Chapter 5 data-centric-Data Access (5), Chapter 5 Access

Chapter 5 data-centric-Data Access (5), Chapter 5 Access
5.2.3 add, delete, and modify operations

The following describes how to add, delete, and modify an object. The following describes how to perform database operations by encapsulating these actions in a class DBHelper.

// Import omitted Public class DBHelper {   Private static final String [] COLS = new String [] {"_ id", "name "}; Private SQLiteDatabase db; Private final DBOpenHelper dbOpenHelper;   Public DBHelper (final Context context ){ This. dbOpenHelper = new DBOpenHelper (context ); EstablishDb (); } /** * Get a writable SQLite database. If the database has not been created, * The DBOpenHelper auxiliary class is responsible for establishing this database. * If the database has been created, a writable database is directly returned. */ Private void establishDb (){ If (this. db = null ){ This. db = this. dbOpenHelper. getWritableDatabase (); } } /** * Shut down the database. */ Public void cleanup (){ If (this. db! = Null ){ This. db. close (); This. db = null; } } /** * Insert a data entry */ Public void insert (String id, String name ){ ContentValues values = new ContentValues (); Values. put ("_ id", id ); Values. put ("name", id ); This. db. insert (DBOpenHelper. TABLE_NAME, null, values ); Cleanup (); } /** * Update a piece of data */ Public void update (String id, String name ){ ContentValues values = new ContentValues (); Values. put ("_ id", id ); Values. put ("name", id ); This. db. update (DBOpenHelper. TABLE_NAME, values, "_ id =" + id, null ); Cleanup (); } /** * Delete a piece of data */ Public void delete (final long id ){ This. db. delete (DBOpenHelper. TABLE_NAME, "_ id =" + id, null ); } /** * Delete all data */ Public void deleteAll (){ This. db. delete (DBOpenHelper. TABLE_NAME, null, null ); } Private void query (){ // Obtain a writable database. SQLiteDatabase db = dbOpenHelper. getReadableDatabase (); // Query the database Cursor cur = db. query (DBOpenHelper. TABLE_NAME, COLS, null ); If (cur! = Null ){ For (cur. moveToFirst ();! Cur. isAfterLast (); cur. moveToNext ()){ Int idColumn = cur. getColumnIndex (COLS [0]); Int nameColumn = cur. getColumnIndex (COLS [1]); String id = cur. getString (idColumn ); String name = cur. getString (nameColumn ); } Cur. close (); } } }

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The Code "Cursor cur = db. query (DBOpenHelper. TABLE_NAME, COLS, null, null);" puts the queried data into a Cursor. This Cursor encapsulates all the columns in TABLE_NAME of the data table. The query () method is quite useful. Here we will briefly talk about the parameters in the query.

The first parameter is the name of the table in the database. In our example, the table name is TABLE_NAME, that is, "myTableName ".

The second parameter is the column information that we want to return data. In this example, we want to obtain the id and name columns. We put the names of these two columns in the string array.

The third parameter is selection, which is equivalent to the where part of the SQL statement. If you want to return all the data, you can directly set it to null.

The fourth parameter is selectionArgs. In the selection section, you may use "?", The string defined in selectionArgs will replace "?" In selection "?".

The fifth parameter is groupBy, which defines whether the queried data is grouped. If it is null, it means no grouping is required.

The sixth parameter is having, which is equivalent to having in an SQL statement.

The seventh parameter is orderBy to describe whether the expected return value needs to be sorted. If it is set to null, it means no sorting is required.

Integernum = cur. getCount (); you can use the getCount () method to obtain the number of data in cursor.

Next we will introduce some common methods of the Cursor class.

5.2.4 Cursor operations-Use Cursor

In databases, cursor is a very important concept. A cursor provides a flexible means to operate the data retrieved from a table. In essence, A cursor is actually a mechanism that can extract a record from a result set that contains multiple data records.

In the example in the previous section, we see the Cursor, which is the Cursor. We can simply think of Cursor as an object pointing to a row of data in the database. In Android, the query database is implemented through the Cursor class. When we use the SQLiteDatabase. query () method, we will get a Cursor object, which points to each piece of data. It provides many query methods, as shown in Table 5-2.

Method	Return Value	Description
Close ()	Void	Close the cursor to release resources
CopyStringToBuffer (int columnIndex, CharArrayBuffer buffer)	Void	Retrieve the text of the requested column in the buffer and store it
GetColumnCount ()	Int	Returns the total number of all columns.
GetColumnIndex (String columnName)	Int	Returns the name of the specified column. If no Column exists,-1 is returned.
GetColumnIndexOrThrow (String columnName)	Int	The specified column name is returned from scratch. If the column name does not exist, an IllegalArgumentException exception is thrown.
GetColumnName (int columnIndex)	String	Returns the column name from the given index.
GetColumnNames ()	String []	Returns the column name of a string array.
GetCount ()	Int	Returns the number of rows in Cursor.
MoveToFirst ()	Boolean	Move the cursor to the first line
MoveToLast ()	Boolean	Move the cursor to the last line
MoveToNext ()	Boolean	Move the cursor to the next row
MoveToPosition (int position)	Boolean	Move the cursor to an absolute position
MoveToPrevious ()	Boolean	Move the cursor to the previous line
IsBeforeFirst ()	Boolean	Returns whether the cursor points to the position of the first row.
IsAfterLast ()	Boolean	Returns whether the cursor points to the position of the last row.
IsClosed ()	Boolean	If true is returned, the cursor is closed.

Table 5-2 Cursor Method

 

The previous code example contains a simple example of Using Cursor.