From http://developer.android.com/guide/topics/providers/content-provider-basics.html

ContentResolver.java

/*
 * This defines a one-element String array to contain the selection argument.
 */
String[] mSelectionArgs = {""};
// 从UI中获取单词
mSearchString = mSearchWord.getText().toString();
// Remember to insert code here to check for invalid or malicious input.
// 如果为空，则获取全部单词
if (TextUtils.isEmpty(mSearchString)) { 
   // 设置选择条件为空，会返回所有
    mSelectionClause = null;
    mSelectionArgs[0] = "";
} else {
// 构造可以匹配用户输入的单词的选择条件
    mSelectionClause = UserDictionary.Words.WORD + " = ?";
// 将用户输入的单词作为选择参数
    mSelectionArgs[0] = mSearchString;
}
// Does a query against the table and returns a Cursor object
mCursor = getContentResolver().query(
    UserDictionary.Words.CONTENT_URI,  // The content URI of the words table
    mProjection,                       // The columns to return for each row
    mSelectionClause                   // Either null, or the word the user entered
    mSelectionArgs,                    // Either empty, or the string the user entered
    mSortOrder);                       // The sort order for the returned rows// Some providers return null if an error occurs, others throw an exception
if (null == mCursor) {
/*
     * Insert code here to handle the error. Be sure not to use the cursor! You may want to
     * call android.util.Log.e() to log this error.
     *
     */// If the Cursor is empty, the provider found no matches
} else if (mCursor.getCount() < 1) {    /*
     * Insert code here to notify the user that the search was unsuccessful. This isn't necessarily
     * an error. You may want to offer the user the option to insert a new row, or re-type the
     * search term.
     */
} else {
    // Insert code here to do something with the results}