Each Student and Teacher have their own UNIQUE UserID
Is it possible to retrieve the values from the three tables at once?
such that: it will display the UserID owned by each Student or Teacher?
I've tried the following query, but it doesn't work:
SELECT u.UserID, StudentID, TeacherID
FROM User u
INNER JOIN (SELECT * FROM Student, Teacher) ss
ON u.UserID = ss.UserID
Maybe this way:
SELECT u.UserID, S.StudentID, T.TeacherID
from User U
left join Student S on S.UserID = u.UserID
left join Teacher T on T.UserID = U.UserID
Related
Dear all I have users table and cars table.
and I have following join query:
select
users.id as user_id,
users.username,
users.job,
cars.id,
cars.brand as car_brand
FROM users
LEFT JOIN cars on users.id = cars.user_id
GROUP BY users.username, users.id, cars.id;
Here is the snapshot:
How to query for users that having cars more than one?
I tried code below but it return empty data:
How to get users that having more than one cars? (username: Ismed)
You can do in this was as well.
select
users.id as user_id,
users.username,
users.job,
cars.id,
cars.brand as car_brand
FROM users
LEFT JOIN cars on users.id = cars.user_id
where exists (select username, count(*) multiplecars
FROM users u
JOIN cars c on u.id = c.user_id
where users.username = u.username
group by
u.username
having count(*) > 1 )
If the users have more than one car (even if same brand then this will bring those records) if you only want users with more than one branded care you can do count(distinct)
The simplest and probably most performance method is to use window functions:
select user_id, username, job, id, brand
from (select u.id as user_id, u.username, u.job,
c.id, c.brand as car_brand,
count(*) over (partition by u.id) as num_cars
from users u join
cars c
on u.id = c.user_id
) uc
where num_cars > 1;
Note that I changed the left join to a join. If you have two matches, you are requiring a match. I also introduced table aliases so the query is easier to write and to read.
SELECT users.username
FROM users
WHERE users.id IN(
select
users.id
FROM users
JOIN cars on users.id = cars.user_id
GROUP BY users.id
HAVING COUNT(*) > 1
);
Filter users first who has more then one car then get corresponding details
I need to print all persons which are not students and not teachers. I have three tables. Oracle database. Code so far:
SELECT PersonID, FirstName, LastName, Gender, DateOfBirth
FROM PERSON
INNER JOIN STUDENT S ON PERSON.PersonID = S.StudentID
INNER JOIN TEACHER T ON PERSON.PersonID = T.TeacherID
WHERE PERSON.PersonID != S.StudentID
AND PERSON.PersonID != T.TeacherID;
I guess my query is wrong because it returns 0 results. Do you have any idea what must I change?
" my query is wrong because it returns 0 results"
Inner joins return records when there is a match. You're trying to find PERSON records which join to neither STUDENT nor TEACHER. So, change your query to use outer joins:
SELECT PersonID, FirstName, LastName, Gender, DateOfBirth
FROM PERSON
LEFT OUTER JOIN STUDENT S ON PERSON.PersonID = S.StudentID
LEFT OUTER JOIN TEACHER T ON PERSON.PersonID = T.TeacherID
WHERE S.StudentID is null
AND T.TeacherID is null;
This is an anti-join: it returns records from PERSON which don't match records in STUDENT and TEACHER.
Use not exists:
select p.*
from person p
where not exists (select 1 from teacher t where t.teacherid = p.personid) and
not exists (select 1 from students s where s.studentid = p.personid);
Although you can write this query with left join, I think the version using not exists is almost a direct translation of the question, making it easier to understand.
In Oracle, you can also write this using minus -- if you want only the id:
select personid
from person
minus
select teacherid
from teacher
minus
select studentid
from student;
--Try this with Left Outer Join...
SELECT
P.PersonID,
P.FirstName,
P.LastName,
P.Gender,
P.DateOfBirth
FROM PERSON P
LEFT OUTER JOIN STUDENT S ON P.PersonID = S.StudentID
LEFT OUTER JOIN TEACHER T ON P.PersonID = T.TeacherID
WHERE S.PersonID is null and T.PersonID is null
I have dbo.Users tables
Id, Name
1, John
2, Mary
3, Michael
Then I have dbo.Phones table
Id, Phonenumber
10, 1234
11, 5555
Then I have dbo.Relationship table
Id, ChildId
1, 10
2, 11
How could I make a query that returns
Id, Name, Phonenumber
1, John, 1234
2, Mary, 5555
3, Michael, NULL
This is what I got so far.
SELECT u.Id, u.Name, p.Phonenumber
FROM dbo.Users as u
LEFT JOIN dbo.Phones as p
-- Something
SQL Fiddle
Think of the Relationship table as the middle-man between your Users and Phones tables here. It is a many-to-many relationship with a mapping table. Join your Users to the Relationship and then the Relationship to your Phones.
SELECT u.Id
,u.Name
,p.PhoneNumber
FROM dbo.Users u
LEFT JOIN dbo.Relationship r ON r.Id = u.Id
LEFT JOIN dbo.Phones p ON p.Id = r.ChildId
Think of it like:
Users: Hello Relationship, I have UserId = 1, what PhoneIds do I have for that UserId?
Relationship: Hi Users. I have PhoneId = 10 for you. I'll go talk to Phones to see what the number is.
Phones: Hi Relationships! I have PhoneNumber 1234 for you. It matches the PhoneId you gave me.
Join them on id field.I would use inner join depending on the requirement
SELECT distinct u.Id, u.Name, p.Phonenumber
FROM dbo.Users as u
LEFT JOIN dbo.Phones as p on u.id = p.id
or---
SELECT distinct u.Id, u.Name, p.Phonenumber
FROM dbo.Users as u
inner join T JOIN dbo.Phones as p on u.id = p.id
inner join dbo.relationship r on r.id = u.id
where----
try this :
Select u.Id, u.Name, p.Phonenumber
From
Users u
Left join Relationship r on r.Id = u.Id
Left join Phones p on r.ChildId = p.Phonenumber
I have two tables (User and Salary). I want to do a left join from User to Salary. For each user I want their name and salary. In case they have no salary that field can be left empty. So far a left join is all we need. But I only want one row per user. Due to some defects there can be several salaries for one user (see table salary). I only want one row per user which can be selected randomly (or top 1). How do I do that? The expected output is presented in the bottom.
User Table:
User Name
1 Adam
2 Al
3 Fred
Salary Table
User Salary
1 1000
2 2000
2 2000
Expected table:
User Name Salary
1 Adam 1000
2 Al 2000
3 Fred null
Changed User to Userid as User is a reserved word in SQL
SELECT u.Userid, u.Name, MAX(S.Salary)
FROM Usertable u
LEFT JOIN Salarytable s ON u.Userid = s.userid
GROUP BY u.userid, u.name
SQL Fiddle: http://sqlfiddle.com/#!6/ce4a8/1/0
Try this:
select U.User, U.Name, min(S.Salary)
from UserTable U
left join SalaryTable S on S.User = U.User
group by U.User, U.Name
You can utilize a ROW_NUMBER to get the max (or min) salary:
SELECT *
FROM Usertable u
LEFT JOIN
(
select Userid, Salary,
row_number()
over (partition by Userid
order by Salary desc) as rn
from Salarytable
) as s
ON u.Userid = s.userid
AND rn = 1
And in Teradata you could apply the rn = 1filter using QUALIFY within the Derived Table:
SELECT *
FROM Usertable u
LEFT JOIN
(
select Userid, Salary,
row_number()
over (partition by Userid
order by Salary desc) as rn
from Salarytable
qualify rn = 1
) as s
ON u.Userid = s.userid
Use a derived table to get distinct rows from salaries table.
select u.userid, u.username, s.salary
from users u left join (select distinct userid, salary from salaries) s
on u.userid = s.userid
Also, renamed tables and columns. Table names should normally end with s (since pluralis.) Columns should not.
Or, do a GROUP BY:
select u.userid, u.username, max(s.salary)
from users u left join salaries s
on u.userid = s.userid
group by u.userid, u.username
Or skip the left join, instead do a correlated sub-query:
select u.userid, u.username, (select max(s.salary) from salaries s
where u.userid = s.userid)
from users
Try this
select distinct U.User, U.Name, S.Salary
from UserTable U
left join SalaryTable S on S.User = U.User
I have 2 tables a user table (user_id, fname, lname, dob, etc) and a are_friends table
(userA_id, userB_id). I have been trying to do this query for a while now, I need it to list all friends for a user_id.
What I have got so far,
SELECT
U.user_id,
U.fname,
U.lname
FROM are_friends A, user U
WHERE
A.user_id = U.user_id
AND (
A.user_id = 1
OR A.user_id IN (SELECT userB_id FROM are_friends WHERE userA_id = 1)
);
Any help will be much appreciated.
Try using an INNER JOIN like this:
SELECT u2.user_id, u2.fname, u2.lname
FROM user u
INNER JOIN are_friends f ON f.userA_id = u.user_id
INNER JOIN user u2 ON u2.user_id = f.userB_id
WHERE u.user_id = 1
You can change the WHERE clause to specifically get the friends of another user id.