How to use WHERE IN with psycopg?-CodePudding

I have a table like this:

id	topic_id
2	'1'
3	'1.1'
...	...
21	'4'

and I would like to get a list of ids corresponding to their topic_ids.

I can query the database directly just fine:

SELECT id FROM topics WHERE topic_id IN ('1', '4')

# returns [2, 21]

But when I do it via psycopg (as recommended here), I get no results:

topic_ids = ['1', '4']

with psycopg.connect(os.environ['DB_CONNECTION']) as conn:
    with conn.cursor() as cur:
        sql = "SELECT id FROM topics WHERE topic_id IN (%(ids)s)"
        cur.execute(sql, {"ids": tuple(topic_ids)})

        cur.fetchall() # returns []

I can only get a single id out with

        cur.execute(sql, {"ids": topic_ids[0]})

        cur.fetchall() # I get [(2,)]

How can I get the whole list out?

CodePudding user response：

You may build a dynamic WHERE IN (...) clause, with a variable number of %s placeholders, and then bind the correct tuple.

where = "WHERE topic_id IN (?"   ", ?"*(len(topic_ids) - 1)   ")"
sql = "SELECT id FROM topics "   where
cur.execute(sql, tuple(topic_ids))
cur.fetchall()

CodePudding user response：

Instead of using IN, you can use =ANY and then have python bind the entire array to one placeholder.

sql = "SELECT id FROM topics WHERE topic_id =ANY (%(ids)s)"
cur.execute(sql, {"ids": topic_ids})