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
Related
I have 3 tables
User Table
id
Name
1
Mike
2
Sam
Score Table
id
UserId
CourseId
Score
1
1
1
5
2
1
1
10
3
1
2
5
Course Table
id
Name
1
Course 1
2
Course 2
What I'm trying to return is rows for each user to display user id and user name along with the sum of the maximum score per course for that user
In the example tables the output I'd like to see is
Result
User_Id
User_Name
Total_Score
1
Mike
15
2
Sam
0
The SQL I've tried so far is:
select TOP(3) u.Id as User_Id, u.UserName as User_Name, SUM(maxScores) as Total_Score
from Users as u,
(select MAX(s.Score) as maxScores
from Scores as s
inner join Courses as c
on s.CourseId = c.Id
group by s.UserId, c.Id
) x
group by u.Id, u.UserName
I want to use a having clause to link the Users to Scores after the group by in the sub query but I get a exception saying:
The multi-part identifier "u.Id" could not be bound
It works if I hard code a user id in the having clause I want to add but it needs to be dynamic and I'm stuck on how to do this
What would be the correct way to structure the query?
You were close, you just needed to return s.UserId from the sub-query and correctly join the sub-query to your Users table (I've joined in reverse order to you because to me its more logical to start with the base data and then join on more details as required). Taking note of the scope of aliases i.e. aliases inside your sub-query are not available in your outer query.
select u.Id as [User_Id], u.UserName as [User_Name]
, sum(maxScore) as Total_Score
from (
select s.UserId, max(s.Score) as maxScore
from Scores as s
inner join Courses as c on s.CourseId = c.Id
group by s.UserId, c.Id
) as x
inner join Users as u on u.Id = x.UserId
group by u.Id, u.UserName;
So I have two tables: Please see the ER diagram here
I want to use SELECT to create one table with "name" from the USER table, "id" as the foreign key for the two tables, and the count of friend_id as the number of friends each user has.
Here is my code:
SELECT name, id, (SELECT count(friend_id) as number
FROM friend
GROUP BY user_id)
FROM user
ORDER BY number DESC
I'm wondering what's the problem with these lines. Thank you!
You can use a subquery to calculate the count.
SELECT name, id, COALESCE(f.Count, 0) AS friend_count
FROM user u
LEFT JOIN (
SELECT user_id, COUNT(DISTINCT friend_id) AS Count
FROM friend
GROUP BY user_id
) f ON f.user_id = u.id
ORDER BY friend_count DESC
I used a LEFT JOIN so that if a user doesn't have a row in friend, it will still return a row with a friend count of 0 (thanks to COALESCE). I also added a DISTINCT so that if the friend has duplicates the friend is counted only one, might not be necessary especially if you have a UNIQUE INDEX setup on columns user_id, friend_id
Just add where to find only one id and remove group by because you have only one id for one or more friends as your diagram says.
SELECT name, id, (SELECT count(friend_id) as number
FROM friend
WHERE user_id = user.id)
FROM user
ORDER BY number DESC
I think this will be correct for you puprose
CREATE TABLE #user(
id VARCHAR(22),
[name] VARCHAR(255),
)
CREATE TABLE #friend(
user_id VARCHAR(22),
friend_id VARCHAR(22)
)
SELECT name, id, (SELECT COALESCE(COUNT(friend_id), 0)
FROM #friend f
WHERE f.user_id = u.id
GROUP BY user_id) as number
FROM #user u
ORDER BY number DESC
--Same query with join:
SELECT u.[name], u.id, COALESCE(COUNT(f.friend_id),0) number
FROM #user u
LEFT JOIN #friend f ON f.user_id = u.id
GROUP BY u.[name], u.id
ORDER BY number
I have a table called users, which has the following columns:
id: INT NOT NULL
face: face_type
face_type is an ENUM type that has the following values: 'square', 'round' and 'triangle'.
And I have another table called houses, which has the following columns:
id: INT NOT NULL
user_id: INT NOT NULL
Now, I want to get all the houses grouped by the different type of face types. So, what I have so far is this:
SELECT users.face_type, COUNT(*)
FROM users
LEFT JOIN houses ON houses.user_id = users.id
GROUP BY users.face_type
The problem is that I also want to get rows for face_type which none of the users have, as well as a result for NULL face_type. So, for example, if I have the following data:
users (id, face_type)
1, 'round'
2, 'triangle'
houses (id, user_id)
1, 1
2, 1
3, 2
I would expect the result to be:
face_type, count
'round' 2
'triangle' 1
'square' 0
null 0
I know how to get all the potential values of the face_type ENUM, by doing :
SELECT unnest(enum_range(NULL::face_type)) AS face_types;
But I don't know how to use that to count all potential face types in the aggregate, as well as also calculating for NULL face types.
You can use LEFT JOIN:
SELECT ft.face_type, COUNT(h.user_id)
FROM (SELECT unnest(enum_range(NULL::face_type)) AS face_types
) ft LEFT JOIN
users u
ON u.face_type = ft.face_type LEFT JOIN
houses h
ON h.user_id = u.id
GROUP BY ft.face_type;
To get NULL, just use UNION ALL:
SELECT ft.face_type, COUNT(h.user_id)
FROM (SELECT unnest(enum_range(NULL::face_type)) AS face_types
UNION ALL
SELECT NULL
) ft LEFT JOIN
users u
ON u.face_type = ft.face_type LEFT JOIN
houses h
ON h.user_id = u.id
GROUP BY ft.face_type;
Of course, the = will not every match. If that is possible, then you want to change the JOIN condition to u.face_type is not distinct from ft.face_type.
to COUNT(houses.*)
SELECT face_type.type, COUNT(houses.*)
FROM (SELECT unnest(enum_range(NULL::face_type))) AS face_type(type)
FULL JOIN users ON users.face_type=face_type.type
LEFT JOIN houses ON houses.user_id = users.id
GROUP BY face_type.type
A LEFT JOIN starting from the ENUM and going to users and houses will allow you to recover totals for each enumerated value. To also display the NULL face types, you can use a UNION query.
SELECT
ft.face_type,
COUNT(ho.user_id) as cnt
FROM
(SELECT unnest(enum_range(NULL::face_type)) AS face_types) ft
LEFT JOIN users us ON us.face_type = ft.fact_type
LEFT JOIN houses ho ON ho.user_id = us.id
GROUP BY ft.face_type
UNION
SELECT
null,
COUNT(ho.user_id)
FROM houses ho
INNER JOIN users us ON ho.user_id = us.id AND us.face_type IS NULL
ORDER BY cnt desc
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
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.