Content Value和Cursor的一些认识及其方法的使用
ContentValue用来向数据库的表中插入新的行。每一个Content Values对象都将一个表行表示为列名到值的映射。
数据库查询作为Cursor对象返回。Cursor是底层数据中的结果集的指针，它没有提取和返回结果值的副本。Cursor为控制在数据库查询的结果集中的位置（行）提供了一种易于管理的方式。
在理解和使用 Android Cursor 的时候你必须先知道关于 Cursor 的几件事情：
Cursor是每行的集合、使用 moveToFirst() 定位第一行、你必须知道每一列的名称、你必须知道每一列的数据类型、Cursor 是一个随机的数据源、所有的数据都是通过下标取得。
在Android 查询数据是通过Cursor 类来实现的。当我们使用 SQLiteDatabase.query()方法时，就会得到Cursor对象， Cursor所指向的就是每一条数据。Cursor 位于 android.database.Cursor类，可见出它的设计是基于数据库服务产生的。
Cursor类包含了多个导航函数，其中包含但不限于以下几种：
moveToFirst（）：把游标移动到查询结果的第一行，返回boolean类型，返回false代表游标为空。
booleanandroid.database.Cursor.moveToFirst()
Movethe cursor to the first row.
Thismethod will return false if the cursor is empty.
Returns:whether the move succeeded.
moveToNext（）：把游标移动到下一行。
booleanandroid.database.Cursor.moveToNext()
Movethe cursor to the next row.
Thismethod will return false if the cursor is already past the last entry in theresult set.
Returns:whether the move succeeded.
moveToPrevious（）：把游标移动到前一行。
booleanandroid.database.Cursor.moveToPrevious()
Movethe cursor to the previous row.
Thismethod will return false if the cursor is already before the first entry in theresult set.
Returns:whether the move succeeded.
getCount（）：返回结果集中的行数。
intandroid.database.Cursor.getCount()
Returnsthe numbers of rows in the cursor.
Returns:the number of rows in the cursor.
getColumnIndexOrThrow（String columnName）throws IllegalArgumentException：返回具有指定名称的列的索引（如果不存在拥有该名称的列，就会抛出异常），索引从0开始计数。
intandroid.database.Cursor.getColumnIndexOrThrow(String columnName) throwsIllegalArgumentException
Returnsthe zero-based index for the given column name, or throwsIllegalArgumentException if the column doesn't exist. If you're not sure if acolumn will exist or not use getColumnIndex(String) and check for -1, which ismore efficient than catching the exceptions.
Parameters:columnName the name of the target column.
Returns:the zero-based column index for the given column name
Throws:IllegalArgumentException - if the column does not exist
SeeAlso: getColumnIndex(String)
getColumnName（int columnIndex）：返回指定列索引的名称。
Stringandroid.database.Cursor.getColumnName(int columnIndex)
Returnsthe column name at the given zero-based column index.
Parameters:columnIndex the zero-based index of the target column.
Returns:the column name for the given column index.
getColumnNames（）：返回当前Cursor中所有列名的字符串数组。
String[]android.database.Cursor.getColumnNames()
Returnsa string array holding the names of all of the columns in the result set in theorder in which they were listed in the result.
Returns:the names of the columns returned in this query.
moveToPosition（int position）：将游标移动到指定行。
booleanandroid.database.Cursor.moveToPosition(int position)
Movethe cursor to an absolute position. The valid range of values is -1 <=position <= count.
Thismethod will return true if the request destination was reachable, otherwise, itreturns false.
Parameters:position the zero-based position to move to.
Returns:whether the requested move fully succeeded.
getPosition（）：返回当前游标位置。
intandroid.database.Cursor.getPosition()
Returnsthe current position of the cursor in the row set. The value is zero-based.When the row set is first returned the cursor will be at positon -1, which isbefore the first row. After the last row is returned another call to next()will leave the cursor past the last entry, at a position of count().
Returns:the current cursor position.
getBlob（int columnIndex）：获取存入数据库的图片（Bitmap）
byte[]android.database.Cursor.getBlob(int columnIndex)
Returnsthe value of the requested column as a byte array.
Theresult and whether this method throws an exception when the column value isnull or the column type is not a blob type is implementation-defined.
Parameters:columnIndex the zero-based index of the target column.
Returns:the value of that column as a byte array.
示例：
publicBitmap getBmp(int position)
{
SQLiteDatabase db = getReadableDatabase();
Cursor cursor = select(TB_NAME);
cursor.moveToPosition(position);
byte[] in =cursor.getBlob(cursor.getColumnIndex(IMAGE));
Bitmap bmpout =BitmapFactory.decodeByteArray(in, 0, in.length);
return bmpout;
}
getString（int columnIndex）：获得指定列的值，以String类型返回。
Stringandroid.database.Cursor.getString(int columnIndex)
Returnsthe value of the requested column as a String.
Theresult and whether this method throws an exception when the column value isnull or the column type is not a string type is implementation-defined.
Parameters:columnIndex the zero-based index of the target column.
Returns:the value of that column as a String.
isBeforeFirst（）：返回游标是否指向第一行之前的位置。
booleanandroid.database.Cursor.isBeforeFirst()
Returnswhether the cursor is pointing to the position before the first row.
Returns:whether the cursor is before the first result.
isAfterLast（）：返回游标是否指向第最后一行之后的位置。
booleanandroid.database.Cursor.isAfterLast()
Returnswhether the cursor is pointing to the position after the last row.
Returns:whether the cursor is after the last result.
isClosed（）：如果返回 true 即表示该游戏标己关闭
booleanandroid.database.Cursor.isClosed()
returntrue if the cursor is closed
Returns:true if the cursor is closed.
访问 Cursor 的下标获得其中的数据：
intnameColumnIndex = cur.getColumnIndex(People.NAME);
Stringname = cur.getString(nameColumnIndex);
现在让我们看看如何循环 Cursor 取出我们需要的数据
while(cur.moveToNext()){
//光标移动成功
String email =cursor.getString(cursor.getColumnIndex(RuiXin.EMAIL));
startManagingCursor(cursor); //查找后关闭游标
//把数据取出
}
当cur.moveToNext() 为假时将跳出循环，即 Cursor 数据循环完毕。
如果你喜欢用 for 循环而不想用While 循环可以使用Google 提供的几下方法：isBeforeFirst()、isAfterLast()、isClosed()
有了以上的方法，可以如此取出数据：
for(cur.moveToFirst();!cur.isAfterLast();cur.moveToNext())
{
int nameColumn =cur.getColumnIndex(People.NAME);
int phoneColumn =cur.getColumnIndex(People.NUMBER);
String name = cur.getString(nameColumn);
String phoneNumber =cur.getString(phoneColumn);
}