如何在（Flutter）中仅将新值存储到 sqflite 数据表中

Question

我已将电话列表存储到数据表中。我只想将新的通话清单数据存储到此数据表中。这意味着，只会保存新数据而跳过现有数据。 请举例说明。 这是我的代码：

这是数据库助手 database_helper.dart

import 'dart:io';
import 'package:path/path.dart';
import 'package:sqflite/sqflite.dart';
import 'package:path_provider/path_provider.dart';

class DatabaseHelper {
  static final _databaseName = "MyDatabase.db";
  static final _databaseVersion = 1;

  static final table = 'my_table';

  static final columnId = '_id';
  static final columnName = 'name';
  static final columnNumber = 'number';
  static final columnType = 'type';
  static final columnDate = 'date';
  static final columnDuration = 'duration';

  // make this a singleton class
  DatabaseHelper._privateConstructor();
  static final DatabaseHelper instance = DatabaseHelper._privateConstructor();

  // only have a single app-wide reference to the database
  static Database _database;
  Future<Database> get database async {
    if (_database != null) return _database;
    // lazily instantiate the db the first time it is accessed
    _database = await _initDatabase();
    return _database;
  }

  // this opens the database (and creates it if it doesn't exist)
  _initDatabase() async {
    Directory documentsDirectory = await getExternalStorageDirectory();
    String path = join(documentsDirectory.path, _databaseName);
    await deleteDatabase(path);
    return await openDatabase(path,
        version: _databaseVersion, onCreate: _onCreate);
  }

  // SQL code to create the database table
  Future _onCreate(Database db, int version) async {
    await db.execute('''
          CREATE TABLE $table (
            $columnId INTEGER PRIMARY KEY,
            $columnName TEXT,
            $columnNumber INTEGER,
            $columnType TEXT,
            $columnDate DATETIME,
            $columnDuration INTEGER
          )
          ''');
  }

  // Helper methods

  // Inserts a row in the database where each key in the Map is a column name
  // and the value is the column value. The return value is the id of the
  // inserted row.
  Future<int> insert(Map<String, dynamic> row, {ConflictAlgorithm conflictAlgorithm = ConflictAlgorithm.replace}) async {
    Database db = await instance.database;
    return await db.insert(table, row, conflictAlgorithm: conflictAlgorithm);
  }

  // All of the rows are returned as a list of maps, where each map is
  // a key-value list of columns.
  Future<List<Map<String, dynamic>>> queryAllRows() async {
    Database db = await instance.database;
    return await db.query(table);
  }

  // All of the methods (insert, query, update, delete) can also be done using
  // raw SQL commands. This method uses a raw query to give the row count.
  Future<int> queryRowCount() async {
    Database db = await instance.database;
    return Sqflite.firstIntValue(
        await db.rawQuery('SELECT COUNT(*) FROM $table'));
  }
  
  // We are assuming here that the id column in the map is set. The other
  // column values will be used to update the row.
  Future<int> update(Map<String, dynamic> row) async {
    Database db = await instance.database;
    int id = row[columnId];
    return await db.update(table, row, where: '$columnId = ?', whereArgs: [id]);
  }

  // Deletes the row specified by the id. The number of affected rows is
  // returned. This should be 1 as long as the row exists.
  Future<int> delete(int id) async {
    Database db = await instance.database;
    return await db.delete(table, where: '$columnId = ?', whereArgs: [id]);
  }
}

这是主文件。我在这里只添加了数据库插入方法。 home.dart

...
Future callLogDB() async {
    Iterable<CallLogEntry> cLog = await CallLog.get();
    final dbHelper = DatabaseHelper.instance;

    cLog.forEach((log) async {
      // row to insert
      Map<String, dynamic> row = {
        DatabaseHelper.columnName: '${log.name}',
        DatabaseHelper.columnNumber: '${log.number}',
        DatabaseHelper.columnType: '${log.callType}',
        DatabaseHelper.columnDate:
            '${DateTime.fromMillisecondsSinceEpoch(log.timestamp)}',
        DatabaseHelper.columnDuration: '${log.duration}'
      };
      await dbHelper.insert(row, conflictAlgorithm: ConflictAlgorithm.replace);
      print('CallLog:: $row');
    });
    return cLog;
  }
...

我的代码有什么问题？

Answer 1

有几种方法可以做到这一点，我将提供的方法不是最好或最好的，但希望他们能有所帮助

1) 只需将所有数据写入表格

您可以将所有数据插入到表中，将 ConflictAlgorithm 设置为 replace 或 ignore

db.insert(table, data, conflictAlgorithm: ConflictAlgorithm.replace);

这将替换/忽略相同的条目

2) 查询、比较、替换

这是一个不太“优雅”的解决方案，您可以先从表中查询所有数据

db.query(table, columns: availableColumns, where: 'columnToQueryBy = ?', whereArgs: [neededValue]);

然后与你拥有的数据进行比较

然后如上使用db.insert()写

我认为在您的情况下，第一个选项更适合，this 示例几乎涵盖了可能对您有所帮助的大部分内容

希望对你有帮助！

Answer 2

如何从 Sqflite 读取数据并在数据表中显示？