I would like to take values from one table column and subtract those values from another column from another table.
I was able to achieve this by joining those tables and then subtracting both columns from each other.
Data from first table:
SELECT max_participants FROM courses ORDER BY id;
Data from second table:
SELECT COUNT(id) FROM participations GROUP BY course_id ORDER BY course_id;
Here is some code:
SELECT max_participants - participations AS free_places FROM
(
SELECT max_participants, COUNT(participations.id) AS participations
FROM courses
INNER JOIN participations ON participations.course_id = courses.id
GROUP BY courses.max_participants, participations.course_id
ORDER BY participations.course_id
) AS course_places;
In general, it works, but I was wondering, if there is some way to make it simplier or maybe my approach isn't correct and this code will not work in some conditions? Maybe it needs to be optimized.
I've read some information about not to rely on natural order of result set in databases and that information made my doubts to appear.
If you want the values per course, I would recommend:
SELECT c.id, (c.max_participants - COUNT(p.id)) AS free_places
FROM courses c LEFT JOIN
participations p
ON p.course_id = c.id
GROUP BY c.id, c.max_participants
ORDER BY 1;
Note the LEFT JOIN to be sure all courses are included, even those with no participants.
The overall number is a little tricker. One method is to use the above as a subquery. Alternatively, you can pre-aggregate each table:
select c.max_participants - p.num_participants
from (select sum(max_participants) as max_participants from courses) c cross join
(select count(*) as num_participants from participants from participations) p;
Related
All i want to do is to join two tables, list ALL the rows from the first table, find the average from the second table from all the rows, then list only the ones that are greater than the average.
This is wahat i have done so far, and i am only getting one greater than the average but there are others.
SELECT winner_age, AVG(actor_age) FROM oscar_winners
INNER JOIN actors ON actors.id = oscar_winners.id
WHERE winner_age > (
SELECT AVG(actor_age)
)
You don't really need a join here:
SELECT o.WINNER_AGE
FROM OSCAR_WINNERS o
WHERE o.WINNER_AGE > (SELECT AVG(a.ACTOR_AGE)
FROM ACTORS a)
Something like this?
SELECT actors.*, (SELECT AVG(actor_age) from actors) as average
FROM oscar_winners
INNER JOIN actors ON actors.id = oscar_winners.id and actors.winner_age > (SELECT AVG(actor_age) from actors)
The problem with your query is because you are using a where clause, while you should probably be using having:
SELECT w.winner_age, AVG(a.actor_age)
FROM oscar_winners w
INNER JOIN actors a
ON actors.id = oscar_winners.id
group by w.winner_age
having w.winner_age > AVG(a.actor_age)
I need to write SQL query like:
Show all countries with more than 1000 users, sorted by user count.
The country with the most users should be at the top.
I have tables:
● Table users (id, email, citizenship_country_id)
● Table countries (id, name, iso)
Users with columns: id, email, citizenship_country_id
Countries with columns: id, name, iso
SELECT countries.name,
Count(users.citiizenship_country_id) AS W1
FROM countries
LEFT JOIN users ON countries.id = users.citizenship_country_id
GROUP BY users.citiizenship_country_id, countries.name
HAVING ((([users].[citiizenship_country_id])>2));
But this does not work - I get an empty result set.
Could you please tell me what I'm doing wrong?
A LEFT JOIN is superfluous for this purpose. To have 1000 users, you need at least one match:
SELECT c.name, Count(*) AS W1
FROM countries c JOIN
users u
ON c.id = u.citizenship_country_id
GROUP BY c.name
HAVING COUNT(*) > 1000;
Notice that table aliases also make the query easier to write and to read.
Group by country name and use HAVING Count(u.citiizenship_country_id)>1000, it filters rows after aggregation:
SELECT c.name,
Count(u.citiizenship_country_id) AS W1
FROM countries c
INNER JOIN users u ON c.id = u.citizenship_country_id
GROUP BY c.name
HAVING Count(u.citiizenship_country_id)>1000
ORDER BY W1 desc --Order top counts first
;
As #GordonLinoff pointed, you can use INNER JOIN instead of LEFT JOIN, because anyway this query does not return counries without users and INNER JOIN performs better because no need to pass not joined records to the aggregation.
i am a total beginner in SQL. So i have two tables ( table a and table b )
table a holds people with unique IDs, table b holds multiple rows for each person of table a and also the persons ID ( for a possible join ) . the rows in table b are sorted by the columnn row_number.
How can i select all people but only the row of table b with the highest row_number ?
i hope you could somewhat understand me.
Cheers
If i got you right:
SELECT a.persons_ID
,b.rn
FROM A
INNER JOIN
(
SELECT MAX(row_number_column) AS rn
,persons_ID
FROM B
GROUP BY persons_ID
) sub
ON sub.persons_ID = A.persons_ID
The subselect in the inner joins groups your data of table B. So there will be just one row for each persons_ID - the row with the highest row_number_column.
Finally just a simple join on persons_ID.
If you don't need any other information than the person ID and the last row_number per person, then it is quite trivial. Let's call the first table person and the second visit:
select person_id,
max(row_number) max_row_number
from visit
group by person_id
If you need some other information from the first table, like person.name, then perform the join:
select person.person_id,
person.name,
max(visit.row_number) max_row_number
from person
inner join visit on visit.person_id = person.person_id
group by person.person_id,
person.name
If you need some other information from the second table, like visit.present, then modern databases support the row_number() window function (not to be confused with the column that you have):
select name,
base.row_number,
present
from (
select person.name,
row_number() over (partition by visit.person_id
order by visit.row_number desc) rn,
visit.row_number,
visit.present
from person
inner join visit on visit.person_id = person.person_id
) base
where rn = 1
NB: I would strongly advise to rename the column row_number to some other name, as row_number is an analytic function in many databases.
I am supposed to use the given Database(Its pretty huge so I used codeshare) to list last names and customer numbers of top 5% of customers for each branch. To find the top 5% of customers, I decided to use the NTILE Function, (100/5 = 20, hence NTILE 20). The columns are pulled from two separate tables so I used Inner joins. For the life of me, I honesly cannot figure out where I am going wrong. I keep getting "missing expression" errors but Do not know what exactly I am missing. Here is the Database
Database: https://codeshare.io/5XKKBj
ERD: https://drive.google.com/file/d/0Bzum6VJXi9lUX1d2ZkhudTE3QXc/view?usp=sharing
Here is my SQL Query so far.
SELECT
Ntile(20) over
(partition by Employee.Branch_no
order by sum(ORDERS.SUBTOTAL) desc
) As Top_5,
CUSTOMER.CUSTOMER_NO,
CUSTOMER.LNAME
FROM
CUSTOMER
INNER JOIN ORDERS
ON
CUSTOMER.CUSTOMER_NO = ORDERS.CUSTOMER_NO
GROUP BY
ORDERS.SUBTOTAL,
CUSTOMER.CUSTOMER_NO,
CUSTOMER.LNAME;
You need to join Employee and the GROUP BY must include all non-aggregated expressions. You can use a subquery to generate the subtotals and get the NTILE in the outer query, e.g.:
SELECT
Ntile(20) over
(partition by BRANCH_NO
order by sum_subtotal desc
) As Top_5,
CUSTOMER_NO,
LNAME
FROM (
SELECT
EMPLOYEE.BRANCH_NO,
CUSTOMER.CUSTOMER_NO,
CUSTOMER.LNAME,
sum(ORDERS.SUBTOTAL) as sum_subtotal
FROM CUSTOMER
JOIN ORDERS
ON CUSTOMER.CUSTOMER_NO = ORDERS.CUSTOMER_NO
JOIN EMPLOYEE
ON ORDERS.EMPLOYEE_NO = EMPLOYEE.EMPLOYEE_NO
GROUP BY
EMPLOYEE.BRANCH_NO,
CUSTOMER.CUSTOMER_NO,
CUSTOMER.LNAME
);
Note: you might want to include BRANCH_NO in the select list as well, otherwise the output will look confusing with duplicate customers (if a customer has ordered from employees in multiple branches).
Now, if you want to filter the above query to just get the top 5%, you can put the whole thing in another subquery and add a predicate on the Top_5 column, e.g.:
SELECT CUSTOMER_NO, LNAME
FROM (... the query above...)
WHERE Top_5 = 1;
I am trying to get the most frequent Zip_Code for the Location ID from table B. Table A(transaction) has one A.zip_code per Transaction but table B(Location) has multiple Zip_code for one area or City. I am trying to get the most frequent B.Zip_Code for the Account using Location_D that is present in both table.I have simplified my code and changed the names of the columns for easy understanding but this is the logic for my query I have so far.Any help would be appreciated. Thanks in advance.
Select
A.Account_Number,
A.Utility_Type,
A.Sum(usage),
A.Sum(Cost),
A.Zip_Code,
( select B.zip_Code from B where A.Location_ID= B.Location_ID having count(*)= max(count(B.Zip_Code)) as Location_Zip_Code,
A.Transaction_Date
From
Transaction_Table as A Left Join
Location Table as B On A.Location_ID= B.Location_ID
Group By
A.Account_Number,
A.Utility_Type,
A.Zip_Code,
A.Transaction_Date
This is what I come up with:
Select tt.Account_Number, tt.Utility_Type, Sum(tt.usage), Sum(tt.Cost),
tt.Zip_Code,
(select TOP 1 l.zip_Code
Location_Table l
where tt.Location_ID = l.Location_ID
group by l.zip_code
order by count(*) desc
) as Location_Zip_Code,
tt.Transaction_Date
From Transaction_Table tt
Group By tt.Account_Number, tt.Utility_Type, tt.Zip_Code, tt.Transaction_Date;
Notes:
Table aliases are a good thing. However, they should be abbreviations for the tables referenced, rather than arbitrary letters.
The table alias qualifies the column name, not the function. Hence sum(tt.usage) rather than tt.sum(usage).
There is no need for a join in the outer query. You are doing all the work in the subquery.
An order by with top seems the way to go to get the most common zip code (which, incidentally, is called the mode in statistics).