如何检查 SqlAlchemy 中的查询是否返回空结果？

Question

I'd like to perform a query like so:

    query = users.select().where(users.column.id == id)
    res = await database.execute(query)

I would then like to add some error handling by checking res . if not res does not seem to be correct. What is the proper way to do this?

Answer 1

You can try taking first object from your query and checking if it is Null like this

result = users.select().where(users.column.id == id).first()
if not result:
    print('query is empty')

The other way editing your code would be

res = query.first()

And then checking if it is Null

Answer 2

Depending on the complexity of your query, it might be cheaper to wrap it inside a SELECT EXISTS(...) than to SELECT... LIMIT 1 , as Antonio describes. PostgreSQL probably knows to treat those queries the same, but according to this answer , not all DBMS might do so.

The beauty of such a solution is that it exists as soon as it finds any result -- it does as little work as possible and is thus as cheap as possible. In SQLAlchemy that would be sa.select([sa.exists(query)])