Postgresql unique constraint for colums of multiple related tables - sql

Question

I have the following tables:
server (server_id as PK)
server_id | server_name
----------+---------------
3251623 | ServerOfDoom
5578921 | BestServerEU
server_groups
Each server can have multiple server groups, every group name within a server must be unique.
id | server_id | group_name
----+-----------+---------------
1 | 3251623 | mods
2 | 5578921 | admins
3 | 5578921 | anothergroup
group_tags
Each group can have multiple tags, a tag can be connected to multiple groups. Each tag name must be unique.
id | tag_name
----+--------------
1 | mods
2 | admins
3 | ag
group_tag_relations
group_id | tag_id
----------+--------
2 | 1
3 | 3
1 | 2
Problem:
I need group tag names and server group names to be unique within one server. So a tag can be connected to multiple groups, but only to one group per server. Or the tag "foo" cannot be connected to the group "bar" of server z because server z already has a group "foo". The same goes for the other way around, so if the group "foo" had the tag "hey" you shouldn't be able to add a group "hey" to the same server.
Basically names should not appear multiple times on a server, no matter if tag or group name.
How would I implement such a Constraint?
EDIT
So basically I'm trying to convert this JSON format into SQL tables:
{
"5578921": {
"Server Name": "Server1",
...
"Groups": {
"Game1": {
"Tags": [
"g1",
"gameone"
], ...
},
"Game2": {
"Tags": [
"g2",
"gametwo"
], ...
}
}
},
"3251623": ...
}
The id's 5578921 and 3251623 should just represent Discord server id's, so every id is for one server my bot is on. The Json file is all the information of my bot for each server, but it's not as reliable and scalable as a database, so I wanted to convert it.
So the upper tables are what I came up with:
A One-To-Many relation between server and server_groups and a Many-To-Many relation between server_groups and group_tags (so instead of storing duplicate tags I can just assign them to different groups).
I just want to make sure there are no duplicate names on one server and asking how to do so for my current tables.

Answer 1

As mentioned before, a stored procedure or function with triggers is probably the way to go.
The code could look something like this:
CREATE FUNCTION public."CHECK_TAG_AND_GROUP_NAME_UNIQUE_PER_SERVER"()
RETURNS trigger
LANGUAGE 'plpgsql'
NOT LEAKPROOF
AS $BODY$
DECLARE
countServerGroupsWithNamePerServer integer;
countGroupTagsWithNamePerServer integer;
BEGIN
-- Count occurrences of name in server_groups table
SELECT COUNT(*)
FROM server_groups
INTO countServerGroupsWithNamePerServer
WHERE "name" = NEW.name
AND "server_id" = NEW.server_id;
-- Check if one exists. If it does, throw error
IF countServerGroupsWithNamePerServer > 0 THEN
RAISE 'Name already exists as a group server name %', NEW.name;
END IF;
-- Count occurrences of name in group_tags table
SELECT COUNT(*)
FROM group_tags
INTO countGroupTagsWithNamePerServer
WHERE "name" = NEW.name
AND "server_id" = NEW.server_id;
-- Check if one exists. If it does, throw error
IF countGroupTagsWithNamePerServer > 0 THEN
RAISE 'Name already exists as a group_tag name %', NEW.name;
END IF;
-- If no error is thrown, insert the new record
RETURN NEW;
END;
$BODY$;
and then you attach the function as a BEFORE INSERT trigger to each of the two tables group_tags and server_groups:
CREATE TRIGGER "BEFORE_INSERT_CHECK_TAG_NAME_UNIQUE_PER_SERVER"
BEFORE INSERT
ON public.group_tags
FOR EACH ROW
EXECUTE PROCEDURE public."CHECK_TAG_AND_GROUP_NAME_UNIQUE_PER_SERVER"();
CREATE TRIGGER "BEFORE_INSERT_CHECK_TAG_NAME_UNIQUE_PER_SERVER"
BEFORE INSERT
ON public.server_groups
FOR EACH ROW
EXECUTE PROCEDURE public."CHECK_TAG_AND_GROUP_NAME_UNIQUE_PER_SERVER"();
Please notice for this example I also added a foreign key column server_id to the group_tags table. Otherwise we are not sure to which server the tag belongs. This is just a rough guide though, please feel free to change it up as much as you want.

Answer 2

After hours of suffering I finally got what I wanted:
Get all Tags of a server
CREATE OR REPLACE FUNCTION get_server_tags(serverid BIGINT)
RETURNS TABLE(group_name VARCHAR(100), tag_name VARCHAR(100), group_id BIGINT, tag_id BIGINT)
AS
$$
SELECT group_name, tag_name, group_id, tag_id FROM group_tag_relations
JOIN server_groups
ON server_groups.server_id = serverid
AND server_groups.id = group_tag_relations.group_id
JOIN group_tags
ON group_tags.id = group_tag_relations.tag_id
$$
language sql
stable;
Get all Groups of a server
CREATE OR REPLACE FUNCTION get_server_group(serverid BIGINT, groupname VARCHAR(100))
RETURNS TABLE(group_name VARCHAR(100), group_id BIGINT)
AS
$$
SELECT group_name, id
FROM server_groups
WHERE server_id = serverid
AND lower(group_name) = lower(groupname);
$$
language sql
stable;
Get a Group by Name (Calling both Functions above)
CREATE OR REPLACE FUNCTION get_group_by_name(serverid BIGINT, groupname VARCHAR(100))
RETURNS TABLE(group_name VARCHAR(100), group_id BIGINT)
AS
$$
BEGIN
RETURN QUERY SELECT get_server_group.group_name, get_server_group.group_id
FROM get_server_group(serverid, groupname);
IF NOT found THEN
RETURN QUERY SELECT get_server_tags.group_name, get_server_tags.group_id
FROM get_server_tags(serverid)
WHERE lower(tag_name) = lower(groupname);
END IF;
END;
$$
language plpgsql
stable;
Update Trigger for server_groups table, checking wether the name is already taken on a server
CREATE OR REPLACE FUNCTION group_name_update()
RETURNS TRIGGER
AS
$$
BEGIN
PERFORM get_group_by_name(OLD.server_id, NEW.group_name);
IF lower(OLD.group_name) = lower(NEW.group_name) THEN
RETURN NEW;
ELSIF found THEN
RETURN OLD;
ELSE
RETURN NEW;
END IF;
END;
$$
language plpgsql
volatile;
CREATE TRIGGER group_name_update_trigger
BEFORE UPDATE ON server_groups
FOR EACH ROW EXECUTE PROCEDURE group_name_update();
Insert Trigger for server_groups table, checking wether the name is already taken on a server
CREATE OR REPLACE FUNCTION group_name_insert()
RETURNS TRIGGER
AS
$$
BEGIN
PERFORM get_group_by_name(NEW.server_id, NEW.group_name);
IF found THEN
RETURN OLD;
ELSE
RETURN NEW;
END IF;
END;
$$
language plpgsql
volatile;
CREATE TRIGGER group_name_insert_trigger
BEFORE INSERT ON server_groups
FOR EACH ROW EXECUTE PROCEDURE group_name_insert();
Insert Trigger for group_tag_relation table, checking wether the tag is already taken on a server (as the tags wont get updated there's no need for an update trigger)
CREATE OR REPLACE FUNCTION group_tag_relation_insert()
RETURNS TRIGGER
AS
$$
BEGIN
PERFORM get_group_by_name((SELECT server_id FROM server_groups WHERE id = NEW.group_id), (SELECT tag_name FROM group_tags WHERE id = tag_id));
IF found THEN
RETURN OLD;
ELSE
RETURN NEW;
END IF;
END;
$$
language plpgsql
volatile;
CREATE TRIGGER group_tag_relation_insert_trigger
BEFORE INSERT ON group_tag_relations
FOR EACH ROW EXECUTE PROCEDURE group_tag_relation_insert();