I'm building this bartering-type function in this site using PHP/MySQL and I'm trying to create a query that responds with the following fields:
owner's username, title, offerer's username, offerer's item
Basically I have three tables here with the following fields:
users
user_id, username, email
items_available
item_number, owner_id (foreign key that links to user_id), title
offers_open
trade_id, offerers_id (fk of user_id), offerers_item(fk with item_number), receivers_id (fk from user_id)
How do I do this? I've seen some references to many-to-many SQL queries but they don't seem to particularly fit what I'm looking for (for example, the offerers_id and the owner_ids refer to different users_id in the Users table, but how do I make them distinguishable in the sql query?)
If I understand correctly, this is what you are looking for:
SELECT owner.username, oferrers.username, ia.title
FROM offers_open o
INNER JOIN users AS offerers
ON o.offerers_id = offerers.user_id
INNER JOIN items_available AS ia
ON o.offerers_item= ia.item_number
INNER JOIN users AS owner
ON ia.owner_id = owner.user_id
I don't see a title on the users table, so didn't include one.
I'm not sure exactly what output you want, but since your users table will appear twice in the query, they need to be aliased like so:
SELECT offerer.username AS offerer_username, title, receiver.username AS receiver_username
FROM users AS owner
JOIN items_available ON owner_id = owner.user_id
JOIN offers_open ON offerers_item = item_number
JOIN users AS receiver ON receivers_id
Again, don't know if that's what you want, but hope you get the idea.
It sounds as you need an alias of the users table.
Something like?
select
u.*, ia.*, oo.*,u2.*
from
users as u,
items_available as ia,
offers_open as oo,
users as u2
where
u.user_id = ia_user_id and
oo.user_id = u2.user_id and
oo.item_id = ia.item_id
Related
I have two tables, Users and Relationships tables. Users table has following columns:
id, name,password,username,email,avatar,followersCount,followingCount,tweetCount.
And the Relationships table has the following columns:
id, followingId, followerId
How should I go about creating a SQL query to extract a user with a specific Id and find id's from Relationships that user is following? So in other words find people that user follows
I've come this far so long
SELECT *
FROM public."Users" JOIN
public."Relationships"
ON (public."Users".id = public."Relationships".id)
If I understand correctly, you want:
SELECT u.*
FROM public."Relationships" r JOIN
public."Users" u
ON u.id = r.followerId
WHERE r.followingId = ?;
? is a parameter placeholder for the user you care about. This returns all the followers of that user.
Do you mean this query
SELECT public."Users".*
FROM public."Users"
JOIN public."Relationships"
ON public."Users".id = public."Relationships".followingId
AND public."Relationships".followerId = a user ID
I am not really clear about followerId and followingId mean but you can change them in the query if it is not what you want.
Good evening,
I have been struggling with what I think should be a fairly basic SQL query, but I can't seem to figure it out. Particularly because it seems as if there are multiple approaches (joins/ sub-queries).
I have three primary tables, and two tables connecting them.
USER
uID - PK
name, email, ...
GROUP
gID - PK
name
ROLE
rID - PK
permission
USERGROUP
uID - FK
gID - FK
GROUPROLE
gID - FK
rID - FK
These are many to many relationships, such that a user can be in multiple groups, and a group can have multiple roles.
I would like to have a query that ultimately gives me all of the roles that are connected to a given user through the groups that he belongs. Thus the uID is known, but nothing else. The output could have as few as one column with the rID, as that is all I really need for the program logic.
Is it possible to perform a query on the result of a previous query? I can easily find all the groups a user is in, and given each group, I can easily find the roles associated, but I get tripped up by the fact that there are many groups associated with a user, not just a single one which would be easier.
I would appreciate any help!
Thanks in advance.
This is just joins and aggregation:
select array_agg(distinct r.permission) as permissions
from usergroups ug join
grouproles gr
on ug.gid = gr.gid join
roles r
on gr.rid = r.id
where ug.uid = ?;
If you want the permissions in separate rows, then:
select distinct r.permission
from usergroups ug join
grouproles gr
on ug.gid = gr.gid join
roles r
on gr.rid = r.id
where ug.uid = ?;
I am leaving the distinct because different groups could share the same permissions.
This is task table:
This is user table:
I want to select user tasks.
I would give from backend ("given_to_user) id.
But The thing is I want that SELECTED data would have usernames instead of Id which is (created_by_user and given_to_user).
SELECTED table would look like this.
Example:
How to achieve what I want?
Or maybe I designed poorly my tables that It is difficult to select data I need? :)
task table has to id values that are foreign keys to user table.
I tried many thinks but couldn't get desired result.
You did not design poorly the tables.
In fact this is common practice to store the ids that reference columns in other tables. You just need to learn to implement joins:
SELECT
task.id, task.title, task.information, user.usename AS created_by, user2.usename AS given_to
FROM
(task INNER JOIN user ON task.created_by_user = user.id)
INNER JOIN user AS user2 ON task.created_by_user = user2.id;
Do you just want two joins?
select t.*, uc.username as created_by_username,
ug.username as given_to_username
from task t left join
users uc
on t.created_by_user = uc.id left join
users ug
on t.given_to_user = ug.id;
This uses left join in case one of the user ids is missing.
I know the title isn't so describing but it's really hard to find something generic to describe my situation. If someone wants to edit, feel free...
So, I have a postgres database, with a users table. I would like to store the users followed by one user, and I really don't see how I could do this. I would like to do like SELECT followed_users FROM users WHERE username='username' and this would return me every usernames, or id, or whatever of each followed users. But I don't see any clean way to do this.
Maybe an example would be more describing: user1 is following user2 and user3.
How to store who user1 is following?
EDIT: I don't know how many users the user will follow.
Thank you for your help.
Expanding on my comment above, since it got wordy:
Create a new table called something like user_follows with columns like
user_id1 | user_id2
or
follower_id | follows_id
Then you can query:
SELECT t1.username as follower_username, t3.username as following_usernae
FROM users t1
INNER JOIN user_follows t2 ON t1.user_id = t2.follower_id
INNER JOIN users t3 ON t2.following_id = t3.user_id
WHERE t1.user_id = <your user>
In the end, think of your tables as "Objects". Then when you are presented with a problem like "How do I add users that are following other users" you can determine if this relationship is a new object, or an attribute of an existing object. Since a user might follow more than one other user than the relationship is not a good attribute for "Users", so it gets its own table user_follows.
Since user_follows is just one type of relationship that two users may have to one another, it might make sense to increase the scope of that object to relationships and store the relationship type as an attribute of the table:
user_id1 | user_id2 | relationship_type
where relationships.relationship_type might have values like follows, student of, sister of etc...
So the new query would be something like:
SELECT t1.username as follower_username, t3.username as following_username
FROM users t1
INNER JOIN relationships t2 ON t1.user_id = t2.user_id1
INNER JOIN users t3 ON t2.user_id2 = t3.user_id
WHERE t1.user_id = <your user> AND t2.relationship_type = 'Follows';
I'd add another table, let's call it following for argument's sake, which saves pairs of users and users they are following:
CREATE TABLE following (
user_id INT NOT NULL REFERENCES users(id),
following_id INT NOT NULL REFERENCES users(id),
PRIMARY KEY (user_id, following_id)
)
Then you could query all the user's a specific user is following by joining with the users table (twice). E.g., to get the names of all the users that I (username "mureinik") am following:
SELECT fu.username
FROM following f
JOIN users u ON f.user_id = u.id
JOIN users fu ON f.user_id = fu.id
WHERE u.username = 'mureinik'
Sorry if that title is a bit convoluted... I'm spoiled by an ORM usually and my raw SQL skills are really poor, apparently.
I'm writing an application that links to a vBulletin forum. Users authenticate with their forum username, and the query for that is simple (selecting by username from the users table). The next half of it is more complex. There's also a subscriptions table that has a timestamp in it, but the primary key for these is a user id, not a username.
This is what I've worked out so far:
SELECT
forum.user.userid,
forum.user.usergroupid,
forum.user.password,
forum.user.salt,
forum.user.pmunread,
forum.subscriptionlog.expirydate
FROM
forum.user
JOIN forum.subscriptionlog
WHERE
forum.user.username LIKE 'SomeUSER'
Unfortunately this returns the entirety of the subscriptionlog table, which makes sense because there's no username field in it. Is it possible to grab the subscriptionlog row using the userid I get from forum.user.userid, or does this need to be split into two queries?
Thanks!
The issue is that you are blindly joining the two tables. You need to specify what column they are related by.
I think you want something like:
SELECT * FROM user u
INNER JOIN subscriptionlog sl ON u.id = sl.userid
WHERE u.username LIKE 'SomeUSER'
select * from user u JOIN subscriptions s ON u.id = s.id where u.username = 'someuser'
The bit in bold is what you want to add, it combines the 2 tables into one that you return results from.
try this
SELECT
forum.user.userid,
forum.user.usergroupid,
forum.user.password,
forum.user.salt,
forum.user.pmunread,
forum.subscriptionlog.expirydate
FROM
forum.user
INNER JOIN forum.subscriptionlog
ON forum.subscriptionlog.userid = forum.user.userid
WHERE
forum.user.username LIKE 'SomeUSER'