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构建Android查询详解

最新推荐文章于 2025-12-08 20:24:14 发布

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#android #ContentProvider #Projection #SelectionClause #SelectionArgs

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本文介绍如何在Android中通过ContentProvider构建查询，包括定义投影、选择子句、选择参数及排序等关键步骤，并给出具体示例。

今天继续讲android ContentProvider，讲构建查询，涉及Projection、SelectionClause、SelectionArgs、SortOrder。

下面是android developer官方的讲解：

构建查询

从提供程序中检索数据的下一步是构建查询。第一个代码段定义某些用于访问用户字典提供程序的变量：

// A "projection" defines the columns that will be returned for each row
String[] mProjection =
{
    UserDictionary.Words._ID,    // Contract class constant for the _ID column name
    UserDictionary.Words.WORD,   // Contract class constant for the word column name
    UserDictionary.Words.LOCALE  // Contract class constant for the locale column name
};

// Defines a string to contain the selection clause
String mSelectionClause = null;

// Initializes an array to contain selection arguments
String[] mSelectionArgs = {""};

下一个代码段以用户字典提供程序为例，显示了如何使用 ContentResolver.query()。提供程序客户端查询与 SQL 查询类似，并且包含一组要返回的列、一组选择条件和排序顺序。

查询应该返回的列集被称为投影（变量 mProjection）。

用于指定要检索的行的表达式分割为选择子句和选择参数。选择子句是逻辑和布尔表达式、列名称和值（变量 mSelectionClause）的组合。如果您指定了可替换参数 ? 而非值，则查询方法会从选择参数数组（变量 mSelectionArgs）中检索值。

在下一个代码段中，如果用户未输入字词，则选择子句将设置为 null，而且查询会返回提供程序中的所有字词。如果用户输入了字词，选择子句将设置为 UserDictionary.Words.WORD + " = ?" 且选择参数数组的第一个元素将设置为用户输入的字词。

/*
 * This defines a one-element String array to contain the selection argument.
 */
String[] mSelectionArgs = {""};

// Gets a word from the UI
mSearchString = mSearchWord.getText().toString();

// Remember to insert code here to check for invalid or malicious input.

// If the word is the empty string, gets everything
if (TextUtils.isEmpty(mSearchString)) {
    // Setting the selection clause to null will return all words
    mSelectionClause = null;
    mSelectionArgs[0] = "";

} else {
    // Constructs a selection clause that matches the word that the user entered.
    mSelectionClause = UserDictionary.Words.WORD + " = ?";

    // Moves the user's input string to the selection arguments.
    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

}

此查询与 SQL 语句相似：

SELECT _ID, word, locale FROM words WHERE word = <userinput> ORDER BY word ASC;

在此 SQL 语句中，会使用实际的列名称而非协定类常量。