Ok, I have situation where I need to create SQL query which will return for me ids from table1 (products) which was ordered by table2 (category) and limited by 10 for each category.
So, what I need. Select product ids which was appeared on "top 10" (limited by 10) results in each category after ordering of those products. Each product has some columns and I order by those columns. The same product can appear on different categories on top 10, for example. So I need use distinct for uniq results.
Is there any relationship between Product and Category? What at the Product columns you're ordering by? Is it ok for there to be duplication between different lists of products? You should really post you models/sql tables and more clearly explain what you're trying to do if you want real help.
Assuming they're many-to-many/the relationships are set up in rails and having the same products in multiple lists is ok I would do something like this
top_products = {}
Category.all.each do |cat|
top_products[cat.name] = cat.products.order("some_product_column DESC").limit(10).map{|p| p.id}
end
Related
How to join products and their characteristics
I have two tables.
Product (id, title, price, created_at, updated_at etc)
and
ProductCharacteristic(id, product_id, sold_quantity, date, craated_at, updated_at etc).
I should show products table (header is product.id, product.title, product.price, sold_quantity) for some period of time and ordered by any fields from header.
And I can't write query
Now I have following query
> current_project.products.includes(:product_characteristics).group('products.id').pluck(:title, 'SUM(product_characteristics.sold_quantity) AS sold_quantity')
(45.4ms) SELECT "products"."title", SUM(product_characteristics.sold_quantity) AS sold_quantity FROM "products" LEFT OUTER JOIN "product_characteristics" ON "product_characteristics"."product_id" = "products"."id" WHERE "products"."project_id" = $1 GROUP BY products.id [["project_id", 20]]
Please help me to write query through orm(to add where with dates and ordering) or write raw sql query.
I used pluck. It returns array of arrays (not array of hashes). It's no so good of course.
product_characteristics.date field is unique by scope product_id. But please give me two examples (with this condition and without it to satisfy my curiosity).
And I use postgresql and rails 4.2.x
P.S. By the way the ProductCharacteristic table will have a lot of records(mote than one million). Should I use postgresql table partitioning. Can it improve performance?
Thank you.
You can use select instead of count in that case, and the property will be accessible as product.sold_quantity
The query becomes
products = current_project.products.joins(:product_characteristics).group('products.id').select(:title, 'SUM(product_characteristics.sold_quantity) AS sold_quantity')
products.first.sold_quantity # => works
To order, you can just add an order clause
products = products.order(id: :asc)
or
products = products.order(id: :desc)
for instance
And for the where
products = products.where("created_at > ?", 2.days.ago)
for instance.
You can chain sql clauses after the first line, it does not matter cause the query will only be launched when you actually use the retrieved set.
And so you can also do stuff like
if params[:foo]
products = products.order(:id)
end
I am working on a database with products and lot numbers. Each entry in the Lots table has a Lot Number and a Product description.
Sometimes there are multiple records of the same lot number, for example when an item is repacked a new record is created, but with the same Lot Number and same product description - this is fine. But other times there are problem cases, namely when two different products share the same Lot Number. I am trying to find those.
In other words, there are 3 possibilities:
Lot numbers for which there is only one record in the table.
Lot numbers for which there are multiple records, but the Product description is the same for all of them
Lot numbers for which there are multiple records, and the product descriptions are not all the same.
I need to return only #3, with a separate record for each instance of that Lot Number and product description.
Any help would be greatly appreciated.
Thanks Juan for the sample data. Using this example, I want to return the data contained in Id 2-8, but not 1, 9, 10, 11.
This wasn't easy because lot of time don't use access.
First select unique values using distinct.
Then count how many diferent product appear on each lotnumber using group by
Last join both result and show only the lots with more than one description where total >1
.
SELECT id, Product.lotnumber, Product.Product, total
FROM
Product Inner join
(
SELECT lotnumber, count(*) as total
FROM
(SELECT distinct lotnumber, product
FROM Product)
GROUP BY lotnumber
) SubT On Product.lotnumber = SubT.lotnumber
WHERE total > 1
ORDER BY id
As you can see :
lot 2 have two products (yy and zz)
lot 3 have thre products (aa, bb, cc)
I include my product table:
Sorry for spanish. Field types are Autonumeric, Short Text, and Number
Lets say i have a bunch of products. Each product has and id, price, and long description made up of multiple paragraphs. Each product would also have multiple sku numbers that would represent different sizes and colors.
To clarify: product_id 1 has 3 skus, product_id 2 has 5 skus. All of the skus in product 1 share the same price and description. product 2 has a different price and description than product 1. All of product 2's skus share product 2's price and description.
I could have a large table with different records for each sku. The records would have redundant fields like the long description and price.
Or I could have two tables. One named "products" with product_id, price, and description. And one named "skus" with product_id, sku, color, and size. I would then join the tables on the product_id column.
$query = "SELECT * FROM skus LEFT OUTER JOIN products ON skus.product_id=products.product_id WHERE color='green'";
or
$query = "SELECT * FROM master_table WHERE color='green'";
This is a dumbed down version of my setup. In the end there will be a lot more columns and a lot of products. Which method would have better performance?
So to be more specific: Let's say I want to LIKE search on the long_description column for all of the skus. I am trying to compare having one table that has 5000 long_description and 5000 skus vs OUTER JOINing two tables, one has 1000 long_description records and the other has 5000 skus.
It depends on the usage of those tables - in order to get a definitive answer you should do both and compare using representative data sets / system usage.
The normal approach is to only denormalise data in order to combat specific performance problems that you are having, so in this case my advice would be to default to joining across two tables and only denormalise to using a single table if you have a performance problem and find that denormalisation fixes it.
OLTP normalized tables better
Join them at query, easier data manupulation and good response for short queries
OLAP denormalized tables better
Tables mostly dont change and good for long queries
Can SQL be used to find all the brands that has the most common categories?
For example, the brand "Dove" can have category of Soap, Skin Care, Shampoo
It is to find all the brands that has the most matching categories, in other words, the most similar brands.
It can be done programmatically using Ruby or PHP: just take a brand, and loop through all the other brands, and see how many matching categories there are, and sort by it. But if there are 2000 brands, then there needs to be 2000 queries per brand. (unless we pre-cache all the 2000 query results, so for all 2000 brands, we re-use those results)
Can it be done by SQL / MySQL by 1 query?
Say, the table has:
entities
--------
id
type = brand or category or product
name
entities_parent_child
--------------------
parent_id
child_id
the table above has an entry for each parent = brand and child = product, and also an entry for each parent = category and child = product, so brand has to relate to category by products.
I think the hard part for SQL is: find all the maximum matching counts, and sort by those numbers.
I agree with wuputah's comment. For this problem an "entities" table is not the answer. You've given yourself a hint the design is wrong when you say you cannot form a query to get the answers you want.
Create a proper hierarchy for your data, with separate tables for separate real word entities, yours will be:
[Brands]
[Categories]
[Products]
If you need help with defining trees and hierarchies in SQL I suggest you pick up a copy of Celko's Trees and Hierarchies in SQL for Smarties.
SQL has no concept of polymorphism so don't try to design your database to fit your programming language. Databases work with sets, so think in sets.
To find similar brands join your tables and use grouping:
SELECT Brands.brand_name, COUNT(Categores.category_name) as category_count
FROM Brands INNER JOIN Categories
ON Brands.brand_name = Categories.brand_name
GROUP BY Brands.brand_name
ORDER BY Brands.brand_name, COUNT(Categores.category_name) -- add DESC if you want largest count at the top
That gives you the basic idea, if you can expand on the requirement:
...find all the maximum matching
counts, and sort by those numbers
Then I can help redesign the query and, if necessary, the schema design.
Imagine I have table like this:
id:Product:shop_id
1:Basketball:41
2:Football:41
3:Rocket:45
4:Car:86
5:Plane:86
Now, this is an example of large internet mall, where there are shops which sell to one customer, so customer can choose more products from each shop and buy it in one basket.
However, I am not sure if there is any SQL syntax which allows me to simply get unique shop_ids and total number of those shops' products in customer basket. So I'd get something like:
Shop 41 has 2 products
Shop 45 one product
Shop 86 two product
I can make SQL queries to scoop through table to make some kind of ['shop_id']['number_of_products'] array variable that would store all products' shop_ids, then "unique them" - up and count how many times I had to cut one more shop_id out to have some remaining but that just seems as a lot of useless scripting.
If you got some nice and neat idea, please, let me know.
This is exactly the sort of thing that aggregate functions are for. You make one row of output for each group of rows in the table. Group them by shop_id and count how many rows are in each group.
select shop_id, count(1) from TABLE_NAME
group by shop_id