7.53. SQLAlchemy Use Cases

7.53.1. Use Case - 0x01

  • SQLAlchemy to Pandas

from sqlalchemy import create_engine
import pandas as pd


# DATABASE powinen zaczynać się od sqlite:///
DATABASE = 'sqlite:///myfile.db'

SQL = """

    SELECT *
    FROM astronauts;

"""

# %%timeit -r 1 -n 1
# with create_engine(DATABASE).connect() as db:
#    df = pd.read_sql(SQL, db)
# 2.47 ms ± 0 ns per loop (mean ± std. dev. of 1 run, 1 loop each)
#
# %%
# df.info(memory_usage='deep')

with create_engine(DATABASE).connect() as db:
    df = pd.read_sql(SQL, db)

print(df)

7.53.2. Use Case - 0x02

from datetime import date

from sqlalchemy import create_engine, select, func
from sqlalchemy import Column, String, Integer, Date
from sqlalchemy.ext.declarative import declarative_base
from sqlalchemy.orm import sessionmaker



DATABASE = 'sqlite:///:memory:'
engine = create_engine(DATABASE, future=True)
Session = sessionmaker(engine)
Model = declarative_base()


class User(Model):
    __tablename__ = 'users'
    uid = Column(Integer, autoincrement=True, primary_key=True)
    firstname = Column(String, nullable=False, index=True)
    lastname = Column(String, nullable=False)
    born = Column(Date, default=date.today)

    def __repr__(self):
        return f'{self.firstname} {self.lastname}'

with Session.begin() as db:
    Model.metadata.create_all(engine)
    db.add_all([
        User(firstname='Mark', lastname='Watney'),
        User(firstname='Melissa', lastname='Lewis'),
        User(firstname='Rick', lastname='Martinez'),
        User(firstname='Alex', lastname='Vogel'),
        User(firstname='Beth', lastname='Johanssen'),
        User(firstname='Chris', lastname='Beck'),
    ])


query_select = (
    select(User.firstname,
           User.lastname,
           User.born).

    where((User.born >= date(1969,7,21))
        & (User.born <= date.today())
        & (User.born.between('2022-01-01', '2022-02-23'))
        & (User.firstname != None)
        & (User.lastname.in_(['Watney', 'Lewis', 'Martinez']))
        & (User.firstname.in_(
            select(User.firstname).
            where(User.firstname.startswith('M')).
            distinct()))
        & (User.firstname.like('Mel__%'))).

    order_by(User.firstname.desc().nulls_first(),
             User.lastname.asc()).

    group_by(User.firstname).
    having(func.count(User.firstname == 1)).
    limit(10).
    offset(5)
)

print(query_select.compile(engine, compile_kwargs={"literal_binds": True}))
# SELECT users.firstname,
#        users.lastname,
#        users.born
# FROM users
# WHERE users.born >= '1969-07-21'
#   AND users.born <= '2022-02-22'
#   AND users.born BETWEEN '2022-01-01' AND '2022-02-23'
#   AND users.firstname IS NOT NULL
#   AND users.lastname IN ('Watney', 'Lewis', 'Martinez')
#   AND users.firstname IN (
#       SELECT DISTINCT users.firstname
#       FROM users
#       WHERE (users.firstname LIKE 'M' || '%'))
#   AND users.firstname LIKE 'Mel__%'
# GROUP BY users.firstname
# HAVING count(users.firstname = 1)
# ORDER BY users.firstname DESC NULLS FIRST,
#          users.lastname ASC
# LIMIT 10
# OFFSET 5


with Session.begin() as db:
    result = db.execute(query_select)
    for row in result.all():
        print(row)

# (1, 'Mark', 'Watney')
# (2, 'Melissa', 'Lewis')
# (3, 'Rick', 'Martinez')
# (4, 'Alex', 'Vogel')
# (5, 'Beth', 'Johanssen')
# (6, 'Chris', 'Beck')