relational-database

问题 For a school project, I'm supposed to design the systems of a previous semester's project. We're using UML, creating an extremely simple use case diagram (no <<extend>> type nonsense, just actors pointing at use cases inside of a system), from which we make a class diagram, then a relational model. Use Case and Class Diagram attempts: The last diagram is just me eventually thinking that maybe my use cases were the issue. My application lets people/restaurants list food they're willing to

问题 I am trying to construct a MySQL query that will allow me to pull the song title of a track in the database that has a genre of both Pop and Electronic. +----------------------------------------------------+ TABLE: SONG +----------------------------------------------------+ song_id | title | 1 | song name | +----------------------------------------------------+ TABLE: GENRE +----------------------------------------------------+ genre_id | name | 1 | Pop | 2 | Electronic | +-------------------

问题 I want to build an application that allows a user to build its own form(s). A user can input data into its form and query the data too. The form can be edited after being created and used ( add/remove fields from it ). The form can have small text box, big text box, single list, check box, or a drop down menu. I've been reading and asking around about which approach is better for building this kind of databases and I got no straight/good answer. I read that in this kind of application, the

问题 I've been looking at examples of ER diagrams to better understand them. I came across an ER diagram that I am not sure is correct. Here is the question/specification: UPS prides itself on having up-to-date information on the processing and current location of each shipped item. To do this, UPS relies on a company-wide information system. Shipped items are the heart of the UPS product tracking information system. Shipped items can be characterized by item number (unique), weight, dimensions,

问题 For my project I am using oracle database where I have to work with 2 different database schema. Consider the following scenario please - I have a schema A and in it I have a table table_a with a primary key apk And I have another schema B and in it I have a table table_b with a primary key bpk If both of these tables are in a same database then I can easily make a primary key - foreign key relationship. But can I make a primary key - foreign key relation ship (or something like this) between

问题 Recently I am building a REST api as part of an assignment where I am suppose to increment a counter in the database table, assuming the table has only one column, I am suppose to fire like 1000 requests per second to this REST api to increment the counter and at the end the data should be consistent i.e. if initially the counter value in DB is 0 then after the succesful run of 1000 requests concurrently it should be 1000. No worries so far I achieved it via database row-level locking, other

问题 I have two tables Country and Region. A Region is a set of one or several Countries. Some Countries are in no Region. How should I represent this in a relational database? I thought of the two following possibilities: Country has a Edition column that includes either null or the Edition that it belongs to. My problem with this: I have been taught nulls are evil in a database. Edition has a Countries column that is an array of Country. My problem with this: I have been taught arrays are evil

问题 In this book: Regina Obe & Leo Hsu , PostgreSQL Up & Running, p. 101. It is written as an introduction to PostgreSQL XML data-type: The XML data type, similar to JSON, is “controversial” in a relational database because it violates principles of normalization. Without further explanation. Could someone detail what is normalization principles and why XML does violate some of those principles. 回答1: There are many tutorials on relational database normalization in books and on the web. See for

问题 Lets jump to an example that illustrates a table referencing multiple tables : CREATE TABLE Corses ( ID int PRIMARY KEY, ..... ) CREATE TABLE Questions ( ID int PRIMARY KEY, ..... ) CREATE TABLE Answers ( ID int PRIMARY KEY, ..... ) CREATE TABLE Files ( ID INT PRIMARY KEY, Corse_ID INT, Question_ID INT, Answer_ID INT, FOREIGN KEY (Corse_ID) REFERENCES Corses(ID), FOREIGN KEY (Question_ID) REFERENCES Questions(ID), FOREIGN KEY (Answer_ID) REFERENCES Answers(ID) ) The example above illustrates

问题 Suppose I perform A natural join B, where: A's schema is: (aID), where (aID) is a primary key. B's schema is: (aID,bID), where (aID, bID) is a composite primary key. Would performing the natural join work in this case? Or is it necessary for A to have both aID and bID for this to work? 回答1: NATURAL JOIN returns rows with one copy each of the common input table column names and one copy each of the column names that are unique to an input table. It returns a table with all such rows that can