What is SQLAlchemy and how is it used in Flask?

SQLAlchemy is Python's most popular ORM (Object-Relational Mapper). Flask-SQLAlchemy integrates it with Flask, providing a convenient setup. Install: pip install flask-sqlalchemy. Configure: app.config['SQLALCHEMY_DATABASE_URI'] = 'sqlite:///db.sqlite3'; db = SQLAlchemy(app). Define a model: class User(db.Model): id = db.Column(db.Integer, primary_key=True); name = db.Column(db.String(100), nullable=False). Create tables: db.create_all(). Query: User.query.all(), User.query.filter_by(name='Alice').first(). Insert: db.session.add(user); db.session.commit(). Flask-SQLAlchemy automatically handles connection pooling, session lifecycle tied to the request context, and supports multiple databases.