I'm struggling with writing this basic SQL query.
I have two tables. person and address_join.
create table person (id bigint);
create table person_address (person_id bigint, address_id bigint);
Given one person's ID I want to find other people they share an address with. It's also worth noting that a person can have more than one address.
How can I make this query return other person records that share the same address_id?
select * from person
join person_address on person_address.person_id = person.id
where person.id = ?;
You can answer this with a self join on person_address:
select pa2.*
from person_address pa1 join
person_address pa2
on pa1.address_id = pa2.address_id and
pa1.person_id <> pas2.person_id
where pa1.person_id = ?
select * from person p
join person_address a on a.person_id = p.id
WHERE EXISTS (
SELECT * FROM person_address x
WHERE x.person_id = ?
AND x.address_id = a.address_id
AND x.person_id <> p.person_id
);
Related
I have 3 tables:
Teacher Table (t_id, email, ...)
Student Table (s_id, email, ...)
Teaching Table (t_id, s_id, class_time, ...)
I have a task which is, given two t_id, find the common students that these 2 teachers have taught.
Is it possible to accomplish this in strictly SQL? If not I might try to retrieve out the student records individually based on different teacher, and do a search to see which students they have in common. This seems a bit overkill for something that seems possible to write a SQL query for.
You can self join to get students for both teachers.
DECLARE #TeacherID1 INT = 1
DECLARE #TeacherID2 INT = 2
SELECT
StudentID = T1.s_id,
Teacher1 = T1.t_id,
Teacher1ClassTime = T1.class_time ,
Teacher2 = T2.t_id,
Teacher2ClassTime = T2.class_time
FROM
TeachingTable T1
INNER JOIN TeachingTable T2 ON T2.s_id=T1._sid AND T2.t_id=#TeacherID2
WHERE
T1.t_id = #TeacherID1
ORDER BY
T1.ClassTime
select s_id
from student a
inner join teaching b on a.s_id = b.s_id
where t_id = 'First give t_id'
INTERSECT
select s_id
from student a
inner join teaching b on a.s_id = b.s_id
where t_id = 'Second give t_id'
This work with MS DB, but probably not with others.
select s_id
from student a
inner join teaching b on a.s_id = b.s_id
where b.t_id = 'First give t_id'
and s_id in (
select s_id
from student c
inner join teaching d on c.s_id = d.s_id
where d.t_id = 'Second give t_id'
)
the second one should work with any DB.
I need some help with a SQL query. I have a table of courses and a table that contains user id and course id, denoting courses that user has taken (might not have taken any; no entry in that table for that user id).
I need a query to return the list of courses not taken.
Course Category table
CategoryID
Caegory
Courses table
CourseID
CategoryID
CourseName
...
UserCourse table
UserID
CourseID
you can use not exists
Select *
From Courses c
Where Not Exists (Select 1 From UserCourse uc Where uc.CourseID = c.CourseID)
This will just list the course
select *
from Courses C
Left join CourseCategory cc on
cc.CategoryID = c.CategoryID
where CourseID not in (Select CourseID from UserCourse where UserID = 14)
what i need is for a given user id and course category, what courses within that category have not been taken by this user
(This should have been in the request by the way.)
So:
Select from courses.
Limit to the desired category.
Limit to courses not in the set of courses taken by the user.
The query:
select *
from courses
where categoryid = 123
and courseid not in (select courseid from usercourse where userid = 456);
Another way of writing same query, which will perform faster.
select C.CourseID,C.CategoryID
from Courses C
Left join CourseCategory cc on
cc.CategoryID = c.CategoryID
left join UserCourse uc
on C.CourseID=uc.CourseID
where uc.CourseID is null
How to find city when ContactID is provided and condition is if ContactID is coming as 123 then it will look whether it is P or C, If P then it will go to Person table and returns City(USA) as output and If C then it will go to Company table and gives City(AUS) as output.
NB: all tables contain thousands of record and City value comes from run time.
Unless you're dynamically generating the query (i.e. using some language other than SQL to execute it) then you need to join on both tables anyway. If you're joining on both tables then there's no need for a CASE statement:
select *
from contacts co
left outer join person p
on co.contactid = p.contactid
and co.person_company = 'P'
left outer join company c
on co.contactid = c.contactid
and co.person_company = 'C'
You'll start noting an issue here, for every column from PERSON and COMPANY you're going to have to add some business logic to work out which table you want the information from. This can get very tiresome
select co.contactid
, case when p.id is not null then p.name else c.name end as name
from contacts co
left outer join person p
on co.contactid = p.contactid
and co.person_company = 'P'
left outer join company c
on co.contactid = c.contactid
and co.person_company = 'C'
Your PERSON and COMPANY tables seem to have exactly the same information in them. If this is true in your actual data model then there's no need to split them up. You make the determination as to whether each entity is a person or a company in your CONTACTS table.
Creating additional tables to store data in this manner is only really helpful if you need to store additional data. Even then, I'd still put the data that means the same thing for a person or a companny (i.e. name or address) in a single table.
If there's a 1-2-1 relationship between CONTACTID and PID and CONTACTID and CID, which is what your sample data implies, then you have a number of additional IDs, which have no value.
Lastly, if you're not restricting that only companies can go in the COMPANY table and individuals in the PERSON table. You need the PERSON_COMPANY column to exist in both PERSON and COMPANY, though as a fixed string. It would be more normal to set up this data model as something like the following:
create table contacts (
id integer not null
, contact_type char(1) not null
, name varchar2(4000) not null
, city varchar2(3)
, constraint pk_contacts primary key (id)
, constraints uk_contacts unique (id, contact_type)
);
create table people (
id integer not null
, contact_type char(1) not null
, some_extra_info varchar2(4000)
, constraint pk_people primary key (id)
, constraint fk_people_contacts
foreign key (id, contact_type)
references contacts (id, contact_type)
, constraint chk_people_type check (contact_type = 'P')
);
etc.
you can LEFT JOIN all 3 tables and the using a CASE statement select the one that you need based on the P or C value
SELECT
CASE c.[Person/Company]
WHEN 'P' THEN p.NAME
WHEN 'C' THEN a.Name
END AS Name
FROM Contact c
LEFT JOIN Person p on p.ContactId = c.ContactId
LEFT JOIN Company a on a.ContachId = c.ContactId
Ben's answer is almost right. You might want to check that the first join has no match before doing the second one:
select c.*, coalesce(p.name, c.name) as p.name
from contacts c left outer join
person p
on c.contactid = p.contactid and
c.person_company = 'P' left join
company co
on c.contactid = co.contactid and
c.person_company = 'C' and
p.contactid is null;
This may not be important in your case. But in the event that the second join matches multiple rows and the first matches a single row, you might not want the additional rows in the output.
I am only a beginner in SQL, and I have problem that I can not solve.
The problem is the following:
i have four tables
Student: matrnr, name, semester, start_date
Listening: matrnr<Student>, vorlnr<Subject>
Subject: vorlnr, title, sws, teacher<Professor>
Professor: persnr, name, rank, room
I need to list all the students that are listening the Subject of some Professor with samo name.
EDIT:
select s.*
from Student s, Listening h
where s.matrnr=h.matrnr
and h.vorlnr in (select v.vorlnr from Subject v, Professor p
where v.gelesenvon=p.persnr and p.name='Kant');
This is how i solved it but i am not sure is it optimal solution.
Your approach is good. Only, you want to show students, but join students with listings thus getting student-listing combinations.
Moreover you use a join syntax that is out-dated. It was replaced more than twenty years ago with explicit joins (INNER JOIN, CROSS JOIN, etc.)
You can do it with subqueries only:
select *
from Students,
where matrnr in
(
select matrnr
from Listening
where vorlnr in
(
select vorlnr
from Subject
where gelesenvon in
(
select persnr
from Professor
where name='Kant'
)
)
);
Or join the other tables:
select *
from Students
where matrnr in
(
select l.matrnr
from Listening l
inner join Subject s on s.vorlnr = l.vorlnr
inner join Professor p on p.persnr = s.gelesenvon and p.name='Kant'
);
Or with EXISTS:
select *
from Students s
where exists
(
select *
from Listening l
inner join Subject su on su.vorlnr = l.vorlnr
inner join Professor p on p.persnr = su.gelesenvon and p.name='Kant'
where l.matrnr = s.matrnr
);
Some people like to join everthing and then clean up in the end using DISTINCT. This is easy to write, especially as you don't have to think your query through at first. But for the same reason it can get complicated when more tables and more logic are involved (like aggregations) and it can become quite hard to read, too.
select distinct s.*
from Students s
inner join Listening l on l.matrnr = s.matrnr
inner join Subject su on su.vorlnr = l.vorlnr
inner join Professor p on p.persnr = su.gelesenvon and p.name='Kant';
At last it is a matter of taste.
When you have an SQL problem, a good way of presenting the problem is to show us the tables as CREATE TABLE statements. Such statements show details such as the types of the columns and which columns are primary keys. Additionally this allows us to actually build a little database in order to reproduce a faulty behavior or just to test our solutions.
CREATE TABLE Student
(
matrnr NUMBER(9) PRIMARY KEY,
name NVARCHAR2(50),
semester NUMBER(2),
start_date DATE
);
CREATE TABLE Listening
(
matrnr NUMBER(9), -- Student
vorlnr NUMBER(9), -- Subject
CONSTRAINT PK_Listening PRIMARY KEY (matrnr, vorlnr)
);
CREATE TABLE Subject
(
vorlnr NUMBER(9) PRIMARY KEY,
title NVARCHAR2(50),
sws NVARCHAR2(50),
teacher NUMBER(9) -- Professor
);
CREATE TABLE Professor
(
persnr NUMBER(9) PRIMARY KEY,
name NVARCHAR2(50),
rank NUMBER(3),
room NVARCHAR2(50)
);
Using this schema, my solution would look like this:
SELECT *
FROM
Student
WHERE
matrnr IN (
SELECT L.matrnr
FROM
Listening L
INNER JOIN Subject S
ON L.vorlnr = S.vorlnr
INNER JOIN Professor P
ON S.teacher = P.persnr
WHERE P.name = 'Kant'
);
You can find it here: http://sqlfiddle.com/#!4/5179dc/2
Since I didn't insert any records, the only thing it is testing is the syntax and the correct use of table and column names.
Your solution is suboptimal. It does not differentiate between joining of tables and additional conditions specified as where-clause. It can produce several result records per student if they attend several courses of the professor. Therefore my solution puts all the other tables into the sub-select.
select st.name
from student st
join listening l on l.matrnr = st.matrnr
join subject su on su.vorlnr = l.vorlnr
join professor p on su.teacher = p.persnr
where p.name = 'some name'
SELECT *
FROM student
INNER JOIN listening ON student.matrnr = listening.matrnr
INNER JOIN subject ON listening.vorlnr = subject.vorlnr
INNER JOIN professor ON subject.teacher = professor.name
WHERE professor.name = 'some name'
Table A: Person: id, name
Table B: Toys: id, person_id, toy_name
I have a search screen that includes a dropdown of fixed toy names.
A search is found if a subset of the total set of toys for a person is matched.
Example, a person name=bob has toys: doll, car, house, hat
A search is done for person name=bob and toys=doll, hat.
I want to return bob and ALL of his toys, not just what toys were searched for(doll, hat).
Bob is found because a subset of his toys are a match.
I don't know what the most efficient/least db calls way to accomplish this.
I can do a search for bob and get all of his toys, then parse through the result set to see if the searched for toys find a match, but that seems wrong, that the db call could return rows for which no match is found (and that seems wrong?).
okay,
select
p.id,
p.name,
t.id as toyid,
t.toy_name
from
person p
join
toys t
on p.id = t.person_id
where
p.id in (
select person_id from toys where toy_name = 'doll'
intersect
select person_id from toys where toy_name = 'hat');
Fiddle Here
If you normalise your schema a little further,
create table Person
(
Id int,
Name varchar(100)
);
create table Toy
(
Id int,
Name varchar(100)
);
create table PersonToy
(
Id int,
PersonId int,
ToyId int
);
It should make the complexity of the problem clearer. It will also save some space. A statement of the form,
select
p.Name PersonName,
t.Name ToyName
from
Person p
join
PersonToy pt
on pt.PersonId = p.Id
join
Toy t
on t.Id = pt.ToyId
where
p.Id in
(
select PersonId from PersonToy where ToyId = 1
intersect
select PersonId from PersonToy where ToyId = 4
);
will work efficiently.
Updated Fiddle
Here's one way to do it using a subquery and checking for the existence of Hat and Doll in the HAVING clause:
select p.id, p.name,
t.id as toyid, t.name as toyname
from person p
inner join toys t on p.id = t.person_id
inner join (
select person_id
from toys
group by person_id
having sum(name = 'hat') > 0 and
sum(name = 'doll') > 0
) t2 on p.id = t2.person_id
SQL Fiddle Demo