Assum I have three tables User, Product and UserProduct. Once a user buys a product a record in the UserProduct table is created.
I would like to calculate an average number of products being bought by users. I can do this with plain SQL on mySql RDBMS like as follows:
select avg(c) from (select count(*) as c from user_product up join user u on (up.user_id = u.id) group by u.id) as res
Any chance I can do the same with JPA?
Update: I know that subqueries may be used in the WHERE or HAVING clause only, but still...
Related
I have a user table and another order table. A user can have many orders. How do I get the last 1000 users and those users use my last 1000 orders for each one?
Query - Get users
select distinct users.id, users.first_name, users.last_name
from users
limit 2;
Query - Get orders
select distinct orders.id, orders.user_id
from orders
limit 2;
First you need to know.. When it comes to query data don't do separate operation.. If you put out All Users and Orders Your data will be unordered and not consistency.. So you need to make Join to see All User with Order they have.. And as stated by #Sharon you need to add column date_ordered to see that order make.. I am assume you already have that column but i will call that column with date_ordered..
And your query will be :
select
users.id,
users.first_name,
users.last_name,
orders.id
from
users
inner join orders on users.id = orders.user_id
order by
orders.date_ordered desc
limit 1000
By order date_ordered use desc you will get the latest all the user with their order.. And i assume user_id column in table orders have constraint foreign key references to table users with column id..
Postgres allows you to create a table using inheritance. We have a design where we have 1400 tables that inherit from one main table. These tables are for each of our vendor's inventory.
When I want to query stock for a vendor, I just query the main table. When running Explain, the explanation says that it is going through all 1400 indexes and quite a few of the inherited tables. This causes the query to run very slowly. If I query only the vendor's stock table, I cut the query time to less than 50% of the time by querying the main table.
We have a join on another table that pulls identifiers for the vendor's partner vendors and we also want to query their stock. Example:
SELECT
(select m2.company from sup.members m2 where m2.id = u.id) as company,
u.id,
u.item,
DATE_PART('day', CURRENT_TIMESTAMP - u.datein::timestamp) AS daysinstock,
u.grade as condition,
u.stockno AS stocknumber,
u.ic,
CASE WHEN u.rprice > 0 THEN
u.rprice
ELSE
NULL
END AS price,
u.qty
FROM pub.net u
LEFT JOIN sup.members m1
ON m1.id = u.id OR u.id = any(regexp_split_to_array(m1.partnerslist,','))
WHERE u.ic in ('01036') -- part to query
AND m1.id = 'N40' -- vendor to query
The n40_stock table has stock for the vendor with id = N40 and N40's partner vendors (partnerslist) are G01, G06, G21, K17, N49, V02, M16 so I would also want
to query the g01_stock, g06_stock, g21_stock, k17_stock, n49_stock, v02_stock, and m16_stock tables.
I know about the ONLY clause but is there away to modify this query to get the data from ONLY the specific inherited tables?
Edit
This decreases the time to under 800ms, but I'd like it less:
WITH cte as (
SELECT partnerslist as a FROM sup.members WHERE id = 'N40'
)
SELECT
(select m2.company from sup.members m2 where m2.id = u.id) as company,
u.id,
u.item,
DATE_PART('day', CURRENT_TIMESTAMP - u.datein::timestamp) AS daysinstock,
u.grade as condition,
u.stockno AS stocknumber,
u.ic,
CASE WHEN u.rprice > 0 THEN
u.rprice
ELSE
NULL
END AS price,
u.qty
FROM pub.net u
WHERE u.ic in ('01036') -- part to query
AND u.id = any(regexp_split_to_array('N40,'||(select a from cte), ','))
I cannot retrieve the company from sup.members in the cte because I need the one from the u.id, which is different when the partner changes in the where clause.
Inherited table lookups are based on the actual WHERE clause, which maps to the CHECK table constraint. Simply inheriting tables is not good enough.
https://www.postgresql.org/docs/9.6/static/ddl-partitioning.html
Caveat, you can not use a dynamically created variables where the actual value is not implemented in the raw query. This results in a check of all inherited tables.
Okay. So the question that I got asked by the teacher was this:
(5 marks) Construct a SQL query on the dvdrental database that uses a natural join of two or more tables and an additional where condition. (E.g. find the titles of films rented by a particular customer.) Note the hints on the course news page if your query returns nothing.
Here is the layout of the database im working with:
http://www.postgresqltutorial.com/wp-content/uploads/2013/05/PostgreSQL-Sample-Database.png
The hint to us was this:
PostgreSQL hint:
If a natural join doesn't produce any results in the dvdrental DB, it is because many tables have the last update: timestamp field, and thus the natural join tries to join on that field as well as the intended field.
e.g.
select *
from film natural join inventory;
does not work because of this - it produces an empty table (no results).
Instead, use
select *
from film, inventory
where film.film_id = inventory.film_id;
This is what I did:
select *
from film, customer
where film.film_id = customer.customer_id;
The problem is I cannot get a particular customer.
I tried doing customer_id = 2; but it returns a error.
Really need help!
Well, it seems that you would like to join two tables that have no direct relation with each other, there's your issue:
where film.film_id = customer.customer_id
To find which films are rented by which customer you would have to join customer table with rental, then with inventory and finally with film.
The task description states
Construct a SQL query on the dvdrental database that uses a natural join of two or more tables and an additional where condition.quote
I'm working on a query that will eventually be used as the record source for a report.
I have a customers and orders table. I want to show customer_id, order_id, and order_date in a query, but I only want to show data associated with the earliest order date for each customer. Basically, I need to show the order_id field without including it in the group by clause. If I include it in the group by clause, I get a lot more records than I want. Based on my research, the code below will work in mysql, but not ms access.
Select customer.customer_id, order.order_id, min(order.order_dt)
From customer inner join order on customer.customer_id = order.customer_id
Group by customer.customer_id
I've tried grouping by order_id in a sub query and ordering by customer then date, then using the first function in the outer query. Unfortunately, the first function doesn't work as advertised.
Any help is greatly appreciated!
Does this work for you? It should bring up the earliest orders by order date for each customer. If there is more than one order on the earliest order date for a customer, all of those orders will be shown, though, so keep it in mind.
SELECT c.customer_id, o.order_id, o.order_dt
FROM customers AS c INNER JOIN (orders AS o INNER JOIN (SELECT customer_ID, MIN([order_dt]) AS MinOrder_dt FROM Orders GROUP BY customer_id) AS d ON (o.Customer_ID = d.customer_id) AND (o.[order_dt] = d.MinOrder_dt)) ON c.customer_id = o.customer_id;
I am deriving a table with just the customer_id and the min order_dt and joining customers and orders to that to only bring up the oldest orders.
Postgresql provides a parameter join_collapse_limit = 1 to disable order of joins. But when I set the parameter and reset the server, the query plan is not changed and there still is order join optimization. My query FROM table is like this
FROM
student as group_A,
student as group_B,
student as group_C
WHERE ...
If users want to use join_collapse_limit = 1 to disable join reordering, explicit JOINS must be provided in the query. For example, the above FROM table should be
FROM
student as group_A CROSS JOIN
student as group_B CROSS JOIN
student as group_C
WHERE ...