Selecting distinct column values in SQLAlchemy/Elixir
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29-04-2021 - |
Question
In a little script I'm writing using SQLAlchemy and Elixir, I need to get all the distinct values for a particular column. In ordinary SQL it'd be a simple matter of
SELECT DISTINCT `column` FROM `table`;
and I know I could just run that query "manually," but I'd rather stick to the SQLAlchemy declarative syntax (and/or Elixir) if I can. I'm sure it must be possible, I've even seen allusions to this sort of thing in the SQLAlchemy documentation, but I've been hunting through that documentation for hours (as well as that of Elixir) and I just can't seem to actually figure out how it would be done. So what am I missing?
Solution
You can query column properties of mapped classes and the Query class has a generative distinct()
method:
for value in Session.query(Table.column).distinct():
pass
OTHER TIPS
For this class:
class Assurance(db.Model):
name = Column(String)
you can do this:
assurances = []
for assurance in Assurance.query.distinct(Assurance.name):
assurances.append(assurance.name)
and you will have the list of distinct values
for user in session.query(users_table).distinct():
print user.posting_id
I wanted to count the distinct values, and using .distinct()
and .count()
would count first, resulting in a single value, then do the distinct. I had to do the following
from sqlalchemy.sql import func
Session.query(func.count(func.distinct(Table.column))
For class,
class User(Base):
name Column(Text),
id Column(Integer, primary_key=True)
Method 1: Using load_only
from sqlalchemy.orm import load_only
records= (db_session.query(User).options(load_only(name)).distinct().all())
values = [record[0] if len(record) == 1 else record for record in records] #list of distinct values
Method2: without any imports
records = db_session.query(User.name).distinct().all()
l_values = [record.__dict__[l_columns[0]] for record in records]