I have 3 tables
Users - columns id, userName,...
Comment - columns id, text, userId, roomId
CommentRate - columns userId, commmentId
CommentRate table contain userId's that liked this comment identified by commentId.
I want to select all comments (id, text, roomId, userId, userName, []ListOfLikes) from Comment table by roomId where []ListOfLikes should contain userId, userName.
You can refer the same table ( USER in this case) more times with different join criteria using aliases, but It is not possible to convert values in field names. This statement will extract all that you need, but repeating rows and not adding columns:
SELECT C.ID, C.TEXT, C.ROOMID, U.USERNAME, U2.USERNAME
FROM COMMENT C
INNER JOIN COMMENTRATE CR ON C.ID = CR.COMMENTID
INNER JOIN USERS U ON C.USERID = U.ID
INNER JOIN USERS U2 ON CR.USERID = U2.ID
WHERE U.USERNAME IN ('username1', 'username2')
ORDER BY C.ROOMID
If you want to filter by users that likes, you could replace U with U2 in the WHERE line of code.
Related
I have 3 tables user, student_data, teacher_data. A user can be either student or a teacher. If it is the teacher I want to join user and teacher_data. And if it is a student then I want to join user with student_data.
How I can do this join with the condition.
I'd combine the two data tables in a sub-query, and then join the users to that.
SELECT
*
FROM
usr u
LEFT JOIN
(
SELECT user_id, datum, xxx, NULL AS yyy FROM student_data
UNION ALL
SELECT user_id, datum, NULL, yyy FROM teacher_data
)
d
ON d.user_id = u.id
https://dbfiddle.uk/?rdbms=oracle_21&fiddle=9b801ea739d42fe50c00ef4e17eaf143
NOTES:
The columns selected from the two data tables must match
Any unmatched columns must either be skipped or filled with NULL
Please don't call a table user, it's a reserved keyword and Oracle won't allow it.
You can write it like this:
select u.user_id,
s.student_id,
t.teacher_id
from usr u
left join student_data s on u.user_id=s.student_id
left join teacher_data t on u.user_id=t.teacher_id
where s.student_id is not null or t.teacher_id is not null
order by u.user_id
For every user_id check if he is a student or teacher, if he is student get his student column values else null, if he is a teacher get his teacher column values else null.
maybe try a union - something like this
select user_id, user_other_stuff
from user, student_data
where user.user_id = student_data.user_id
UNION
select user_id, user_other_stuff
from user, teacher_data
where user.user_id = teacher_data.user_id
I have two tables to join.
First table has two columns
User ID of the record owner.
User ID of the assigned user.
And the second table has userId and username columns. These two columns of first table refers to the same column of the same table. I want to see both user names in a view but I can't get those at the same time. How should I do that?
Edit
select
TASKID,
CREATED_BY,
CREATED_AT,
ASSIGNED_USER,
USERNAME ,
DEADLINE,
USERNAME
from
TASKS
inner join
USERS on ASSIGNED_USER = USERID
I can get username of ASSIGNED_USER's username with the query above. I've tried the second for getting both usernames but it didn't work
select
TASKID,
CREATED_BY,
CREATED_AT,
ASSIGNED_USER,
USERNAME ,
DEADLINE,
USERNAME
from
TASKS
inner join
USERS on ASSIGNED_USER = USERID and CREATED_BY = USERID
It returns an empty result.
you have to join to user twice once for CREATED_BY and once for ASSIGNED_USER:
select
TASKID,
CREATED_BY,
CREATED_AT,
ASSIGNED_USER,
USERNAME ,
DEADLINE,
USERNAME
from TASKS
inner join USERS u1 on ASSIGNED_USER = u1.USERID
inner join USERS u2 on CREATED_BY = u2.USERID
Join USERS twice to decode different references
select TASKID, CREATED_BY, CREATED_AT, ASSIGNED_USER, au.USERNAME assignedUser_Name, DEADLINE, uc.USERNAME createdByUser_Name
from TASKS t
inner join USERS au
on t.ASSIGNED_USER = au.USERID
inner join USERS uc
on t.CREATED_BY = uc.USERID
Maybe for some people it might look very simple, but I just cant get it.
My tables are:
CREATE TABLE USERS (user_ID number PRIMARY KEY, username varchar2(32), password varchar2(32));
CREATE TABLE VIDEOS (video_ID number PRIMARY KEY, title varchar(64), description varchar(128));
CREATE TABLE VIEWS (view_ID number PRIMARY KEY, user_ID number, video_ID number);
CREATE TABLE FAVORITES (fav_ID number PRIMARY KEY, user_ID number, video_ID number);
I ve created those separated queries:
SELECT u.username AS "Username", count(*) AS "Views"
FROM Views v, Videos vd, Users u
WHERE v.user_id = u.user_id
AND v.video_id = vd.video_id
GROUP BY u.username
SELECT u.username AS "Username", count(*) AS "Favorites"
FROM Favorites f, Videos vd, Users u
WHERE f.user_id = u.user_id
AND f.video_id = vd.video_id
GROUP BY u.username
And I want a query to show something like that in only one simple query:
Username Views Favorites
-------------------------------
Person1 12 1
Person2 234 21
...
I Googled bunch of similar questions but I couldnt make any of them to work.
So any help is greatly appreciated.
You are progressing on the right track. -> You got two queries and you wish to see them together. You could perform a full outer join to get your results you are looking for as below.
with fave
as (
SELECT u.username AS "Username"
, count(*) AS "Favorites"
FROM Favorites f
JOIN Videos vd
ON f.video_id = vd.video_id
JOIN Users u
ON f.user_id = u.user_id
GROUP BY u.username
)
,views
as (SELECT u.username AS "Username"
, count(*) AS "Views"
FROM Views v
JOIN Videos vd
ON v.video_id = vd.video_id
JOIN Users u
ON v.user_id = u.user_id
GROUP BY u.username
)
select isnull(f.username,v.username) as username
,f.favourites
,v.views
from fave f
full outer join views v
on f.username=v.username
Since you know your data better, you could optimize the query further. Eg: it could be a rule that user who has set a favourite would also have viewed the video. If this is true then you can write a better query to optimize the dataset in a single block, instead of two blocks using full outer join
Aggregate separately in Views:
select user_id, count(*) counter
from Views
group by user_id
and Favorites
select user_id, count(*) counter
from Favorites
group by user_id
and finally LEFT join Users to the above queries:
select u.username,
coalesce(v.counter, 0) Views,
coalesce(f.counter, 0) Favorites
from users u
left join (
select user_id, count(*) counter
from Views
group by user_id
) v on v.user_id = u.user_id
left join (
select user_id, count(*) counter
from Favorites
group by user_id
) f on f.user_id = u.user_id
I used LEFT joins because there may exist users that did not see any video or do not have any favorites. In any of these cases COALESCE() will return 0 instead of null.
The table Videos is not needed.
I got 3 tables, Users, courses and course realation tables. I want to get users who aren't on specific course. So I figure I need somehow merge 2 selects with right join. How could I make one select from 2 selects?
SELECT ID, NAME, LASTNAME, ROLE FROM COURSERELATION JOIN USERS ON
ID_USER = ID WHERE ID_COURSE = ?
RIGTH JOIN
SELECT ID, NAME, LASTNAME, ROLE from COURSERELATION JOIN USERS ON
ID_USER = ID WHERE ID_COURSE != ?
You need to extract users for which it doesn't exist a record of that user for the specific course. You can filter the rows using a NOT EXISTS clause over a subquery.
Please try below query:
SELECT u.ID,
u.NAME,
u.LASTNAME,
u.ROLE
FROM USERS u
WHERE NOT EXISTS (SELECT 1
FROM COURSERELATION s
WHERE s.id_user = u.id
AND s.id_course = 'YOUR_COURSE_ID_HERE' )
I have 3 tables (user, relationship and user_type) with these data (the relevant ones):
USER TABLE:
id, username, avatar, user_type_id
RELATIONSHIP TABLE:
id, user_id1, user_id2, relationship_points
USER_TYPE
id
I'm trying to create a single entry per relationship so, user with id "1" could be in user_id1 OR user_id2 in a relationship, so, I don't have to duplicate unnecessary data.
I already created (thanks to another StackOverflow answer) a query to select all relationship details from an user, but only if it's id is in "user_id1".
SELECT
r.id AS relationship_id,
r.relationship_points AS points,
u.username AS username,
u.avatar_url AS avatar
FROM
relationship AS r
INNER JOIN
user AS u
ON
r.user_id2 = u.id
INNER JOIN
user_type AS t
ON
u.user_type_id = t.id
WHERE
r.user_id1 = ?
But, as you can see, if the user is in "user_id2", it doesn't work.
I know I could make another query, but I think it's the "easy, lazy" way, and I would love to learn how to do this in a single query.
Let me know if this makes the trick:
SELECT
r.id AS relationship_id,
r.relationship_points AS points,
u.username AS username,
u.avatar_url AS avatar
FROM relationship AS r
INNER JOIN user u
ON r.user_id2 = u.id OR r.user_id1 = u.id
INNER JOIN user_type AS t
ON u.user_type_id = t.id
WHERE u.id = ?