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How do I do a semijoin using SQLAlchemy?


How do I do a semijoin using SQLAlchemy?

By : user3860711
Date : November 18 2020, 07:00 PM
With these it helps Let's assume you have models classes A and B mapped to corresponding tables.
The simplest case is when you have relation in A pointing to B, let's name it A.b. Then you just use either A.b.has(type='some type') or A.b.any(type='some type') (depending on whether A.b is scalar or represent a collection) as condition when querying A model.
code :


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How semijoin is different from natural join + projection?

How semijoin is different from natural join + projection?


By : agdrago
Date : March 29 2020, 07:55 AM
Does that help It isn't. A semijoin can be defined as a projection on a natural join.
See Wikipedia: Relational algebra / Semijoin
postgres/sqlalchemy/python: converting sqlalchemy.sql.selectable.Select to sqlalchemy.orm.query.Query?

postgres/sqlalchemy/python: converting sqlalchemy.sql.selectable.Select to sqlalchemy.orm.query.Query?


By : Spooner
Date : March 29 2020, 07:55 AM
will be helpful for those in need I'm working on an existing application that runs SQL postgres queries by creating sqlalchemy.sql.selectable.Select objects and running their execute() method. , You can sorta-kinda convert statements to Query objects like this:
code :
stmt = select([foo.c.id, foo.c.bar])
query = session.query(*stmt.inner_columns).from_statement(stmt)
conn = engine.connect().execution_options(stream_results=True)
for row in conn.execute(stmt):
    ...
conn = engine.connect()
with conn.connection.connection.cursor(name="foo") as cur:
    cur.itersize = 10000
    compiled = stmt.compile(engine)
    cur.execute(compiled.string, compiled.params)
    for row in cur:
        pass
BigQuery - CASE WHEN x IN (field from other table), results in Semijoin error

BigQuery - CASE WHEN x IN (field from other table), results in Semijoin error


By : jlmjlm1000
Date : March 29 2020, 07:55 AM
it helps some times I am trying to segment website visitors by whether or not they have ever purchased from us before. When I put in the below field in the SELECT statement, I get the further below error: , I suggest joining a summarized result along these lines:
code :
SELECT 
    CASE WHEN p.has_purchased = 1 THEN "Purchased"
         ELSE "Never Purchased" 
    END AS Previous_Purchase
FROM existing_from_table f
LEFT JOIN (
    SELECT fullVisitorId, 1 has_purchased
    FROM [xxxx-0000:36363.unique_FullVisitorIds_of_past_purchasers_table]
    GROUP BY fullVisitorId
    ) p on f.fullVisitorId = p.fullVisitorId
Flask, Python, SQLalchemy, SQLalchemy-searchable doesn't return matching text

Flask, Python, SQLalchemy, SQLalchemy-searchable doesn't return matching text


By : Wiggles
Date : March 29 2020, 07:55 AM
With these it helps I'm wondering if you need to initialize the new entry by specifying an init method for your model. Where are the "name" and "content" kwargs coming from?
Semijoin expression must be a part of logical AND

Semijoin expression must be a part of logical AND


By : EOS
Date : March 29 2020, 07:55 AM
hop of those help? BigQuery indeed currently has limitation that IN subqueries must be connected with ANDs. As a workaround, you may execute each SELECT separately, and then UNION the results, i.e.
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