python sqlalchemy distinct column values

Question

I have 6 tables in my SQLite database, each table with 6 columns(Date, user, NormalA, specialA, contact, remarks) and 1000+ rows.

How can I use sqlalchemy to sort through the Date column to look for duplicate dates, and delete that row?

Answer 1

Assuming this is your model:

class MyTable(Base):
    __tablename__ = 'my_table'
    id = Column(Integer, primary_key=True)
    date = Column(DateTime)
    user = Column(String)
    # do not really care of columns other than `id` and `date`
    # important here is the fact that `id` is a PK

following are two ways to delete you data:

1. Find duplicates, mark them for deletion and commit the transaction
2. Create a single SQL query which will perform deletion on the database directly.

For both of them a helper sub-query will be used:

# helper subquery: find first row (by primary key) for each unique date
subq = (
    session.query(MyTable.date, func.min(MyTable.id).label("min_id"))
    .group_by(MyTable.date)
) .subquery('date_min_id')

Option-1: Find duplicates, mark them for deletion and commit the transaction

# query to find all duplicates
q_duplicates = (
    session
    .query(MyTable)
    .join(subq, and_(
        MyTable.date == subq.c.date,
        MyTable.id != subq.c.min_id)
    )
)

for x in q_duplicates:
    print("Will delete %s" % x)
    session.delete(x)
session.commit()

Option-2: Create a single SQL query which will perform deletion on the database directly

sq = (
    session
    .query(MyTable.id)
    .join(subq, and_(
        MyTable.date == subq.c.date,
        MyTable.id != subq.c.min_id)
    )
).subquery("subq")

dq = (
    session
    .query(MyTable)
    .filter(MyTable.id.in_(sq))
).delete(synchronize_session=False)

Answer 2

Inspired by the Find duplicate values in SQL table this might help you to select duplicate dates:

query = session.query(
    MyTable
).\
    having(func.count(MyTable.date) > 1).\
    group_by(MyTable.date).all()

If you only want to show unique dates; distinct on is what you might need