Querying the database

Introduction to Databases in Python

Jason Myers

Co-Author of Essential SQLAlchemy and Software Engineer

Part 3: answering data science questions with queries

Determine average age for males and females

from sqlalchemy import select

stmt = select([census.columns.sex,
         (func.sum(census.columns.pop2008 * 
          census.columns.age) /
          func.sum(census.columns.pop2008)
        ).label('average_age')])

stmt = stmt.group_by('census.columns.sex')

results = connection.execute(stmt).fetchall()

Part 3: answering data science questions with queries

Determine the percentage of Females for each state

from sqlalchemy import case, cast, Float

stmt = select([
         (func.sum(
             case([
                 (census.columns.state == 'New York', 
                  census.columns.pop2008)
             ], else_=0)) /
          cast(func.sum(census.columns.pop2008), 
               Float) * 100).label('ny_percent')])

Part 3: answering data science questions with queries

Determine the top 5 states by population change from 2000 to 2008

stmt = select([census.columns.age,           
         (census.columns.pop2008-
         census.columns.pop2000).label('pop_change')
     ])

stmt = stmt.order_by('pop_change')
stmt = stmt.limit(5)

Let's practice!

Introduction to Databases in Python