I have trouble understanding row flags. The below question can clear it for me:
Is it possible to store a name and its flag in the same cell in SQL?
Consider:
If you have a table known as cars with the columns number_plate, colour, and brand_name. The brand_name has a name and a flag.
How would one store that in a single column? If it is not possible or advised, explain why and how to do it.
How would you then get the number of cars from a given country (based on the unique number_plate(primary key)) and the country flag?
I think you are trying to design a schema but haven't quite got the hang of foreign keys.
In your example, you'd have the following tables:
country:
country_id name continent
-----------------------------------
1 Germany Europe
2 Japan Asia
3 USA N.America
Brand
Brand_id name country_id (foreign key)
---------------------------------------------
1 Mercedes 1
2 Toyota 2
3 BMW 1
4 Chrysler 3
Car
Number_plate colour brand_id
------------------------------------------
xxx-yy-zz Green 1
aa-bb-cc Red 1
kkk-l-mmm Orange 2
....
To find the number of cars, based on the country where the brand is based, you'd do something like:
select country.name,
count(*)
from car
inner join brand on car.brand_id = brand.brand_id
inner join country on brand.country_id = country.country_id
group by country.name
Let's say name and flag are two separate columns. Using concat function they can be stored into a single column named brand_name.
select number_plate, colour, concat(name,' ',flag) as brand_name from cars
To get the count of cars(unique) based on a flag
select * from
(select
distinct number_plate,
colour,
concat(name,' ',flag) as brand_name from cars
) a
where brand_name like '%UK%'
Demo
Related
A restaurant provides wine pairings for most food items on its menu. The structure of two of the tables containing this information is shown below
Join these two tables by their id columns to find the country that the recommended wine is produced in.
Here is the code I have tried:
SELECT country, item
FROM regions
INNER JOIN pairing
regions.id = pairing.id
ORDER BY item
LIMIT 5;
But the compiler gives the solution as:
SELECT country, item
FROM regions
INNER JOIN pairing
USING(id)
ORDER BY item
LIMIT 5;
OUTPUT:
country
item
France
caviar
Italy
curry
Italy
grilled vegetables
Argentina
lamb
Germany
roast duck
Doubt:
I want to clear if there is any difference bwtween USING and equal statement on id or they are same?
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
This is a heavily simplified version of an SQL problem I'm dealing with. Let's say I've got a table of all the cities in the world, like this:
country city
------------
Canada Montreal
Cuba Havanna
China Beijing
Canada Victoria
China Macau
I want to count how many cities each country has, so that I would end up with a table as such:
country city_count
------------------
Canada 50
Cuba 10
China 200
I know that I can get the distinct country values with SELECT distinct country FROM T1 and I suspect I need to construct a subquery for the city_count column. But my non-SQL brain is just telling me I need to loop through the results...
Thanks!
Assuming the only reason for a new row is a unique city
select country, count(country) AS City_Count
from table
group by country
I want to do some statistic for the Point in my appliation,this is the columns for Point table:
id type city
1 food NewYork
2 food Washington
3 sport NewYork
4 food .....
Each point belongs to a certain type and located at the certain city.
Now I want to caculate the numbers of points in different city for each type.
For example, there are two types here :food and sport.
Then I want to know:
how many points of `food` and `sport` at NewYork
how many points of `food` and `sport` at Washington
how many points of `food` and `sport` at Chicago
......
I have tried this:
select type,count(*) as num from point group by type ;
But I can not group the by the city.
How to make it?
Update
id type city
1 food NewYork
2 sport NewYork
3 food Chicago
4 food San
And I want to get something like this:
NewYork Chicago San
food 2 1 1
sport 1 0 0
I will use the html table and chart to display these datas.
So I need to do the counting, I can use something like this:
select count(*) from point where type='food' and city ='San'
select count(*) from point where type='food' and city ='NewYork'
....
However I think this is a bad idea,so I wonder if I can use the sql to do the counting.
BTW,for these table data,how do people organization their structure using json?
this's what you want:
SELECT city,
COUNT(CASE WHEN [type] = 'food' THEN 1 END) AS FoodCount,
COUNT(CASE WHEN [type] = 'sport' THEN 1 END) AS SportCount
FROM point
GROUP BY city
UPDATE:
To get the results in an aggregated row/column format you need to use a pivot table. In Access it's called a Crosstab query. You can use the Crosstab query wizard to generate the query via a nice UI or cut straight to the SQL:
TRANSFORM COUNT(id) AS CountOfId
SELECT type
FROM point
GROUP BY type
PIVOT city
The grouping is used to count the number of Id's for each type. The additional PIVOT clause groups the data by city and displays each grouping in a separate column. The end result looks something like this:
NewYork Chicago San
food 2 1 1
sport 1 0 0
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.