含的水印id
THEME_TABLE_MUSIC_ID + " TEXT," + // 主题包含的配乐id
THEME_TABLE_ICON_URL + " TEXT ," + // 主题icon的url
THEME_TABLE_ICON_PATH + " TEXT " +   // 主题icon的下载路径
"FOREIGN KEY ("+THEME_TABLE_WATERMARK_ID+") REFERENCES"+TABLE_WATERMARK+" ("+WATERMARK_TABLE_WATERMARK_ID+")" +
"FOREIGN KEY ("+THEME_TABLE_MUSIC_ID+") REFERENCES"+TABLE_MUSIC+" ("+MUSIC_TABLE_MUSIC_ID+")" +
");"
);
}