Finding matching columns from two tables - Mailing list pgsql-sql

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_vBIGINT;   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
SELECTgroup_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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