relational-database | 易学教程

Database changes for products in woocommerce 3

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问题 Woocommerce v2.6 stores the following meta_key values in the wp_postmeta table: _sku _price _regular_price _sale_price _manage_stock _stock_status _featured Does Woocommerce v3.x, still store are all of the above in an identical manner as v2.6 or have any of the above been relocated to another table and/or modified in any way? 回答1: In WooCommerce 3+ everything listed is the same, except for _featured that doesn't work any more. The " feature " product functionality in WooCommerce 3+: Since

How to convert SQL to Relational Algebra in case of SQL Joins?

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问题 I am working on SQL and Relational Algebra these days. And I am stuck on the below questions. I am able to make a SQL for the below questions but somehow my Relational Algebra that I have made doesn't looks right. Below are my tables- Employee ( EmployeeId , EmployeeName, EmployeeCountry) Training ( TrainingCode , TrainingName, TrainingType, TrainingInstructor) Outcome ( EmployeeId, TrainingCode , Grade) All the keys are specified with star *. Below is the question and its SQL query as well

How to organise a many to many relationship in MongoDB

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I have two tables/collections; Users and Groups. A user can be a member of any number of groups and a user can also be an owner of any number of groups. In a relational database I'd probably have a third table called UserGroups with a UserID column, a GroupID column and an IsOwner column. I'm using MongoDB and I'm sure there is a different approach for this kind of relationship in a document database. Should I embed the list of groups and groups-as-owner inside the Users table as two arrays of ObjectIDs? Should I also store the list of members and owners in the Groups table as two arrays,

What's the point of a candidate key?

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问题 I'm fairly new to database management and this question never seems to be answered in more than one sentence. All other SO answers say "A candidate key is a minimal super key." That means nothing to me. A candidate key is supposed to specify uniqueness of a db record, correct? And a primary key is a candidate key. If a primary key already specifies uniqueness, what's point of adding more candidate keys? I have seen example records like the following: Employee(ID, Name, PhoneNumber) where ID

Use cases for NoSQL [closed]

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NoSQL has been getting a lot of attention in our industry recently. I'm really interested in what peoples thoughts are on the best use-cases for its use over relational database storage. What should trigger a developer into thinking that particular datasets are more suited to a NoSQL solution. I'm particularly interested in MongoDB and CouchDB as they seem to be getting the most coverage with regard to PHP development and that is my focus. Just promise yourself that you will never try to map a relational data model to a NoSQL database like MongoDB or CouchDB... This is the most common mistake

Storing time-series data, relational or non?

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I am creating a system which polls devices for data on varying metrics such as CPU utilisation, disk utilisation, temperature etc. at (probably) 5 minute intervals using SNMP. The ultimate goal is to provide visualisations to a user of the system in the form of time-series graphs. I have looked at using RRDTool in the past, but rejected it as storing the captured data indefinitely is important to my project, and I want higher level and more flexible access to the captured data. So my question is really: What is better, a relational database (such as MySQL or PostgreSQL) or a non-relational or

Minimum no of tables that exists after decomposing relation R into 1NF?

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Consider the relation R(A, B, C, D, E, F, G) with the following types of attributes:- Total No of Keys = 1 = {A} Set of Simple (or) Atomic (or) Single Valued Attributes = {B, C} Set of Multivalued Attributes = {D, E} Set of Composite Attributes = { F, G} What would be the minimum no of tables that exists after decomposing relation R into 1NF? (A) 3 (B) 2 (C) 4 (D) 5 My attempt: We needed different table for each multivalued attributes with given key(A), total = 2 Similarly, we needed different table for each composite attributes, total = 2. There are total 4 such attribute. I give 4 tables

Difference between using REFERENCES with and without FOREIGN KEY?

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问题 Basically I'd like to know the difference between using REFERENCES with or without a foreign key. I have these 2 examples: CREATE TABLE Employee ( id INT, name VARCHAR(50), birthYear INT, boss INT REFERENCES Employees(id), worksAt INT NOT NULL REFERENCES Department(id) ON DELETE CASCADE, PRIMARY KEY (id,worksAt) ); Example 2: CREATE TABLE Department ( id INT PRIMARY KEY, name VARCHAR(50), numberOfEmployees INT, location INT NOT NULL, country INT NOT NULL, manager INT, FOREIGN KEY (location

SQLite many-to-many relationship?

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问题 I'm trying to set up a SQLite3 database with foo s and bar s and a many-to-many relation between them. This is what I've got so far: CREATE TABLE foo( id INTEGER PRIMARY KEY NOT NULL, foo_col INTEGER NOT NULL ); CREATE TABLE bar( id INTEGER PRIMARY KEY NOT NULL, bar_col TEXT NOT NULL ); CREATE TABLE foobar( foo_id INTEGER, bar_id INTEGER, FOREIGN KEY(foo_id) REFERENCES foo(id) ON DELETE CASCADE, FOREIGN KEY(bar_id) REFERENCES bar(id) ON DELETE CASCADE ); CREATE INDEX fooindex ON foobar(foo_id

When I should use one to one relationship?

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Sorry for that noob question but is there any real needs to use one-to-one relationship with tables in your database? You can implement all necessary fields inside one table. Even if data becomes very large you can enumerate column names that you need in SELECT statement instead of using SELECT * . When do you really need this separation? Branko Dimitrijevic 1 to 0..1 The "1 to 0..1" between super and sub-classes is used as a part of "all classes in separate tables" strategy for implementing inheritance . A "1 to 0..1" can be represented in a single table with "0..1" portion covered by NULL