如何在创建表之前检查数据库是否存在？

Question

在使用sqflite为Flutter中的数据库创建表之前如何检查数据库是否存在？

例如，如果我要创建数据库doggie_database.db，如何在创建表时过早检查它的存在？

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) 检查数据库是否存在。

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);

但我认为您担心再次调用 CREATE TABLE 语句。如果您指定version，则不必担心这一点。在内部保留版本，如果已指定 onCreate，则不会调用它。

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