# sqlalchemy

***

**1. Create a new database and table:**

```python
from sqlalchemy import create_engine, MetaData, Table, Column, Integer, String
engine = create_engine('postgresql://user:password@localhost:5432/my_database')
metadata = MetaData()
users = Table('users', metadata,
    Column('id', Integer, primary_key=True),
    Column('name', String(255), nullable=False),
    Column('email', String(255), nullable=False))
metadata.create_all(engine)
```

**2. Insert data into a table:**

```python
from sqlalchemy.orm import sessionmaker
Session = sessionmaker(bind=engine)
session = Session()
new_user = User(name='John Doe', email='john@example.com')
session.add(new_user)
session.commit()
```

**3. Query data from a table:**

```python
users = session.query(User).all()
for user in users:
    print(user.name, user.email)
```

**4. Update data in a table:**

```python
user = session.query(User).filter_by(id=1).first()
user.name = 'Jane Doe'
session.commit()
```

**5. Delete data from a table:**

```python
session.delete(user)
session.commit()
```

**6. Join tables:**

```python
from sqlalchemy.orm import aliased
user_alias = aliased(User)
orders = session.query(User, user_alias.name).join(user_alias, User.id == user_alias.id).all()
```

**7. Group by:**

```python
orders = session.query(User.name, func.count(Order.id)).group_by(User.name).all()
```

**8. Order by:**

```python
orders = session.query(User.name).order_by(User.name.desc()).all()
```

**9. Limit results:**

```python
orders = session.query(User.name).limit(10).all()
```

**10. Offset results:**

```python
orders = session.query(User.name).offset(10).all()
```

**11. Use raw SQL:**

```python
from sqlalchemy import text
results = session.execute(text('SELECT * FROM users'))
```

**12. Bulk operations:**

```python
from sqlalchemy.orm import load_only
results = session.query(User).options(load_only('name')).all()
```

**13. Caching:**

```python
from sqlalchemy.orm import sessionmaker
Session = sessionmaker(bind=engine, expire_on_commit=False)
session = Session()
```

**14. Relationships:**

```python
from sqlalchemy import ForeignKey
orders = Table('orders', metadata,
    Column('id', Integer, primary_key=True),
    Column('user_id', Integer, ForeignKey('users.id')))
```

**15. Eager loading:**

```python
users = session.query(User).options(joinedload('orders')).all()
```

**16. Lazy loading:**

```python
user = session.query(User).first()
user.orders  # Lazy load the orders
```

**17. Pagination:**

```python
from sqlalchemy.orm import paginate
page = paginate(session.query(User), page=1, per_page=10)
```

**18. Custom types:**

```python
from sqlalchemy.types import TypeDecorator
class MyType(TypeDecorator):
    impl = String

    def process_bind_param(self, value, dialect):
        return value.upper()

    def process_result_value(self, value, dialect):
        return value.lower()
```

**19. Event listeners:**

```python
from sqlalchemy import event
@event.listens_for(User, 'after_insert')
def my_after_insert_listener(mapper, connection, target):
    print('New user inserted:', target)
```

**20. Custom filters:**

```python
from sqlalchemy import func
@func.scalar
def my_custom_filter(user_id):
    return User.id == user_id
```

**21. Custom validators:**

```python
from sqlalchemy.ext.declarative import declared_attr
from werkzeug.security import generate_password_hash, check_password_hash
class User:
    @declared_attr
    def password_hash(cls):
        return Column(String(128), nullable=False)
    def set_password(self, password):
        self.password_hash = generate_password_hash(password)
    def check_password(self, password):
        return check_password_hash(self.password_hash, password)
```

**22. Custom query functions:**

```python
from sqlalchemy import func
@func.scalar
def my_custom_query_function(user_id):
    return session.query(User.name).filter_by(id=user_id).scalar()
```

**23. Custom expression types:**

```python
from sqlalchemy import Column, Integer, expression
class MyExpressionType(expression.ColumnElement):
    def __init__(self, name):
        self.name = name

    def get_bind(self, compiler, **kw):
        return compiler.visit_column(self)
```

**24. Custom connection processors:**

```python
from sqlalchemy.engine import Connection
def my_connection_processor(connection):
    connection.execute('SET TIME ZONE "Europe/London"')
```

**25. Custom pool pre-pingers:**

```python
from sqlalchemy.engine import Engine
def my_pre_pinger(connection):
    connection.execute('SELECT 1')
```

**26. Custom dialect:**

```python
from sqlalchemy.engine import default
class MyDialect(default.DefaultDialect):
    default_paramstyle = 'named'
```

**27. Custom type coercion:**

```python
from sqlalchemy.types import TypeDecorator
class MyTypeCoercer(TypeDecorator):
    impl = String

    def coerce_result_value(self, value, dialect):
        return value.upper()
```

**28. Custom dialect-specific compiler:**

```python
from sqlalchemy.engine.base import Compiled

```