relational-database

问题 I don't know how to force read-only columns in Hibernate. I would like to set idgroup as read only column. Even if I set insertable=false and updatable=false , in the hibernate SQL I can read: Hibernate: insert into groups (description, name, account_idaccount, idgroup) values (?, ?, ?, ?) but I would like to obtain: insert into groups (description, name, account_idaccount) values (?, ?, ?) Here are my classes: @Entity @Table(name = "groups") public class Group implements java.io.Serializable

问题 I've gone through internet and books and still have some difficulties on how to determine the normal form of this relation R(a, b, c, d, e, f, g, h, i) FDs = B→G BI→CD EH→AG G→DE So far I've got that the only candidate key is BHI (If I should count with F, then BFHI). Since the attribute F is not in use at all. Totally independent from the given FDs. What am I supposed to do with the attribute F then? How to determine the highest normal form for the realation R? 回答1: What am I supposed to do

问题 We have 3-Relation: Students(sid, sname) Courses(cid, cname, dept) take(sid, cid, grade) We want to find student numbers of students whose these students take all courses that present in 'CS' department . why (line 4) is the answer of this query ? anyone could say differ from (line 1) to (line 3). I read this in Solved-Ex in DB. 回答1: Another way to phrase the fourth line is: all students except ( all combinations of students and CS courses except all taken courses ) The set between brackets

问题 I am new to using UML and I am not sure if my diagram is correct. I would like some advice from someone more experienced than me. The statement of my problema says: Students study courses and each student can take several courses "studies" each. Any course can have more than one student studying it. Students, who are identified by a numerical code, have a name, date of birth and one or more nationalities. A student who studies can receive scholarships. A Student can recieve more than one

问题 I am creating a database for a Basketball Season. At this point, I am keeping it simple, and storing tables: - League id[PK], name->(NBA, NCAAM, etc) - Year id[PK], league_id[FK], year->(2012-2013, 2011-2012, etc) - Team id[PK], league_id[FK], name->(Bulls, Lakers, Etc) - Game id[PK], home_team[FK], away_team[FK] - Score id[PK], game_id[FK], home_score, away_score My question is about table structure. I am somewhat new to sql, but I want to be able to make calcuations based on database info.

问题 I have a medium size (~100mb) read-only database that I want to put on google app engine. I could put it into the datastore, but the datastore is kind of slow, has no relational features, and has many other frustrating limitations (not going into them here). Another option is loading all the data into memory, but I quickly hit the quota imposed by google. A final option is to use django-nonrel + djangoappengine, but I'm afraid that package is still in its infancy. Ideally, I'd like to create

问题 I know that for relational database like Postgresql using separated tables would be more efficient but I'm concerning for performance issues because the most executed query will fetch rows from multiple tables using UNION ALL . I have to option to handle this problem. First one is: table1 -> column1, column2 table2 -> column1, column2 table3 -> column1, column2, column3 In this solution I have to use 3 different query merged with UNION ALL in production and this query will be performed a user

问题 In our Rails app, there are 3 models: class User < ActiveRecord::Base has_many :administrations, dependent: :destroy has_many :calendars, through: :administrations end class Administration < ActiveRecord::Base belongs_to :user belongs_to :calendar end class Calendar < ActiveRecord::Base has_many :administrations, dependent: :destroy has_many :users, through: :administrations end And here are the corresponding migrations: class CreateUsers < ActiveRecord::Migration def change create_table

问题 This question is not limited to Power BI, but it will help me explain my problem. If you have more than one table in Power BI, you can establish a relationship between them by dragging a column from one table to the other like this: And you can edit that relationship by clicking the occuring line: And by the way, here are the structures of the two tables: # Table1 A,B 1,abc 2,def 3,ghi 4,jkl # Table2 A,C 1,abc 1,def 2,ghi 3,ghit This works fine since column A in Table1 consists of unique

问题 I currently have a site with Users, Meals, and Orders tables. Users table holds the User information Meals table holds the Meal name, description, img(url) I am currently displaying the Meals using an iteration: <% @meals.each do |meal| %> <%= image_tag(meal.img, alt: meal.name) %> <span class="mealname"><%= meal.name %></span> <p><%= meal.description %><p> <div class="qty"> INPUT FOR ORDER QTYS HERE </div> <% end %> Orders table holds the qty's that the user wants for each Meal Orders