selecting records from parent / child tables - sql

Question

I have parent / child tables that look like this:
CREATE TABLE users(
id integer primary key AUTOINCREMENT,
pnum varchar(10),
dloc varchar(100),
cc varchar(10),
name varchar(255),
group active bit(1)
);
CREATE TABLE group_members(
id integer primary key AUTOINCREMENT,
group_id integer,
member_id integer,
FOREIGN KEY (group_id) REFERENCES users(id),
FOREIGN KEY (member_id) REFERENCES users(id)
);
Users Data looks like:
ID PNUM DLOC CC NAME GRP
86|23101|dloc 89| | |0
87|23101|dloc 90| | |0
88|23102|dloc 91| | |0
590|12345|Group | |Test Group|1
591|90000|dloc 1 | | |0
group_members data looks like:
ID GROUP_ID
1 |590 | 87
2 |590 | 88
Based on the PNUM, I would like to be able to get the dloc values for all users, whether its a group or not.
So for example, if someone requests pnum 23101, I would like to get back
"dloc 89" and
"dloc 90"
But if they request 12345, I would like to get back
"dloc 90", and
"dloc 91"
So far, I have come up with this query:
SELECT users.dloc
FROM users
WHERE users.id IN
(SELECT group_members.member_id
FROM group_members
INNER JOIN users on users.id = group_members.group_id
WHERE users.pnum='12345');
That works for groups, but it won't return any results if i run the same query with pnum 23101.
What I've tried so far
I tried to see if I could use OR like so:
SELECT users.dloc
FROM users
WHERE users.id in
(SELECT group_members.member_id
FROM group_members
INNER JOIN users on users.id = group_members.group_id
WHERE users.pnum='12345');
OR
(SELECT users.dloc
FROM users
WHERE pnum='12345')
Any suggestions would be appreciated.

Answer 1

You may use a UNION to execute both queries, one of which matches directly on PNUM for values where GROUP <> 1 and the other which matches on PNUM and joins through group_members for values where GROUP = 1.
In the second part, you need to join twice to users to get the members of the group matching the original PNUM.
Since the GROUP condition is opposite in each, only one part of the UNION will ever return results.
/* First part of UNION directly matches PNUM for GROUP = 0 */
SELECT dloc
FROM users
WHERE
PNUM = 23101
AND `group` <> 1
UNION
/**
Second part of UNION matches GROUP = 1
and joins through group_members back to users
to get member dloc (from the second users join)
*/
SELECT uu.dloc
FROM users u
INNER JOIN group_members m ON u.`GROUP` = 1 AND u.id = m.group_id
INNER JOIN users uu ON m.member_id = uu.id
WHERE
u.PNUM = 23101
This does unfortunately require placing the PNUM value twice in the query, once per UNION part, but that isn't so bad.
Here it is in action (using MySQL rather than SQLite, but that doesn't really matter)
Using your original method with OR and an IN() subquery, it can also be done, but I've added WHERE conditions for GROUP = 0 and GROUP = 1.
SELECT users.dloc
FROM users
WHERE users.id in (
SELECT group_members.member_id
FROM group_members
INNER JOIN users on users.id = group_members.group_id
WHERE users.pnum='23101' AND `GROUP` = 1
)
OR users.id IN (
SELECT users.ID
FROM users
WHERE pnum='23101' AND `GROUP` = 0
);
And here's the alternative method in action...