如何在创建表之前检查数据库是否存在？

Question

在Flutter使用中为数据库创建表之前如何检查数据库是否存在sqflite？

例如，如果我要创建数据库doggie_database.db，我如何过早地检查它在表创建中的存在？

final Future<Database> database = openDatabase(
  // Set the path to the database. 
  join(await getDatabasesPath(), 'doggie_database.db'),
  // When the database is first created, create a table to store dogs.
  onCreate: (db, version) {
    // Run the CREATE TABLE statement on the database.
    return db.execute(
      "CREATE TABLE dogs(id INTEGER PRIMARY KEY, name TEXT, age INTEGER)",
    );
  },
  // Set the version. This executes the onCreate function and provides a
  // path to perform database upgrades and downgrades.
  version: 1,
);

Answer 1

您可以使用databaseExists(String path).

https://github.com/tekartik/sqflite/blob/master/sqflite/lib/sqflite.dart（第 174 行）

/// Check if a database exists at a given path.
///
Future<bool> databaseExists(String path) =>
    databaseFactory.databaseExists(path);

但我认为您对CREATE TABLE再次调用该声明感到担忧。如果您指定version. 在内部，版本被保留，onCreate如果已经指定则不会被调用。

来自同一个文件：

/// If [version] is specified, [onCreate], [onUpgrade], and [onDowngrade] can
/// be called. These functions are mutually exclusive — only one of them can be
/// called depending on the context, although they can all be specified to
/// cover multiple scenarios
///
/// [onCreate] is called if the database did not exist prior to calling
/// [openDatabase]. You can use the opportunity to create the required tables
/// in the database according to your schema