Filtering and targeting data

Introduction to Databases in Python

Jason Myers

Co-Author of Essential SQLAlchemy and Software Engineer

Where clauses

stmt = select([census])

stmt = stmt.where(census.columns.state == 'California')

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

for result in results:
    print(result.state, result.age)

California 0
California 1
California 2
California 3
California 4
California 5
...

Where clauses

Restrict data returned by a query based on Boolean conditions
Compare a column against a value or another column
Often use comparisons ==, <=, >=, or !=

Expressions

Provide more complex conditions than simple operators
E.g.in_(), like(), between()
Many more in documentation
Available as method on a Column

Expressions

stmt = select([census])

stmt = stmt.where(census.columns.state.startswith('New'))

for result in connection.execute(stmt):
    print(result.state, result.pop2000)

New Jersey 56983
New Jersey 56686
New Jersey 57011
...

Conjunctions

Allow us to have multiple criteria in a where clause
Eg. and_(), or_(), not_()

Conjunctions

from sqlalchemy import or_

stmt = select([census])

stmt = stmt.where(
    or_(census.columns.state == 'California',
        census.columns.state == 'New York'
    )
)

for result in connection.execute(stmt):
    print(result.state, result.sex)

New York M
...
California F

Let's practice!

Introduction to Databases in Python