Question

How do I get the MIN() of a datetime column and a literal datetime in SQL Alchemy v0.6.4? I want to clamp a datetime result to a specific range.

I’ve been using sqlalchemy.func.min(column, literal_datetime). This seems to work fine in SQLite but not at all with MySQL, which no doubt means I’m going about this wrong. The error from MySQL is:

sqlalchemy.exc.ProgrammingError: (ProgrammingError) (1064, "You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the right syntax to use near ' '2011-06-14 12:30:00') AS min_1 \nFROM example' at line 1") 'SELECT min(example.my_datetime, %s) AS min_1 \nFROM example' (datetime.datetime(2011, 6, 14, 12, 30),)

• How can I clamp datetime results in a portable way?
• (Do I really have to do it in my application?)

Here’s what I’ve been using to explore the problem – works fine as presented here (using a memory-based SQLite DB), doesn’t work with MySQL:

#!/usr/bin/env python

import random
from datetime import datetime

import sqlalchemy as sa
import sqlalchemy.orm as orm
from sqlalchemy.ext.declarative import declarative_base

orm_base = declarative_base()

class MyClass( orm_base ):
    __tablename__ = "example"
    pri_key  = sa.Column(sa.Integer(), primary_key=True)
    my_datetime  = sa.Column(sa.DateTime(), index=True)

engine = sa.create_engine("sqlite:///:memory:", echo=True)

Session = orm.sessionmaker( bind=engine )

orm_base.metadata.bind = engine
orm_base.metadata.create_all()

# Create test-data

session = Session()

random.seed(1234567980)
for i in range(100):
    month = random.randint(1, 12)
    day = random.randint(1, 28)
    hour = random.randint(0, 23)
    minute = random.randint(0, 59)

    my_instance = MyClass()
    my_instance.my_datetime = datetime(2011, month, day, hour, minute)

    session.add( my_instance )

session.commit()
session.close()

# Problem starts here

session = Session()
literal_datetime = datetime(2011, 06, 14, 12, 30)
print session.query( sa.func.min(MyClass.my_datetime) ).one()  # OK
print session.query( sa.func.min(MyClass.my_datetime, literal_datetime) ).all()  # Error in MySQL

I’m not an SQL expert but database portability is important to me, so any tips on how to avoid pitfalls like this in the future are welcome.

Answer 1

Use sqlalchemy.sql.expression.case expression, which is supported on both mySql and SQLite.

The min/max functions of SQLite support non-aggregate operation on multiple values. This is not supported on mySql though.