Android SQLite Google 官方用法

SQL官方用法

Saving data to a database is ideal for repeating or structured data, such as contact information.The APIs you’ll need to use a database on Android are available in the android.database.sqlite package.

Define a Schema and Contract(計劃和契約)

One of the main principles of SQL databases is the schema: a formal declaration of how the database is organized. The schema is reflected in the SQL statements that you use to create your database. You may find it helpful to create a companion class, known as a contract class, which explicitly specifies the layout of your schema in a systematic and self-documenting way.

A contract class is a container for constants that define names for URIs, tables, and columns. The contract class allows you to use the same constants across all the other classes in the same package. This lets you change a column name in one place and have it propagate throughout your code.

A good way to organize a contract class is to put definitions that are global to your whole database in the root level of the class. Then create an inner class for each table that enumerates its columns.

Note: By implementing the BaseColumns interface, your inner class can inherit a primary key field called _ID that some Android classes such as cursor adaptors will expect it to have. It’s not required, but this can help your database work harmoniously with the Android framework.

public final class StudentContract {

    // To prevent someone from accidentally instantiating the contract class,
    // give it an empty constructor.
    public StudentContract(){
    }

    /* Inner class that defines the table contents */
    public static abstract class StudentEntry implements BaseColumns{
        public static final String TABLE_NAME="entry";
        public static final String COLUMN_NAME_ENTRY_ID="entryid";
        public static final String COLUMN_NAME_NAME="name";
        public static final String COLUMN_NAME_AGE="age";
        public static final String COLUMN_NAME_HEIGHT="height";
    }
}

Create a Database Using a SQL Helper

Once you have defined how your database looks, you should implement methods that create and maintain the database and tables. Here are some typical statements that create and delete a table:

private static final String TEXT_TYPE=" TEXT";
private static final String INTEGER_TYPE=" INTEGER";
private static final String COMMA_SEP=",";
private static final String SQL_CREATE_ENTRY="CREATE TABLE if not exits "+
        StudentContract.StudentEntry.TABLE_NAME+" ("+
        StudentContract.StudentEntry.COLUMN_NAME_ENTRY_ID+" INTEGER PRIMARY KEY,"+
        StudentContract.StudentEntry.COLUMN_NAME_NAME+TEXT_TYPE+COMMA_SEP+
        StudentContract.StudentEntry.COLUMN_NAME_AGE+INTEGER_TYPE+COMMA_SEP+
        StudentContract.StudentEntry.COLUMN_NAME_HEIGHT+INTEGER_TYPE+" )";

private static final String SQL_DELETE_INTRIES="DROP TABLE IF EXISTS "+
        StudentContract.StudentEntry.TABLE_NAME;

Just like files that you save on the device’s internal storage, Android stores your database in private disk space that’s associated application. Your data is secure, because by default this area is not accessible to other applications.

A useful set of APIs is available in the SQLiteOpenHelper class. When you use this class to obtain references to your database, the system performs the potentially long-running operations of creating and updating the database only when needed and not during app startup. All you need to do is call getWritableDatabase() or getReadableDatabase().

Note: Because they can be long-running, be sure that you call getWritableDatabase() or getReadableDatabase() in a background thread, such as with AsyncTask or IntentService.

To use SQLiteOpenHelper, create a subclass that overrides the onCreate(), onUpgrade() and onOpen() callback methods. You may also want to implement onDowngrade(), but it’s not required.

public class DBHelper extends SQLiteOpenHelper {

    private static final int DATABASE_VERSION=1;
    private static final String DATABASE_NAME="studentInfo.db";

    public DBHelper(Context context, String name, SQLiteDatabase.CursorFactory factory, int version) {
        super(context, DATABASE_NAME, null, DATABASE_VERSION);
    }

    @Override
    public void onCreate(SQLiteDatabase db) {
        db.execSQL(SQL_CREATE_ENTRY);
    }

    @Override
    public void onUpgrade(SQLiteDatabase db, int oldVersion, int newVersion) {
        // This database is only a cache for online data, so its upgrade policy is
        // to simply to discard the data and start over
        if(oldVersion>newVersion) return;
        db.execSQL(SQL_DELETE_INTRIES);
        onCreate(db);
    } 
}

To access your database, instantiate your subclass of SQLiteOpenHelper:

 DBHelper mdbHelper=new DBHelper(MainActivity.this);

Put Information into a Database

insert data into the database by passing a ContentValues object to the insert() method:

// Gets the data repository in write mode
SQLiteDatabase db=mdbHelper.getWritableDatabase();

// Create a new map of values, where column names are the keys
ContentValues values=new ContentValues();
values.put(StudentContract.StudentEntry.COLUMN_NAME_ENTRY_ID,1);
values.put(StudentContract.StudentEntry.COLUMN_NAME_NAME,"Eric");
values.put(StudentContract.StudentEntry.COLUMN_NAME_AGE,"20");
values.put(StudentContract.StudentEntry.COLUMN_NAME_HEIGHT,"130");

// Insert the new row, returning the primary key value of the new row
long newRowId=db.insert(StudentContract.StudentEntry.TABLE_NAME,
        StudentContract.StudentEntry.COLUMN_NAME_NULLABLE,values);

The first argument for insert() is simply the table name. The second argument provides the name of a column in which the framework can insert NULL in the event that the ContentValues is empty (if you instead set this to “null”, then the framework will not insert a row when there are no values).

Read Information from a Database

To read from a database, use the query() method, passing it your selection criteria and desired columns. The method combines elements of insert() and update(), except the column list defines the data you want to fetch, rather than the data to insert. The results of the query are returned to you in a Cursor object.

SQLiteDatabase db=mdbHelper.getReadableDatabase();

// Define a projection that specifies which columns from the database
// you will actually use after this query.
String []projection={
        StudentContract.StudentEntry.COLUMN_NAME_ENTRY_ID,
        StudentContract.StudentEntry.COLUMN_NAME_NAME,
        StudentContract.StudentEntry.COLUMN_NAME_AGE,
        StudentContract.StudentEntry.COLUMN_NAME_HEIGHT
};

// How you want the results sorted in the resulting Cursor
String sortOrder="age"+" DESC";

Cursor cursor=db.query(
        StudentContract.StudentEntry.TABLE_NAME,    // The table to query
        projection,                                 // The columns to return
        "time" + "=?",                              // The columns for the WHERE clause
        new String[]{"20"},                         // The values for the WHERE clause
        null,                                       // don't group the rows
        null,                                       // don't filter by row groups
        sortOrder                                   // The sort order
);

To look at a row in the cursor, use one of the Cursor move methods, which you must always call before you begin reading values. Generally, you should start by calling moveToFirst(), which places the “read position” on the first entry in the results. For each row, you can read a column’s value by calling one of the Cursor get methods, such as getString() or getLong(). For each of the get methods, you must pass the index position of the column you desire, which you can get by calling getColumnIndex() or getColumnIndexOrThrow().

cursor.moveToFirst();
String name=cursor.getString(cursor.getColumnIndex("name"));
System.out.println(name);

Delete Information from a Database

To delete rows from a table, you need to provide selection criteria that identify the rows. The database API provides a mechanism for creating selection criteria that protects against SQL injection. The mechanism divides the selection specification into a selection clause and selection arguments. The clause defines the columns to look at, and also allows you to combine column tests. The arguments are values to test against that are bound into the clause. Because the result isn’t handled the same as a regular SQL statement, it is immune to SQL injection.

// Define 'where' part of query.
String selection= StudentContract.StudentEntry.COLUMN_NAME_NAME+" LIKE ?";
// Specify arguments in placeholder order.
String selectionArgs[]={"Eric"};
// Issue SQL statement.
db.delete(StudentContract.StudentEntry.TABLE_NAME,
        selection,selectionArgs);

Update a Database

When you need to modify a subset of your database values, use the update() method.
Updating the table combines the content values syntax of insert() with the where syntax of delete().

// New value for one column
ContentValues contentValues=new ContentValues();
contentValues.put(StudentContract.StudentEntry.COLUMN_NAME_NAME,"Eric2");

// Which row to update, based on the ID
String selection= StudentContract.StudentEntry.COLUMN_NAME_ENTRY_ID+" LIKE ?";
String []selectionArgs={String.valueOf(1)};

int count=db.update(
        StudentContract.StudentEntry.TABLE_NAME,
        contentValues,
        selection,
        selectionArgs
);