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
Related
I'm new to SQL and am a bit confused on how I would write a query in order to get the count of state in a different table.
Ie i have this table [student]
id
school_code
0
0123
1
2345
2
2345
And this other table [school]
school_code
name
State
0123
xxyy
New Jersey
2345
xyxy
Washington
3456
yxyx
Colarado
I want to find out how I would get this table which tells me the entries for state by checking each student and making a count of how often that state occurs, ordered by most occurrences in student table.
State
No. times occured (iterating through student)
Washington
2
New Jersey
1
SELECT school.state, count(school.state)
FROM student, school
WHERE student.school_code = school.school_code
GROUP BY school.state
ORDER BY count(school.state)`
I'm not sure whether this would be iterating through each student and counting them?
Or just natural-joinging student and school and then counting all the states
When I run this on data supplied, the numbers of times occurred is a really low number which doesn't seem right?
We can simply JOIN the two tables and COUNT the school code in the students table, with GROUP BY state:
SELECT
sc.state, COUNT(st.school_code)
FROM
school sc
JOIN student st
ON sc.school_code = st.school_code
GROUP BY sc.state;
We can try out here: db<>fiddle
I'm very sorry to bother with minor problem, but I tried to search old answers for this one and since my skills in SQL are complete 0, I didn't even understand the answers :/! Neither is my English terminology great enough for properly searching.
I have these 2 tables: Cities and Flights.
Cities
+----+-------------+
|id | name |
+----+-------------+
|1 | Oslo |
|2 | New York |
|3 | Hong Kong |
+----+-------------+
Flights
+----+--------------------+-------------------+
|id | wherefrom_id | whereto_id |
+----+--------------------+-------------------+
|1 | 3 | 2 |
|2 | 3 | 1 |
|3 | 1 | 3 |
+----+--------------------+-------------------+
Now I have to write code where I need to make city ID's merge to wherefrom_id and whereto_id, in that manner that the answer shows table where you can see list of Flights (FROM/TO).
Example:
ANSWER:
+-----------+----------------+
|HONG KONG | NEW YORK |
+-----------+----------------+
|HONG KONG | OSLO |
+-----------+----------------+
|OSLO | HONG KONG |
+-----------+----------------+
This is what I wrote:
SELECT C.name, C.name
FROM Cities C, Flights F
WHERE C.id = F.wherefrom_id AND C.id = F.whereto_id;
For some reason this doesnt seem to work and I get nothing showing on my practice program. There is no error or anything it just doesnt show anything on the test answer. I really hope you get what I mean, English is not my first language and I truly tried my best to make it clear as possible :S
First things first - it's a lot easier to code in standard SQL join syntax. Converting your above to that is
SELECT C.name, C.name
FROM Cities C
INNER JOIN Flights F ON C.id = F.wherefrom_id AND C.id = F.whereto_id;
The question you've been asked requires logic people don't usually use at first so it can be confusing the first time you encounter it.
I will run through the logic jump in a moment.
Imagine your Flights table has the City names in it (not IDs).
It would have columns, say, FlightID, From_City_Name, To_City_Name.
An example row would be 1, 'Oslo', 'Prague'.
Getting the data for this would be easy e.g., SELECT Flight_ID, From_City_Name, To_City_name FROM Flights.
However, this has many problems. As your question has done, you decide to pull out the cities into their own reference tables.
For this first example, however, you decided to have two extra tables as reference tables: From_City and To_City. These would both have an ID and city name. You then change your Flights to refer to these.
Your code would look like
SELECT F.ID, FC.Name AS From_City, TC.Name AS To_City
FROM Flights
INNER JOIN From_City AS FC ON Flights.From_City_ID = FC.ID
INNER JOIN To_City AS TC ON Flights.To_City_ID = TC.ID
Notice how there are two joins there - one to From_City and one to To_City? That is because the From and To cities are referring to different things in the data.
So, then the final part of the issue: why have two city tables (from and to). Why not have one? Well, you can. If you create just one table, and modify the above, you get something like this:
SELECT F.ID, FC.Name AS From_City, TC.Name AS To_City
FROM Flights
INNER JOIN City AS FC ON Flights.From_City_ID = FC.ID
INNER JOIN City AS TC ON Flights.To_City_ID = TC.ID
Note that all that has changed is that the From_City and To_City references have been pointed to a different table City. However, the rest is the same.
And that, actually, would be your answer. The complex part that most people don't get to straight away, is having two joins to the same table.
As an aside, your original code is technically valid.
SELECT C.name, C.name
FROM Cities C
INNER JOIN Flights F ON C.id = F.wherefrom_id AND C.id = F.whereto_id;
However, what it's effectively saying is to get the city names where the From_City is the same as the To_City - which is obviously not what you want (unless you're looking for turnbacks).
What you're doing is an old SQL way of expressing joins. The standard now has better ways to declare the relationships within the from clause and I take it that your material has postponed that slightly:
There are people who will yell at you for using this ancient syntax but the answer is easy enough:
SELECT C1.name, C2.name
FROM Cities C1, Cities C2, Flights F
WHERE C1.id = F.wherefrom_id AND C2.id = F.whereto_id
You can think of this as creating a "cross product" of all city-pair combinations and matching up the ones that match actual flights. The key is to references Cities twice by using different aliases (or correlation names.)
I think this is what you are looking for..
SELECT wf.name "wherefrom", wt.name "whereto"
FROM Flights f
JOIN Cities wf
ON f.wherefrom_id = wf.id
JOIN Cities wt
ON f.whereto_id = wt.id
order by f.id
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
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)
I have a relatively large (as in >10^6 entries) table called "things" which represent locateable objects, e.g. countries, areas, cities, streets, etc. They are used as a tree of objects with a fixed depth, so the table structure looks like this:
id
name
type
continent_id
country_id
city_id
area_id
street_id
etc.
The association inside "things" is 1:n, i.e. a street or area always belongs to a defined city and country (not two or none); the column city_id for example contains the id of the "city" thing for all the objects which are inside that city. The "type" column contains the type of thing (Street, City, etc) as a string.
This table is referenced in another table "actions" as "thing_id". I am trying to generate a table of action location statistics showing the number of active and inactive actions a given location has. A simple JOIN like
SELECT count(nullif(actions.active, 1)) AS icount,
count(nullif(actions.active, 0)) AS acount,
things.name AS name, things.id AS thing_id, things.city_id AS city_id
FROM "actions"
LEFT JOIN things ON actions.thing_id = things.id
WHERE UPPER(substring(things.name, 1, 1)) = UPPER('A')
AND actions.datetime_at BETWEEN '2012-09-26 19:52:14' AND '2012-10-26 22:00:00'
GROUP BY things.name, things.id ORDER BY things.name
will give me a list of "things" (starting with 'A') which have actions associated with them and their active and inactive count like this:
icount | acount | name | thing_id | city_id
------------------------------------------------------------------
0 5 Brooklyn, New York City | 25 | 23
1 0 Manhattan, New York City | 24 | 23
3 2 New York City | 23 | 23
Now I would like to
only consider "city" things (that's easy: filter by type in "things"), and
in the active/inactive counts, use the sum of all actions happening in this city - regardless of whether the action is associated with the city itself or something inside the city (= having the same city_id). With the same dataset as above, the new query should result in
icount | acount | name | thing_id | city_id
------------------------------------------------------------------
4 7 New York City | 23 | 23
I do not need the thing_id in this table (since it would not be unique anyway), but since I do need the city's name (for display), it is probably just as easy to also output the ID, then I don't have to change as much in my code.
How would I have to modify the above query to achieve this? I'd like to avoid additional trips to the database, and advanced SQL features such as procedures, triggers, views and temporary tables, if possible.
I'm using Postgres 8.3 with Ruby 1.9.3 on Rails 3.0.14 (on Mac OS X 10.7.4).
Thank you! :)
You need to count actions for all things in the city in an independent subquery and then join to a limited set of things:
SELECT c.icount
,c.acount
,t.name
,t.id AS thing_id
,t.city_id
FROM (
SELECT t.city_id
,count(nullif(a.active, 1)) AS icount
,sum(a.active) AS acount
FROM things t
LEFT JOIN actions a ON a.thing_id = t.id
WHERE t.city_id = 23 -- to restrict results to one city
GROUP BY t.city_id
) c -- counts per city
JOIN things t USING (city_id)
WHERE t.name ILIKE 'A%'
AND t.datetime_at BETWEEN '2012-09-26 19:52:14'
AND '2012-10-26 22:00:00'
ORDER BY t.name, t.id;
I also simplified a number of other things in your query and used table aliases to make it easier to read.