name | course
Jay | LAWS0001
Mark | LAWS0002
Sam | LAWS0002
Alice | LAWS0001
Ryan | LAWS0001
Ryan | LAWS0002
Hey guys, I've got this database and I want to only select the names that take both 'LAWS0001' and 'LAWS0002'. So from this example, it should select 'Ryan' because he's the only person to take both courses.
I tried IN operator:
SELECT name
FROM student
WHERE course IN ('LAWS0001', 'LAWS0002')
but this takes everyone because everyone is taking either of the courses.
Is there an operator for my problem?
You can use your existing query, using a GROUP BY clause to COUNT the number of distinct courses each student is taking in the set ('LAWS0001', 'LAWS0002') and only selecting those students where the count is 2:
SELECT name
FROM student
WHERE course IN ('LAWS0001', 'LAWS0002')
GROUP BY name
HAVING COUNT(DISTINCT course) = 2
Demo on SQLFiddle
Related
This has been driving me and my team up the wall. I cannot compose a query that will strict match a single record that has a specific permutation of look ups.
We have a single lookup table
room_member_lookup:
room | member
---------------
A | Michael
A | Josh
A | Kyle
B | Kyle
B | Monica
C | Michael
I need to match a room with an exact list of members but everything else I've tried on stack overflow will still match room A even if I ask for a room with ONLY Josh and Kyle
I've tried queries like
SELECT room FROM room_member_lookup
WHERE member IN (Josh, Michael)
GROUP BY room
HAVING COUNT(1) = 2
However this will still return room A even though that has 3 members I need a exact member permutation and that matches the room even not partials.
SELECT room
FROM room_member_lookup a
WHERE member IN ('Monica', 'Kyle')
-- Make sure that the room 'a' has exactly two members
and (select count(*)
from room_member_lookup b
where a.room=b.room)=2
GROUP BY room
-- and both members are in that room
HAVING COUNT(1) = 2
Depending on the SQL dialect, one can build a dynamic table (CTE or select .. union all) to hold the member set (Monica and Kyle, for example), and then look for set equivalence using MINUS/EXCEPT sql operators.
I know similiar questions have been asked and answered before, I have reviewed them but still can't quite wrap my head around how to do this in my case.
I would like to create a query (I use postgreSQL) that would return users from my database filtered by name, sorted by the number of friends in common with a given user (the user sending the request).
The data structure is as follows:
I have a users table, that has a column called search_full_name which stores name + surname in the format of "ADAM SMITH". This is what I filter with.
I have a user_friends table that stores information about who is friends with whom. So I have two columns in there: user_id and friend_id . The data is symmetric, i.e. for every (1,3) there is a (3,1) entry.
So far in the friend search I was just using a query like
select * from users where users.search_full_name like '%query%'
But now, I would like to additionally order the result by the amount of friends in common with the user asking, so my query would have two inputs: query and userId.
Turns out I am not as good with sql as I thought, and I would really appreciate your help, it would be great to see some explanations too.
I imagine the desired output as:
+---------+------------------+----------------------+--+
| user_id | search_full_name | common_friends_count | |
+---------+------------------+----------------------+--+
| 45 | Adam Smith | 14 | |
| 123 | Adam Cole | 11 | |
| 12 | Adamic Kapi | 0 | |
+---------+------------------+----------------------+--+
for a query like 'Adam'
I have been trying this for a whole day now and I feel my brain has exploded.
Please help, thanks
The basic idea is a self-join. The following gets a match on users who share friends with the specified user:
select uf2.user_id, count(*) as num_friends
from user_friends uf join
user_friends uf2
on uf2.friend_id = uf.friend_id and
uf2.user_id <> uf2.user_id
where uf2.user_id = ?
group by uf2.user_id
order by count(*) desc; -- the user you care about
Ok, so after a few hours I came up with a query that works :) Here it is for future reference:
select u.id, u.search_full_name, count(uf.friend_id) as common_friend_count
from users u left join user_friends uf on (u.id = uf.user_id and uf.friend_id in (select friend_id from user_friends where user_id = ?))
where u.search_full_name like ?
group by u.search_full_name, u.id
order by common_friend_count desc;
Here is what I am trying to do: Get all people with the same boss and email the boss. I can only use the table below and I would like to add another column showing the boss email which is found in the table.
id name email boss
-------------------------------------------
1 Apple Apple#alo.com Rita
2 Bob Bob#alo.com Rita
3 Charlie Charlie#alo.com Nuna
4 Dan Dan#alo.com Rita
5 Rita Rita#alo.com Sheeba
6 Nuna Nuna#alo.com Sheeba
Thanks for any help.
You use a join to do that. It looks like the code below. When joining to the same table you have to use an alias.
SELECT base.name, boss.email as boss_email
from the_table_name_you_did_not_say as base
join the_table_name_you_did_not_say as boss on base.boss = boss.name
Technically you don't have to use an alias on the base table if you select a wildcard * for some platforms
At the moment I am busy with two tables, Students and Classes. These two both contain a column project_group, a way to categorize multiple students from one class into smaller groups.
In the Students table there is a column City that states in which town/city students live, from the rows that have been filled there are already several cities occurring multiple times. The code I used to check how many times a city is being showed is this:
SELECT City, count(*)
FROM Students
GROUP BY City
Now the next thing I want to do is show per class in which cities the students live and how many live there, so for example a result like:
A | - | 2
A | New York | 3
A | Los Angeles | 1
B | - | 1
B | Miami | 2
B | Seattle | 1
Students and Classes can join each other on the column project_group but what I'm mostly interested in his using both the GROUP BY mentioned earlier, using the JOIN and also showing the results per class.
Thanks in advance,
KRAD
I'm not sure what the column name is for A and B in your example. I'm assuming Classes.Class in the following:
SELECT
C.Class
, S.City
, COUNT(S.*) AS Count
FROM
Classes AS C INNER JOIN
Students AS S ON C.Project_Group = S.Project_Group
GROUP BY
C.Class
, S.City
I managed to get it working. While doing some tests to see which exact error message it was that I got, I used this and managed to get it working. I now get an overview per class that shows how many people live in which city. This is the code used.
SELECT class_id, city, count(*) AS amount
FROM students, classes
WHERE students.project_group = classes.project_group
GROUP BY class_id, city
ORDER BY class_id
I have taken a good look around and not been able to find any questions that match mine. Maybe I am not using the right language when searching or whatever, but here goes.
I have an SQL table called Classes that looks something like this
Student_Name | Class
--------------------
Edgar | Chemistry
Allan | Chemistry
Burt | Chemistry
Edgar | Math
Sue | Math
Hamilton | Math
Edgar | English
Sue | English
Edgar | German
Ben | German
I want to count how many students are taking both Math and German.
Assuming the following in this example:
- Student names are unique
- One student can have many classes
Logically I would use a select statement to get a result set of students who are taking Math. Then I would go through each Student_Name from the result set and check them against the table to see how many are taking German.
In this case I would expect a return of 1 as only Edgar is taking both Math and German.
Here are some of the queries I have tried so far to no avail :-(
This one was after doing some research on DISTINCT:
SELECT COUNT(DISTINCT Student_Name) FROM Classes WHERE Class = 'Math' AND Class = 'German';
And this one was after finding out more about GROUP BY:
SELECT COUNT(*) FROM (
SELECT DISTINCT Student_Name FROM Classes
WHERE Class IN ( 'Math', 'German' )
GROUP BY Student_Name
);
Neither of these came out quite right any help would be highly appreciated.
SELECT COUNT(*) totalStudent
FROM
(
SELECT student_name
FROM Classes
WHERE class IN ('Math','German')
GROUP BY student_name
HAVING COUNT(*) = 2
) subAlias
SQLFiddle Demo
OUTPUT
╔══════════════╗
║ TOTALSTUDENT ║
╠══════════════╣
║ 1 ║
╚══════════════╝
Could also do the following:
select count(distinct a.Student_name)
from Classes a inner join Classes b on
a.Class = 'German' and
b.Class = 'Math' and
a.Student_Name = b.Student_name;
This solves the problem where the table contains duplicate rows (as pointed out by a commenter to another answer)