8.6. Core Operators¶

= - equals
!= - not equals
> - greater then
>= - greater or equal to
< - less then
<= - less or equal to
between() - in between two values or dates
Use != and == instead is and is not
There is no in and not in operator overload

8.6.1. SetUp¶

>>> from sqlalchemy import create_engine, MetaData, Table, Column
>>> from sqlalchemy import Integer, String, Date, Numeric, Enum
>>>
>>>
>>> engine = create_engine('sqlite:///:memory:', future=True)
>>> metadata = MetaData()
>>>
>>> astronaut = Table('astronaut', metadata,
...     Column('id', Integer, primary_key=True),
...     Column('firstname', String(50), nullable=False),
...     Column('lastname', String(50), nullable=False),
...     Column('agency', Enum('NASA', 'ESA', 'POLSA')),
...     Column('birthdate', Date),
...     Column('age', Integer),
...     Column('height', Numeric(3,2)),
...     Column('weight', Numeric(3,2)),
... )
>>>
>>> with engine.begin() as db:
...     metadata.create_all(db)

8.6.2. Attributes¶

>>> astronaut.c.firstname
Column('firstname', String(length=50), table=<astronaut>, nullable=False)

8.6.3. Operators¶

>>> astronaut.c.firstname == 'Mark'  
<sqlalchemy.sql.elements.BinaryExpression object at 0x...>

>>> expression = (astronaut.c.firstname == 'Mark')
>>>
>>> expression.operator
<built-in function eq>
>>>
>>> expression.left
Column('firstname', String(length=50), table=<astronaut>, nullable=False)
>>>
>>> expression.right  
BindParameter('%(... firstname)s', 'Mark', type_=String(length=50))
>>>
>>> print(expression)
astronaut.firstname = :firstname_1

Bound Parameters are generated from the Python expression. It prevents from SQL injection attacks. Bound parameter sanitization and escaping is typically done by the database driver. Bounds parameters allows also for caching.

>>> expression = (astronaut.c.firstname == 'Mark')
>>> compiled = expression.compile()
>>>
>>> compiled.string
'astronaut.firstname = :firstname_1'
>>>
>>> compiled.params
{'firstname_1': 'Mark'}

8.6.4. Conjunction Options¶

ColumnElements are the basic building block of SQL statement objects. To compose more complex criteria, and_() and or_() for example provide the basic conjunctions of AND and OR. [1]

>>> from sqlalchemy import and_, or_
>>>
>>> criteria = or_(
...     astronaut.c.firstname == 'Mark',
...     astronaut.c.firstname == 'Melissa',
... )
>>>
>>> print(criteria)
astronaut.firstname = :firstname_1 OR astronaut.firstname = :firstname_2

>>> criteria = and_(
...     astronaut.c.lastname == 'Watney',
...     or_(astronaut.c.firstname == 'Mark',
...         astronaut.c.firstname == 'Melissa'),
... )
>>>
>>> print(criteria)
astronaut.lastname = :lastname_1 AND (astronaut.firstname = :firstname_1 OR astronaut.firstname = :firstname_2)

8.6.5. Comparison Operators¶

= - equals
!= - not equals
> - greater then
>= - greater or equal to
< - less then
<= - less or equal to
between() - in between two values or dates

>>> criteria = and_(
...     astronaut.c.id >= 5,
...     astronaut.c.firstname != 'Mark',
...     astronaut.c.birthdate.between('1994-10-01', '1994-10-31'),
... )
>>>
>>> print(criteria)
astronaut.id >= :id_1 AND astronaut.firstname != :firstname_1 AND astronaut.birthdate BETWEEN :birthdate_1 AND :birthdate_2

8.6.6. Null Checking¶

Note != and == instead is and is not
You can easily overload != and == operators

Compare to None produce IS NULL / IS NOT NULL

>>> criteria = and_(
...     astronaut.c.firstname != None,
...     astronaut.c.agency == None,
... )
>>>
>>> print(criteria)
astronaut.firstname IS NOT NULL AND astronaut.agency IS NULL

8.6.7. Numerical Operators¶

Operators may also be type sensitive.
+ with numbers means 'addition'.

>>> criteria = astronaut.c.id + 5
>>>
>>> print(criteria)
astronaut.id + :id_1

8.6.8. String Operators¶

+ with strings means 'concatenation'.

>>> criteria = astronaut.c.firstname + 'Jr.'
>>>
>>> print(criteria)
astronaut.firstname || :firstname_1

8.6.9. Membership Operators¶

Note, there is no in and not in operator overload

The IN operator generates a special placeholder that will be filled in when the statement is executed.

>>> criteria = astronaut.c.firstname.in_(['Mark', 'Melissa', 'Rick'])
>>>
>>> print(criteria)
astronaut.firstname IN (__[POSTCOMPILE_firstname_1])

When it is executed, bound parameters are generated:

>>> result = criteria.compile(compile_kwargs={'render_postcompile': True})
>>>
>>> print(result)
astronaut.firstname IN (:firstname_1_1, :firstname_1_2, :firstname_1_3)

When given an empty collection, the placeholder generates a SQL subquery that represents an 'empty set'. This is due to that every database has a different syntax to search for an 'empty set'.

>>> criteria = astronaut.c.firstname.in_([])
>>> result = criteria.compile(compile_kwargs={'render_postcompile': True})
>>>
>>> print(result)
astronaut.firstname IN (NULL) AND (1 != 1)