CODEWITHSUNDEEP
pythonsqlsqlalchemy
i_trope
i_trope

Reputation: 1604

SQLAlchemy - Select rows with the same column values, excluding those with values from different column

I am inexperienced in SQL / SQLAlchemy, and am trying to write a SQLAlchemy query from a table called Event with data such as this:

I would like to select the rows with Type equal to "Viewed Tab", but only if there does not exists a row with the same Session value that has a Type equal to "Mobile View". So in the sample data above I would want the query to return the rows with EventId equal to 150 and 154, but not 147.

Thank you.

Upvotes: 1

Views: 2570

Answers (2)

van
van

Reputation: 76992

Assuming the table is defined as per below:

class Event(Base):
    __tablename__ = 'events'
    EventId = Column(Integer, primary_key=True)
    Session = Column(Integer)
    Type = Column(String)

the query to produce the desired result can be written as:

viewed = aliased(Event, name='viewed')
mobile = aliased(Event, name='mobile')
qry = (session.query(viewed)
       .filter(viewed.Type == 'Viewed Tab')
       .outerjoin(mobile, and_(
           viewed.Session == mobile.Session,
           mobile.Type == 'Mobile View')
       )
       .filter(mobile.Session == None)
       )

This will produce a query without any aggregations:

SELECT  viewed."EventId" AS "viewed_EventId",
        viewed."Session" AS "viewed_Session",
        viewed."Type" AS "viewed_Type"

FROM    events AS viewed

LEFT OUTER JOIN
        events AS mobile
    ON  viewed."Session" = mobile."Session"
    AND mobile."Type" = 'Mobile View'

WHERE   viewed."Type" = 'Viewed Tab'
    AND mobile."Session" IS NULL

Upvotes: 2

Brian DeMilia
Brian DeMilia

Reputation: 13248

Try:

select e.*
  from event e
  join (select eventid, session
          from event
         group by eventid, session
        having sum(case when type = 'Viewed Tab' then 1 else 0 end) > 0
           and sum(case when type = 'Mobile View' then 1 else 0 end) = 0) v
    on e.eventid = v.eventid

I'm not sure what syntax limitations there are with sql alchemy, however.

Upvotes: -1

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