how can I save data with key in sqflite package in flutter?

The sqflite package uses SQLite, which is a relational database, not a key-value pair database. But you can technically replicate the behavior of a key-value database in it by creating a single table with all the data.

Please note that this is NOT a recommended way to use SQL. If you want a key-value db, just use hive.

First, create the database with the table

Database database = await openDatabase(path, version: 1,
  onCreate: (Database db, int version) async {
    // When creating the db, create the table
    await db.execute(
      'CREATE TABLE Data (_id INTEGER PRIMARY KEY, key TEXT, value TEXT)');
});

Putting the data into the database:

await database.transaction((txn) async {
  await txn.rawInsert(
      'INSERT INTO Data(key, value) VALUES("key", "value")');
});

Getting data from the database:

String value = await database.rawQuery('SELECT "key" FROM Data').first['value'];

Again, this is NOT recommended. Use Hive or other key-value db.