# SQLAlchemy

```python
import pandas as pd
from sqlalchemy import create_engine, text, inspect
sqlalchemy.__version__
```

```bash
pip install sqlalchemy-cockroachdb
```

#### Create engine

```python
from sqlalchemy import create_engine, text, inspect
engine = create_engine('sqlite:///mydb.db')
```

#### Inspector

```python
from sqlalchemy import create_engine, text, inspect
engine = create_engine('sqlite:///mydb.db')

inspector = inspect(engine)
schemas = inspector.get_schema_names()
schemas

schema = schemas[0]
table_names = inspector.get_table_names(schema=schema)
table_names

table_name = table_names[0]
columns = inspector.get_columns(table_name, schema=schema)
columns
```

```python
if inspect(engine).has_table(table_name):
    print("Has table")
```

```python
query = text("""
CREATE DATABASE mydb
""")
with engine.connect() as conn:
  conn.execute(text(query))
```

```python
url = "https://raw.githubusercontent.com/ywchiu/riii/master/data/house-prices.csv"
df = pd.read_csv(url)
df

df.to_sql('house_prices', index=False, if_exists='replace', con=engine)
```

```python
query = text("""
SELECT 
    * 
FROM 
    house_prices 
LIMIT 
    20
""")
df = pd.read_sql(query, con=engine)
df
```

## Session

```python
from sqlalchemy import create_engine
from sqlalchemy.orm import sessionmaker

engine = create_engine('postgresql+psycopg2://username:password@localhost:port/database')

Session = sessionmaker(engine)

session = Session()
try:
    user1 = User(name="user1")
    session.add(user1)
    
    session.execute()
    session.delete(obj1)

    session.commit()
except Exception as e:
    session.rollback()
    raise e
finally:
    session.close()
```

## Object Relational Mapping (ORM)

```python
from sqlalchemy import create_engine
from sqlalchemy import func, or_, not_, and_
from sqlalchemy.orm import sessionmaker
from sqlalchemy.ext.automap import automap_base

engine = create_engine('postgresql+psycopg2://username:password@localhost:port/database')

Base = automap_base()
Base.prepare(engine, reflect=True)

# map existing table
User = Base.classes.user
Acnt = Base.classes.account
Txn = Base.classes.transaction

Session = sessionmaker(engine)
session = Session()

def get_user(user_id: int):
        user = session.query(User).filter(
                User.id == user_id
        ).first()
        return user

last_txn_id = session.query((func.max(Txn.id)).group_by(User.id).subquery()
last_txn_by_users = session.query(User, Acnt, Txn).filter(
        User.id == Txn.user_id,
        User.id == Acnt.user_id,
        User.is_active == 1,
        Acnt.deleted_at.is_(None),
        or_(Acnt.expire_date.is_(None), str(datetime.now()) < func.date(Acnt.expire_date)),
        Txnid.in_(last_txn_id)
).all()

# session.add(row)
# session.commit()
# session.refresh(row)
# .offset(skip) .limit
```


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