Calculating values in a query

Introduction to Databases in Python

Jason Myers

Co-Author of Essential SQLAlchemy and Software Engineer

Math operators

addition +
subtraction -
multiplication *
division /
modulus %
Work differently on different data types

Calculating difference

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

stmt = stmt.group_by(census.columns.age)
stmt = stmt.order_by(desc('pop_change'))

stmt = stmt.limit(5)

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

[(61, 52672), (85, 51901), (54, 50808), (58, 45575),
(60, 44915)]

Case statement

Used to treat data differently based on a condition
Accepts a list of conditions to match and a column to return if the condition matches
The list of conditions ends with an else clause to determine what to do when a record doesn't match any prior conditions

Case example

from sqlalchemy import case

stmt = select([
        func.sum(
            case([
                (census.columns.state == 'New York', 
                 census.columns.pop2008)
            ], else_=0))])

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

[(19465159,)]

Cast statement

Converts data to another type
Useful for converting...
- integers to floats for division
- strings to dates and times
Accepts a column or expression and the target Type

Percentage example

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')])

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

[(Decimal('6.4267619765'),)]

Let's practice!

Introduction to Databases in Python