I have the following pseudo-SQL schema:
table flight
id int primary key
date timestamp
aircraft_id int (foreign key to aircraft.id)
table flight_component
flight_id int (foreign key to flight.id)
component_id int (foreign key to component.id)
duration double
If I convert this to using CoreData, is there an equivalent way to get the total duration for each component with another query predicate? I think the SQL equivalent would be
select component_id, sum(duration)
from flight_component
join flight on (flight_component.flight_id = flight.id)
where flight.date between ? and ?
group by component_id
Or am I going to have to make my query and then sum up all the components myself?
You can query for #"someArray.#sum". There's a list of such operators on the Collection Operators page.
Related
I have 2 tables in my postgresql timescaledb database (version 12.06) that I try to query through inner join.
Tables' structure:
CREATE TABLE currency(
id serial PRIMARY KEY,
symbol TEXT NOT NULL,
name TEXT NOT NULL,
quote_asset TEXT
);
CREATE TABLE currency_price (
currency_id integer NOT NULL,
dt timestamp WITHOUT time ZONE NOT NULL,
open NUMERIC NOT NULL,
high NUMERIC NOT NULL,
low NUMERIC NOT NULL,
close NUMERIC,
volume NUMERIC NOT NULL,
PRIMARY KEY (
currency_id,
dt
),
CONSTRAINT fk_currency FOREIGN KEY (currency_id) REFERENCES currency(id)
);
The query I'm trying to make is:
SELECT currency_id AS id, symbol, MAX(close) AS close, DATE(dt) AS date
FROM currency_price
JOIN currency ON
currency.id = currency_price.currency_id
GROUP BY currency_id, symbol, date
LIMIT 100;
Basically, it returns all the rows that exist in currency_price table. I know that postgres doesn't allow select columns without an aggregate function or including them in "group by" clause. So, if I don't include dt column in my select query, i receive expected results, but if I include it, the output shows rows of every single day of each currency while I only want to have the max value of every currency and filter them out based on various dates afterwards.
I'm very inexperienced with SQL in general.
Any suggestions to solve this would be very appreciated.
There are several ways to do it, easiest one comes to mind is using window functions.
select *
from (
SELECT currency_id,symbol,close,dt
,row_number() over(partition by currency_id,symbol
order by close desc,dt desc) as rr
FROM currency_price
JOIN currency ON currency.id = currency_price.currency_id
where dt::date = '2021-06-07'
)q1
where rr=1
General window functions:
https://www.postgresql.org/docs/9.5/functions-window.html
works also with standard aggregate functions like SUM,AVG,MAX,MIN and others.
Some examples: https://www.postgresqltutorial.com/postgresql-window-function/
CREATE TABLE orders
(
id bigint NOT NULL,
...
created_on date NOT NULL,
quantity int NOT NULL,
...
CONSTRAINT orders_pkey PRIMARY KEY (id)
)
SELECT DATE(o.created_on) AS date, sum(quantity)
FROM orders o
GROUP BY date
ordersItemsQuery.groupBy(_.createdOn).map{
case (created, group) => (created, group.map(_.quantity).sum)
}
notice quantity is not null column, group.map(_.quantity).sum returns Rep[Option[Int]] but not Rep[Int] why?
The Slick method sum evaluates Option[T], and shouldn't be confused with the standard Scala collections method sum that returns a non-optional value.
Slick's sum is optional because a query may produce no results. That is, if you run SELECT SUM(column) FROM table and there are no rows, you do not get back zero from the database. Instead, you get back no rows. Slick is being consistent with this behaviour. Or rather: the sum is happening in SQL, on the database server, and doesn't produce a result when there are no rows.
In contrast to the way a database works, Scala's sum does allow you to sum an empty list (List[Int]().sum) and get back zero.
I have an entity price in my schema it has an attribute amount which is of a custom type money_with_currency.
The money_with_currency is basically type (amount Big Int, currency char(3)).
The price entity belongs to a product. What I want to do is, create a unique constraint on the combination of product_id(foreign key) + currency . How can I do this?
Referencing a single field of a record type is a bit tricky:
CREATE TYPE money_with_currency AS (amount bigint, currency char(3));
CREATE TABLE product_price
(
product_id integer not null references product,
price money_with_currency not null
);
CREATE UNIQUE INDEX ON product_price(product_id, ((price).currency));
I have a table consisting of products (with ID's, ~15k records) and another table price_changes (~88m records) recording a change in the price for a given productID at a given changedate.
I'm now interested in the price for each product at given points in time (say every 2 hours for a year, so altogether ~ 4300 points; altogether resulting in ~64m data points of interest). While it's very straight forward to determine the price for a given product at a given time, it seems to be quite time-consuming to determine all 64m data points.
My approach is to pre-populate a new target table fullprices with the data points of interest:
insert into fullprices(obsdate,productID)
select obsdate, productID from targetdates, products
and then update each price observation in this new table like this:
update fullprices f set price = (select price from price_changes where
productID = f.productID and date < f.obsdate
order by date desc
limit 1)
which should give me the most recent price change in each point in time.
Unfortunately, this takes ... well, ages. Is there any better way to do it?
== Edit: My tables are created as follows: ==
CREATE TABLE products
(
productID uuid NOT NULL,
name text NOT NULL,
CONSTRAINT products_pkey PRIMARY KEY (productID )
);
CREATE TABLE price_changes
(
id integer NOT NULL,
productID uuid NOT NULL,
price smallint,
date timestamp NOT NULL
);
CREATE INDEX idx_pc_date
ON price_changes USING btree
(date);
CREATE INDEX idx_pc_productID
ON price_changes USING btree
(productID);
CREATE TABLE targetdates
(
obsdate timestamp
);
CREATE TABLE fullprices
(
obsdate timestamp NOT NULL,
productID uuid NOT NULL,
price smallint
);
Here is what I have so far:
INSERT INTO Tenants (LeaseStartDate, LeaseExpirationDate, Rent, LeaseTenantSSN, RentOverdue)
SELECT CURRENT_DATE, NULL, NewRentPayments.Rent, NewRentPayments.LeaseTenantSSN, FALSE from NewRentPayments
WHERE NOT EXISTS (SELECT * FROM Tenants, NewRentPayments WHERE NewRentPayments.HouseID = Tenants.HouseID AND
NewRentPayments.ApartmentNumber = Tenants.ApartmentNumber)
So, HouseID and ApartmentNumber together make up the primary key. If there is a tuple in table B (NewRentPayments) that doesn't exist in table A (Tenants) based on the primary key, then it needs to be inserted into Tenants.
The problem is, when I run my query, it doesn't insert anything (I know for a fact there should be 1 tuple inserted). I'm at a loss, because it looks like it should work.
Thanks.
Your subquery was not correlated - It was just a non-correlated join query.
As per description of your problem, you don't need this join.
Try this:
insert into Tenants (LeaseStartDate, LeaseExpirationDate, Rent, LeaseTenantSSN, RentOverdue)
select current_date, null, p.Rent, p.LeaseTenantSSN, FALSE
from NewRentPayments p
where not exists (
select *
from Tenants t
where p.HouseID = t.HouseID
and p.ApartmentNumber = t.ApartmentNumber
)