Question

I would like to know how to pass a list/set/tuple from python (via psycopg2) to a postgres query as a one-column table. For example, if the list is ['Alice', 'Bob'], I want the table to be:

| Temp  |
+-------+
| Alice |
| Bob   |

If anybody has alternate suggestions to achieve my result after reading the section below, that would be fine as well.

Background

I have an SQL table which has three columns of interest:

ID | Members | Group
---+---------+----------
1  | Alice   | 1
2  | Alice   | 1
3  | Bob     | 1
4  | Charlie | 1
5  | Alice   | 2
6  | Bob     | 2
7  | Alice   | 3
8  | Bob     | 4
9  | Charlie | 3

I want a table of groups with certain combinations of members. Note that a member may have multiple items in a group (e.g. IDs 1 and 2).

For an input of ['Alice'] I would want which groups she is in (present) and which contain only her (unique), as below:

Group | Type
------+--------
1     | present
2     | present
3     | present

For an input of ['Alice', 'Bob']:

Group | Type
------+--------
1     | present
2     | unique

From reading it looks like I am looking for relational division as described here, for which I need to do what the original question asks as the input is taken from a web form processed in python. Again, alternative solutions are also welcome.

Answer 1

You need to make a subquery where you create member counts, then do a simple divisor query with a GROUP BY statement, but against a IN static_set clause instead of against another table. Because this is python, you already know the size of the static set.

I’ll assume you already have a database cursor, and the table is called GroupMembers:

MEMBERSHIP_QUERY = '''
    SELECT gm.group, mc.memberscount = %(len)s AS type
      FROM groupmembers gm
      JOIN (SELECT "group", COUNT(DISTINCT members) as memberscount 
              FROM groupmembers
             GROUP BY "group") mc
      ON gm.group = mc.group
    WHERE gm.members IN %(set)s
    GROUP BY gm.group, mc.memberscount
    HAVING COUNT(DISTINCT gm.members) = %(len)s;
'''

def membership(members):
    # obtain a cursor
    for row in cursor.execute(MEMBERSHIP_QUERY, dict(len=len(members), set=members)):
        yield dict(group=row[0], type=row[1])

There is thus no need to use a TEMP table to execute this query.

If you do need a TEMP table for other purposes, inserting a set of rows is easiest with .executemany():

members = ['Alice', 'Bob']
cursor.execute('CREATE TEMP TABLE tmp_members (member CHAR(255)) ON COMMIT DROP;')
cursor.executemany('INSERT INTO tmp_members VALUES (%s);',
    [(name,) for name in members])

Note that .executemany() expects a sequence of sequences; each entry is a sequence of row data, which in this case only holds one name each. I generate a list of single-item tuples to fill the table.

Alternatively, you can use a sequence of mappings too and use the %(name)s parameter syntax (so the row data sequence becomes [dict(name=name) for name in members]).