安卓 数据库的使用

数据库一无所知，最近用到，百度+问群，刚刚入门，自己总结一下做个笔记：

1，创建数据库：继承SQLiteOpenHelper 

public class DatabaseHelper extends SQLiteOpenHelper {private String Tag = "DatabaseHelper";public DatabaseHelper(Context context, String name,CursorFactory factory, int version) {super(context, name, factory, version);// TODO Auto-generated constructor stub}@Overridepublic void onCreate(SQLiteDatabase db) {// TODO Auto-generated method stubLogUtils.i(Tag, "onCreate");try {//本机处理操作同步到云端后的记录db.execSQL("CREATE TABLE devicesOperation_tb (" + "directory VARCHAR (50)  NOT NULL," + "rootDir VARCHAR (20)  NOT NULL," + "parentDir VARCHAR (20)  NOT NULL,"+ "fileName VARCHAR (50)  NOT NULL,"+ "time VARCHAR (50)  NOT NULL" + ");");//本机所有文件记录，用于和操作后的对比，devicesFile_tb记录没有，devicesOperation_tb记录有的话，则是删除操作db.execSQL("CREATE TABLE devicesFile_tb (" + "directory VARCHAR (50)  NOT NULL," + "rootDir VARCHAR (20)  NOT NULL," + "parentDir VARCHAR (20)  NOT NULL,"+ "fileName VARCHAR (50)  NOT NULL,"+ "time VARCHAR (50)  NOT NULL" + ");");//云端操作记录表db.execSQL("CREATE TABLE serverOperation_tb (" + "fileName VARCHAR (50)  NOT NULL," + "rootDir VARCHAR (20)  NOT NULL," + "parentDir VARCHAR (20)  NOT NULL," + "operation VARCHAR (10)  NOT NULL," + "filePath VARCHAR (100)" + ");");} catch (Exception e) {// TODO: handle exceptionLogUtils.i(Tag, "onCreate:"+e);}}@Overridepublic void onUpgrade(SQLiteDatabase db, int oldVersion, int newVersion) {// TODO Auto-generated method stub}}

db.execSQL("CREATE TABLE devicesOperation_tb (" + "directory VARCHAR (50)  NOT NULL," + "rootDir VARCHAR (20)  NOT NULL," + "parentDir VARCHAR (20)  NOT NULL,"+ "fileName VARCHAR (50)  NOT NULL,"+ "time VARCHAR (50)  NOT NULL" + ");");

这一句是创建数据库，devicesOperation_tb：表名；后面都是字段名，每一句以“，”逗号结束，最后一句不用

2，获取执行创建数据库，得到数据库：

mDatabaseHelper = new DatabaseHelper(mContext, DATABASE_NAME, null, 1);mSqLiteDatabase = mDatabaseHelper.getReadableDatabase();

只new是得不到数据库的，必须要getReadableDatabase()或者getWriteableDatabase()一下才能看到数据库，这句执行后，就可以在data/data/你的项目包名/databases下面看到有一个devicesOperation_tb.db的数据库了

3，数据库添加数据，也就是insert语句：

ContentValues cv = new ContentValues();cv.put(DevicesOperationTB.directory, f.toString());cv.put(DevicesOperationTB.rootDir, FilePath.split("/")[0]);cv.put(DevicesOperationTB.parentDir, FilePath.split("/")[1]);cv.put(DevicesOperationTB.fileName, FilePath.split("/")[2]);mSqLiteDatabase.insert(DevicesFileTB.devicesFile_tb, null, cv);

说明：ContentValues ：作用和amp差不多，放键值对的，当然key值要和你数据库里面的字段对应，要不然就报错了mSqLiteDatabase.insert(DevicesFileTB.devicesFile_tb, null, cv);要插入的表名，后面是cv，插入成功返回行号，失败返回-1

4，数据库查询，query语句:

Cursor mCursor = mSqLiteDatabase.query(DevicesOperationTB.devicesOperation_tb, null, "directory = ?", new String[] {f.toString()}, null, null, null);

参数1：表名
参数2：查询到后返回这条数据那一列的值，如果null则返回整条数据的值
参数3：要查询的列的字段
参数4：要查询的值，参数3和参数4是对应的，如果查询的是二列，后面的参数4也就2个值，
例：参数3是"directory = ? and fileName = ?"  那么参数4就应该是new String[] {f.toString(),“文件名.txt”}
注意：参数3里面是二哥条件的话要用and而不是&

5，遍历：
你查询到结果，你要遍历

Cursor mCursor = mSqLiteDatabase.query(DevicesOperationTB.devicesOperation_tb, null, "directory = ?", new String[] {f.toString()}, null, null, null);//先查询，在遍历if (mCursor != null) {//判断数据库不为空if (mCursor.getCount() != 0) {/*注意，这里才是判断是否查询到数据，只要数据库存在，mCursor 就不会为空，没查询到数据mCursor.getCount()==0的*/ for (mCursor.moveToFirst(); !mCursor.isAfterLast(); mCursor.moveToNext()){LogUtils.i(Tag, "filename: "+mCursor.getString(mCursor.getColumnIndex(DevicesOperationTB.directory)));LogUtils.i(Tag, "fileTime: "+mCursor.getString(mCursor.getColumnIndex(DevicesOperationTB.time)));}}}

6，删除：delete语句

String[] whereArgs = {"测试.txt","add"};mSqLiteDatabase.delete(tb, "fileName=? and operation=?", whereArgs);

参数1：表名
参数2：条件，二个条件用and 我这句意思是：删除fileName = "测试.txt"并且operation="add"的那条数据

7，清空表，也是delete语句，参数不同而已：

mSqLiteDatabase.delete(tb, null, null);

无条件删除，就是清空了


总结先到这里吧，自己做笔记用，数据库小白！