relational-database

In my database I have a table Category, with columns Id, CategoryName, ParentCategoryId, where ParentCategoryId has a constraint on Category.Id. I'm using entity framework code first, where the entity looks like: public class Category { public long Id { get; private set; } public string CategoryName { get; private set; } public long? ParentCategoryId { get; private set; } public Category ParentCategory { get; private set; } public virtual ICollection<Category> SubCategories { get; private set; } } If I try to run a query against this, I get the exception: The relationship 'ComplaintModel.FK

I have a movie in a MySQL database. A movie contains data attributes that will never change, such as the following: Barcode: 025192018626 Format: DVD Runtime: 121 min. Discs: 1 Title: 12 Monkeys Year: 1995 It's one single row in a table. But I want to give my user's complete customization over this information in case something is incorrect according to them or if they just want to modify how the data is displayed in some way. I don't care why, I just want to give my users an option to do what they want. Let's say User #1 wants to change the Title for them to be "12 Monkeys (Shelf 1)" and that

I want to use HSTORE type for a column if it uses PostgreSQL as its backend, or PickleType otherwise. The problem is that we cannot determine which backend will be used when schema is being defined (in Python). How can I determine this and conditionally choose the data type when the table actually is created on the backend database? You can accomplish something like this with TypeEngine.with_variant : from sqlalchemy.types import PickleType from sqlalchemy.dialects import postgresql HybridType = PickleType() HybridType = HybridType.with_variant(postgresql.HSTORE(), 'postgresql') This creates a

What will natural join return in relational algebra if tables don't have attributes with same names? Will it be null or the same as cross-product (Cartesian operator)? If there are no attributes in common between two relations and you perform a natural join , it will return the cartesian product of the two relations. A cartesian product of two tables will be returned.This is because when we perform any JOIN operation on two tables a cartesian product of those tables is performed and then based on any select condition in WHERE clause the resultant rows are returned.But here as there are no

I am new to web programming and need some expert advise. I am using SQL database for one of my website project. My website will display a list of topics with a Like and Dislike button. Logged in users will be able to click on the Like or dislike button for each topic. I am able to keep a count of likes and dislikes for each topics. So if a user clicks on a certain topic it will display the number of Likes and Dislikes for that particular topic, however what I am trying to achieve is when someone clicks on a particular Users profile it should display all the topics that user likes or dislikes.

What would be the best way to model the following situation: Word belongs_to :wordable, :polymorphic => true Phrase has_many :words, :as => :workable belongs_to :story Line has_many :words, :as => :wordable belongs_to :story Story has_many :lines has_many :phrases has_many :words, :through => :phrases has_many :words, :through => :lines I want to be able to do @story.words to get list of all words that are linked to a story either via lines or via phrases... Is that possible? Harish Shetty Try this: class Story has_many :lines has_many :phrases def words(reload=false) @words = nil if reload