Flutter数据库的使用方法(2)-木庄网络博客

查询表中的数据

// Get the records
List<Map> list = await database.rawQuery('SELECT * FROM Test');
List<Map> expectedList = [
  {'name': 'updated name', 'id': 1, 'value': 9876, 'num': 456.789},
  {'name': 'another name', 'id': 2, 'value': 12345678, 'num': 3.1416}
];
print(list);
print(expectedList);

查询表中存储数据的总条数

count = Sqflite.firstIntValue(await database.rawQuery('SELECT COUNT(*) FROM Test'));

关闭数据库

await database.close();

删除数据库

await deleteDatabase(path);

使用SQL助手

创建表中的字段及关联类

//字段
final String tableTodo = 'todo';
final String columnId = '_id';
final String columnTitle = 'title';
final String columnDone = 'done';

//对应类
class Todo {
  int id;
  String title;
  bool done;

  //把当前类中转换成Map，以供外部使用
  Map<String, Object?> toMap() {
    var map = <String, Object?>{
      columnTitle: title,
      columnDone: done == true ? 1 : 0
    };
    if (id != null) {
      map[columnId] = id;
    }
    return map;
  }
  //无参构造
  Todo();
  
  //把map类型的数据转换成当前类对象的构造函数。
  Todo.fromMap(Map<String, Object?> map) {
    id = map[columnId];
    title = map[columnTitle];
    done = map[columnDone] == 1;
  }
}

使用上面的类进行创建删除数据库以及数据的增删改查操作。

class TodoProvider {
  Database db;

  Future open(String path) async {
    db = await openDatabase(path, version: 1,
        onCreate: (Database db, int version) async {
      await db.execute('''
  create table $tableTodo ( 
  $columnId integer primary key autoincrement, 
  $columnTitle text not null,
  $columnDone integer not null)
                        ''');
    });
  }

  //向表中插入一条数据，如果已经插入过了，则替换之前的。
  Future<Todo> insert(Todo todo) async {
    todo.id = await db.insert(tableTodo, todo.toMap()，conflictAlgorithm: ConflictAlgorithm.replace,);
    return todo;
  }

  Future<Todo> getTodo(int id) async {
    List<Map> maps = await db.query(tableTodo,
        columns: [columnId, columnDone, columnTitle],
        where: '$columnId = ?',
        whereArgs: [id]);
    if (maps.length > 0) {
      return Todo.fromMap(maps.first);
    }
    return null;
  }

  Future<int> delete(int id) async {
    return await db.delete(tableTodo, where: '$columnId = ?', whereArgs: [id]);
  }

  Future<int> update(Todo todo) async {
    return await db.update(tableTodo, todo.toMap(),
        where: '$columnId = ?', whereArgs: [todo.id]);
  }

  Future close() async => db.close();
}

=查询表中的所有数据

List<Map<String, Object?>> records = await db.query('my_table');

获取结果中的第一条数据

Map<String, Object?> mapRead = records.first;

上面查询结果的列表中Map为只读数据，修改此数据会抛出异常

mapRead['my_column'] = 1;
// Crash... `mapRead` is read-only

创建map副本并修改其中的字段

// 根据上面的map创建一个map副本
Map<String, Object?> map = Map<String, Object?>.from(mapRead);
// 在内存中修改此副本中存储的字段值
map['my_column'] = 1;

把查询出来的List< map>类型的数据转换成List< Todo>类型,这样我们就可以痛快的使用啦。

// Convert the List<Map<String, dynamic> into a List<Todo>.
  return List.generate(maps.length, (i) {
    return Todo(
      id: maps[i][columnId],
      title: maps[i][columnTitle],
      done: maps[i][columnDown],
    );
  });

批处理

您可以使用批处理来避免dart与原生之间频繁的交互。

batch = db.batch();
batch.insert('Test', {'name': 'item'});
batch.update('Test', {'name': 'new_item'}, where: 'name = ?', whereArgs: ['item']);
batch.delete('Test', where: 'name = ?', whereArgs: ['item']);
results = await batch.commit();

获取每个操作的结果是需要成本的（插入的Id以及更新和删除的更改数）。如果您不关心操作的结果则可以执行如下操作关闭结果的响应

await batch.commit(noResult: true);

事务中使用批处理

在事务中进行批处理操作，当事务提交后才会提交批处理。

await database.transaction((txn) async {
  var batch = txn.batch();
  
  // ...
  
  // commit but the actual commit will happen when the transaction is committed
  // however the data is available in this transaction
  await batch.commit();
  
  //  ...
});

批处理异常忽略

默认情况下批处理中一旦出现错误就会停止（未执行的语句则不会被执行了），你可以忽略错误，以便后续操作的继续执行。

await batch.commit(continueOnError: true);

关于表名和列名

通常情况下我们应该避免使用SQLite关键字来命名表名称和列名称。如：

"add","all","alter","and","as","autoincrement","between","case","check","collate",
"commit","constraint","create","default","deferrable","delete","distinct","drop",
"else","escape","except","exists","foreign","from","group","having","if","in","index",
"insert","intersect","into","is","isnull","join","limit","not","notnull","null","on",
"or","order","primary","references","select","set","table","then","to","transaction",
"union","unique","update","using","values","when","where"