Finding matching columns from two tables

Below can be included in a psql via the \i command.

There are basically two tables (and some others for ref-int): membership
and group_sets.
A user (user_id) is a member of one or more groups (group_id). Groups
are also organized into
group sets (group_set_id) in the group_sets table. The task is to find
the group_set_id for a set
of groups, if it exists, given a user_id, and the associated groups that
user is a member of.

The function, get_group_set_id(), at the end is my attempt. It works
but its rather convoluted and
requires 4 SELECT statements.

Any help.

Thanks.

Richard

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/*
In some cases, the group_ids associated with a group_set_id are the
same as the group_ids associated with a user_id. For a given user_id
determine if there is a corresponding group_set_id.

Find the group_set_id such that for a given user_id (input parameter)
there is a one-to-one correspondence between the group_ids associated
with the user_id and the group_ids associated with the group_set_id;
for every group_id that the user_id has, the group_set_id also has it
and for every group_id that the group_set_id has, the user_id also has
it.
If there is no such group_set_id, then return null.

What query will generate the group_set_id?

The function get_group_set_id() is my attempt. Is there a better way?
The fact that it has 4 SELECT statement in it is, I expect, far from
optimum.

SELECT get_group_set_id(10); -- should be 30
SELECT get_group_set_id(11); -- should be -1
SELECT get_group_set_id(12); -- should be -1
SELECT get_group_set_id(13); -- should be 31
*/

-- all of the users
-- user_id are unique
DROP TABLE users;
CREATE TABLE users (
user_id BIGINT,
-- other columns
PRIMARY KEY (user_id)
);
COPY users FROM stdin USING DELIMITERS ':';
10
11
12
13
\.

-- all of the groups
-- user_id are unique
DROP TABLE groups;
CREATE TABLE groups (
group_id BIGINT,
-- other columns
PRIMARY KEY (group_id)
);
COPY groups FROM stdin USING DELIMITERS ':';
20
21
22
23
24
\.
-- a user can be a member of one or more groups
DROP TABLE membership;
CREATE TABLE membership (
user_id BIGINT,
group_id BIGINT,
-- other columns
FOREIGN KEY (user_id) REFERENCES users (user_id),
FOREIGN KEY (group_id) REFERENCES groups (group_id)
);
COPY membership FROM stdin USING DELIMITERS ':';
10:20
10:21
11:20
12:21
12:22
12:23
13:20
13:22
\.

-- a group set is a set of one or more groups
DROP TABLE group_sets;
CREATE TABLE group_sets (
group_set_id BIGINT,
group_id BIGINT,
-- other columns
FOREIGN KEY (group_id) REFERENCES groups (group_id)
-- FOREIGN KEY (group_set_id) REFERENCES group_sets (group_set_id)
);
COPY group_sets FROM stdin USING DELIMITERS ':';
30:20
30:21
31:20
31:22
32:20
32:21
32:22
32:24
\.CREATE OR REPLACE FUNCTION get_group_set_id (
BIGINT
)
RETURNS BIGINT AS '
DECLARE
-- parameters
user_id_p ALIAS FOR $1;
-- local variables
group_id_count_v INTEGER;
count_v INTEGER;
group_set_id_v BIGINT;
foo group_sets%ROWTYPE;
BEGIN
-- how many groups were given to this user
SELECT INTO group_id_count_v count(*) FROM membership
WHERE user_id = user_id_p;

-- select all that have groups shared by the linked user and
-- have the right number (or more)
FOR foo IN SELECT group_set_id FROM group_sets
WHERE group_id IN
(SELECT group_id FROM membership
WHERE user_id = user_id_p)
GROUP BY group_set_id

HAVING count(*) = group_id_count_v
LOOP

-- make sure it has only the right number of group_ids
SELECT INTO count_v count(*) FROM group_sets
WHERE group_set_id = foo.group_set_id;

IF group_id_count_v = count_v THEN
RETURN foo.group_set_id;
END IF;

END LOOP;

RETURN -1;
END;
' LANGUAGE 'plpgsql';
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