Answers.Body: <h3>What is a knowledge graph?</h3>
  <p>Appendix <a href="https://arxiv.org/pdf/2003.02320.pdf#page=111" rel="nofollow noreferrer"><strong>A.3 &quot;Knowledge Graphs&quot;: 2012 Onwards</strong></a> of the survey <a href="https://arxiv.org/pdf/2003.02320.pdf" rel="nofollow noreferrer">Knowledge Graphs</a> (which is probably the most extensive survey on KGs) states that <strong>knowledge graphs have been defined in different ways</strong> in recent years. Each of these definitions raises questions about the relationship between KGs and other related concepts, like graph databases, knowledge bases, and ontologies.</p>
  <p>One definition of a KG is</p>
  <blockquote>
  <p>a graph where nodes represent entities, and edges represent relationships between those entities. Often a directed edge labelled graph is assumed (or analogously, a set of binary relations, or a set of triples)</p>
  </blockquote>
  <p>The question here is: what's the difference between KGs and graph databases (like Neo4j)? Graph databases have been used to build KGs, but is there any actual difference between these 2 terms?</p>
  <p>Another definition of a KG is</p>
  <blockquote>
  <p>a knowledge graph is a graph-structured knowledge base</p>
  </blockquote>
  <p>So, according to this definition, a KG would be a type of knowledge base (KB).</p>
  <h3>What is a knowledge base?</h3>
  <p>In the same appendix, the authors write</p>
  <blockquote>
  <p>The phrase &quot;knowledge base&quot; was popularised in the 70's (possibly earlier) in the context of <strong>rule-based expert systems</strong> <a href="https://stacks.stanford.edu/file/druid:pj337tr4694/pj337tr4694.pdf" rel="nofollow noreferrer">[72]</a>, and later were used in the context of ontologies and other logical formalisms <a href="http://eolo.cps.unizar.es//Docencia/MasterUPV/Articulos/An%20Overview%20of%20the%20KL-ONE%20Knowledge%20Representation%20System-Brachman1985.PDF" rel="nofollow noreferrer">[68]</a></p>
  </blockquote>
  <p>They conclude that a KB has also been defined in ambiguous ways in the past.</p>
  <p>Norvig and Russell, in chapter 7 (p. 235) of their AIMA book (3rd edition), define a KB as a <strong>set of sentences/facts</strong>, for example, expressed in propositional logic. You then use <strong>inference</strong> techniques to derive new knowledge from this knowledge base. The programming language <a href="https://www.doc.gold.ac.uk/%7Emas02gw/prolog_tutorial/prologpages/" rel="nofollow noreferrer">PROLOG</a> is based on this definition of a KB.</p>
  <h3>What is the difference between a KG and KB?</h3>
  <p>So, there is not a single answer to your question because knowledge graphs (KGs) and knowledge bases (KBs) have been <strong>defined in multiple (often ambiguous) ways</strong> in the past. Some people say that KGs are different from KBs, while other people use the term KG as a synonym for KB or define it as a type of KB.</p>
  <h3>One possible answer</h3>
  <p>However, if we define a KG as a graph with nodes that represent entities (like city and country) and edges that represent the relations between those entities (which is a common definition of a KG), we can view a <strong>KG as a visual representation of a KB</strong>, defined as a set of sentences/facts (for example, expressed in propositional logic). To see why this is the case, consider the following simple KG.</p>
  <p><a href="https://i.sstatic.net/MMgXi.png" rel="nofollow noreferrer"><img src="https://i.sstatic.net/MMgXi.png" alt="enter image description here" /></a></p>
  <p>Let's denote this KG by <span class="math-container">$K = \{E, R\}$</span>, where <span class="math-container">$E = \{ \text{Santiago}, \text{Chile}, \text{Perú}\} = \{ S, C, P\}$</span> is the set of entities (with a property) and <span class="math-container">$R = \{ \text{capital}, \text{borders}\} = \{ c, b \}$</span> is the set of relations. In <span class="math-container">$K$</span>, we have the following relations</p>
  <ul>
  <li>Santiago is the capital of Chile: <span class="math-container">$(S, c, C)$</span></li>
  <li>Chile borders Perú: <span class="math-container">$(C, b, P)$</span></li>
  <li>Perú borders Chile: <span class="math-container">$(P, b, C)$</span></li>
  </ul>
  <p>which can be put into a set of facts <span class="math-container">$F = \{(S, c, C), (C, b, P), (P, b, C) \}$</span>, which is the KB associated with this specific KG.</p>
  <p>In the context of KGs, we also have a task/problem similar to the <strong>inference</strong> (or <strong>knowledge reasoning</strong>) one in the context of KBs, which is known <strong>graph completion</strong>, which can be divided into other subtasks, like <strong>entity prediction</strong>, <strong>relation prediction</strong> and <strong>triple classification</strong>, which use <strong>knowledge graph embeddings</strong>.</p>
  <h3>Further reading</h3>
  <p>You should really read <a href="https://arxiv.org/pdf/2003.02320.pdf#page=108" rel="nofollow noreferrer"><strong>Appendix A</strong></a> and successive appendices of <a href="https://arxiv.org/pdf/2003.02320.pdf#page=111" rel="nofollow noreferrer">the mentioned survey</a> to get more information about the issues concerning the history and definition of knowledge graphs and knowledge bases, and other related concepts, like schemas. You can also read chapter 7 (p. 235) of the AIMA book, 3rd edition to know more about knowledge bases.</p>
  
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Answers.UserDisplayName: nbro
Answers.Body: <p>Based on <a href="https://en.wikipedia.org/wiki/Knowledge_base" rel="nofollow noreferrer">the related Wikipedia</a>, a knowledge base (KB) is:</p>
  
  <blockquote>
    <p>a technology used to store complex structured and unstructured information used by a computer system. The initial use of the term was in connection with expert systems which were the first knowledge-based systems.</p>
  </blockquote>
  
  <p>As there are different representation model for a KB, we can find different terminology in different domains. For example, in some AI articles, it's called <strong>ontology</strong>.</p>
  
  <p><a href="https://en.wikipedia.org/wiki/Knowledge_Graph" rel="nofollow noreferrer">Knowledeg graph</a> (KG) is another object model to KB realization which is introduced by Google for its search engine (as you have mentioned). Hence, KG is a specification of KB. You can find more information in the paper <a href="https://arxiv.org/pdf/2003.02320.pdf" rel="nofollow noreferrer">Knowledge Graphs</a>, such as more history about the KG or a formal definition of that: </p>
  
  <blockquote>
    <p>knowledge graph is a graph of data intended to accumulate and convey knowledge of the real world, whose nodes represent entities of interest and whose edges represent relations between these entities.</p>
  </blockquote>
  
  <p>Moreover, you can find some articles about contextual KG (CKG) in the paper <a href="https://aisel.aisnet.org/cgi/viewcontent.cgi?article=1026&amp;context=pacis2017" rel="nofollow noreferrer">Learning Contextual Embeddings for Knowledge Graph Completion</a> and <a href="https://arxiv.org/pdf/1805.12393.pdf" rel="nofollow noreferrer">KG<span class="math-container">$^2$</span>: Learning to Reason Science Exam Questions with Contextual Knowledge Graph Embeddings</a>.</p>
  
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Answers.Text: I am not sure if an ontology is a synonym for KB. For example, read this article [What’s the Difference Between an Ontology and a Knowledge Graph?](https://enterprise-knowledge.com/whats-the-difference-between-an-ontology-and-a-knowledge-graph/), where the author says that a knowledge graph is an "instantiation" of an ontology. See also **Ontologies and knowledge bases: towards a terminological clarification** (1995) by Nicola Guarino et al. The term ontology has been defined differently in many cases. There's also [this related blog post](https://www.ibm.com/cloud/learn/knowledge-graph).
Answers.UserDisplayName: nbro
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Answers.Text: @nbro You're right. They are not a synonym. It's also a realization of a KB. However, the term "ontology" is mostly used in AI as a reference to a KB
Answers.UserDisplayName: OmG
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Answers.Text: I have to support OmG here. I have always seen the word 'ontology' used in AI. Whether that is 'correct' or not is a different question, sadly.
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Answers.UserDisplayName: OmG
Answers.Body: <p>In <a href="https://rads.stackoverflow.com/amzn/click/com/1680838369" rel="nofollow noreferrer" rel="nofollow noreferrer">a book</a> coauthored by Xin Luna Dong, one of the creators of the Google Knowledge Vault, it reads that:</p>
  <blockquote>
  <p>Over the last decade, large-scale knowledge bases, also known as knowledge graphs, have been automatically constructed from web contents and text sources, and have become a key asset for search engines.</p>
  </blockquote>
  <p>Then I thought knowledge graphs are just large-scale knowledge bases, and this statement can be proved by this snippet in <a href="https://www.7x7.com/googles-emily-moxley-on-the-knowledge-graph-and-women-in-tech-1781625222.html" rel="nofollow noreferrer">this article</a> on Emily Moxley, the product manager of the 2012 knowledge graph project:</p>
  <blockquote>
  <p>Then, a year ago, when the search giant bought a company called Metaweb and its collaborative knowledge base called Freebase, made up of metadata collected from multiple sources, Moxley took on the responsibility to help integrate Freebase into Google’s search UI.</p>
  <p>“At Metaweb, they were collecting factual information about real world things, like baseball teams and tourist destinations and celebrities. Once they came to Google, it was an amazing match, because we have all this data, including Maps, Business Photos, Google Books – gigantic data sets that let us identify real world things.”</p>
  <p>Fast forward to the present, and from the <strong>12 million entities</strong> Metaweb brought with it to Google, the Knowledge Graph has ballooned to include <strong>500 million items and 3.5 billion facts</strong>. As for the related search results that are algorithmically displayed, Google recognizes patterns in search data no competitor can see due to its massive scale.</p>
  </blockquote>
  
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Answers.UserDisplayName: Lerner Zhang
Body: <p>During my readings, I have seen many authors using the two terms interchangeably, i.e. as if they refer to the same thing. However, we all know about Google's first quotation of &quot;knowledge graph&quot; to refer to their <em>new</em> way of making use of their knowledge base. Afterward, other companies are claiming to use knowledge graphs.</p>
  <p>What are the technical differences between the two? Concrete examples will be very useful to understand better the nuances.</p>
  
Tags: <comparison><terminology><knowledge-representation><knowledge-graph><knowledge-base>
Title: What are the differences between a knowledge base and a knowledge graph?
UserDisplayName: Betty
ThreadText: # What are the differences between a knowledge base and a knowledge graph?
  
  (Asked by: username_0 on 2020-06-11)
  
  During my readings, I have seen many authors using the two terms interchangeably, i.e. as if they refer to the same thing. However, we all know about Google's first quotation of "knowledge graph" to refer to their *new* way of making use of their knowledge base. Afterward, other companies are claiming to use knowledge graphs.
  
  What are the technical differences between the two? Concrete examples will be very useful to understand better the nuances.
  
  **Tags:** comparison, terminology, knowledge-representation, knowledge-graph, knowledge-base
  
  
  ---
  
  ## 3 answers
  
  ### Answer by username_1 on 2021-12-07. Score: 2
  
  ### What is a knowledge graph?
  
  Appendix [**A.3 "Knowledge Graphs": 2012 Onwards**](https://arxiv.org/pdf/2003.02320.pdf#page=111) of the survey [Knowledge Graphs](https://arxiv.org/pdf/2003.02320.pdf) (which is probably the most extensive survey on KGs) states that **knowledge graphs have been defined in different ways** in recent years. Each of these definitions raises questions about the relationship between KGs and other related concepts, like graph databases, knowledge bases, and ontologies.
  
  One definition of a KG is
  
  > a graph where nodes represent entities, and edges represent relationships between those entities. Often a directed edge labelled graph is assumed (or analogously, a set of binary relations, or a set of triples)
  
  The question here is: what's the difference between KGs and graph databases (like Neo4j)? Graph databases have been used to build KGs, but is there any actual difference between these 2 terms?
  
  Another definition of a KG is
  
  > a knowledge graph is a graph-structured knowledge base
  
  So, according to this definition, a KG would be a type of knowledge base (KB).
  
  ### What is a knowledge base?
  
  In the same appendix, the authors write
  
  > The phrase "knowledge base" was popularised in the 70's (possibly earlier) in the context of **rule-based expert systems** [[72]](https://stacks.stanford.edu/file/druid:pj337tr4694/pj337tr4694.pdf), and later were used in the context of ontologies and other logical formalisms [[68]](http://eolo.cps.unizar.es//Docencia/MasterUPV/Articulos/An%20Overview%20of%20the%20KL-ONE%20Knowledge%20Representation%20System-Brachman1985.PDF)
  
  They conclude that a KB has also been defined in ambiguous ways in the past.
  
  Norvig and Russell, in chapter 7 (p. 235) of their AIMA book (3rd edition), define a KB as a **set of sentences/facts**, for example, expressed in propositional logic. You then use **inference** techniques to derive new knowledge from this knowledge base. The programming language [PROLOG](https://www.doc.gold.ac.uk/%7Emas02gw/prolog_tutorial/prologpages/) is based on this definition of a KB.
  
  ### What is the difference between a KG and KB?
  
  So, there is not a single answer to your question because knowledge graphs (KGs) and knowledge bases (KBs) have been **defined in multiple (often ambiguous) ways** in the past. Some people say that KGs are different from KBs, while other people use the term KG as a synonym for KB or define it as a type of KB.
  
  ### One possible answer
  
  However, if we define a KG as a graph with nodes that represent entities (like city and country) and edges that represent the relations between those entities (which is a common definition of a KG), we can view a **KG as a visual representation of a KB**, defined as a set of sentences/facts (for example, expressed in propositional logic). To see why this is the case, consider the following simple KG.
  
  [![enter image description here](https://i.sstatic.net/MMgXi.png)](https://i.sstatic.net/MMgXi.png)
  
  Let's denote this KG by $K = \{E, R\}$, where $E = \{ \text{Santiago}, \text{Chile}, \text{Perú}\} = \{ S, C, P\}$ is the set of entities (with a property) and $R = \{ \text{capital}, \text{borders}\} = \{ c, b \}$ is the set of relations. In $K$, we have the following relations
  
  * Santiago is the capital of Chile: $(S, c, C)$
  * Chile borders Perú: $(C, b, P)$
  * Perú borders Chile: $(P, b, C)$
  
  which can be put into a set of facts $F = \{(S, c, C), (C, b, P), (P, b, C) \}$, which is the KB associated with this specific KG.
  
  In the context of KGs, we also have a task/problem similar to the **inference** (or **knowledge reasoning**) one in the context of KBs, which is known **graph completion**, which can be divided into other subtasks, like **entity prediction**, **relation prediction** and **triple classification**, which use **knowledge graph embeddings**.
  
  ### Further reading
  
  You should really read [**Appendix A**](https://arxiv.org/pdf/2003.02320.pdf#page=108) and successive appendices of [the mentioned survey](https://arxiv.org/pdf/2003.02320.pdf#page=111) to get more information about the issues concerning the history and definition of knowledge graphs and knowledge bases, and other related concepts, like schemas. You can also read chapter 7 (p. 235) of the AIMA book, 3rd edition to know more about knowledge bases.
  
  
  ---
  
  ### Answer by username_2 on 2020-06-11. Score: 1
  
  Based on [the related Wikipedia](https://en.wikipedia.org/wiki/Knowledge_base), a knowledge base (KB) is:
  
  > a technology used to store complex structured and unstructured information used by a computer system. The initial use of the term was in connection with expert systems which were the first knowledge-based systems.
  
  As there are different representation model for a KB, we can find different terminology in different domains. For example, in some AI articles, it's called **ontology**.
  
  [Knowledeg graph](https://en.wikipedia.org/wiki/Knowledge_Graph) (KG) is another object model to KB realization which is introduced by Google for its search engine (as you have mentioned). Hence, KG is a specification of KB. You can find more information in the paper [Knowledge Graphs](https://arxiv.org/pdf/2003.02320.pdf), such as more history about the KG or a formal definition of that:
  
  > knowledge graph is a graph of data intended to accumulate and convey knowledge of the real world, whose nodes represent entities of interest and whose edges represent relations between these entities.
  
  Moreover, you can find some articles about contextual KG (CKG) in the paper [Learning Contextual Embeddings for Knowledge Graph Completion](https://aisel.aisnet.org/cgi/viewcontent.cgi?article=1026&context=pacis2017) and [KG$^2$: Learning to Reason Science Exam Questions with Contextual Knowledge Graph Embeddings](https://arxiv.org/pdf/1805.12393.pdf).
  
  > **Comments:**
  > **username_1**: I am not sure if an ontology is a synonym for KB. For example, read this article [What’s the Difference Between an Ontology and a Knowledge Graph?](https://enterprise-knowledge.com/whats-the-difference-between-an-ontology-and-a-knowledge-graph/), where the author says that a knowledge graph is an "instantiation" of an ontology. See also **Ontologies and knowledge bases: towards a terminological clarification** (1995) by Nicola Guarino et al. The term ontology has been defined differently in many cases. There's also [this related blog post](https://www.ibm.com/cloud/learn/knowledge-graph).  
  > **username_2**: @username_1 You're right. They are not a synonym. It's also a realization of a KB. However, the term "ontology" is mostly used in AI as a reference to a KB  
  > **username_3**: I have to support OmG here. I have always seen the word 'ontology' used in AI. Whether that is 'correct' or not is a different question, sadly.  
  
  ---
  
  ### Answer by username_4 on 2022-10-25. Score: 0
  
  In [a book](https://rads.stackoverflow.com/amzn/click/com/1680838369) coauthored by Xin Luna Dong, one of the creators of the Google Knowledge Vault, it reads that:
  
  > Over the last decade, large-scale knowledge bases, also known as knowledge graphs, have been automatically constructed from web contents and text sources, and have become a key asset for search engines.
  
  Then I thought knowledge graphs are just large-scale knowledge bases, and this statement can be proved by this snippet in [this article](https://www.7x7.com/googles-emily-moxley-on-the-knowledge-graph-and-women-in-tech-1781625222.html) on Emily Moxley, the product manager of the 2012 knowledge graph project:
  
  > Then, a year ago, when the search giant bought a company called Metaweb and its collaborative knowledge base called Freebase, made up of metadata collected from multiple sources, Moxley took on the responsibility to help integrate Freebase into Google’s search UI.
  >
  > “At Metaweb, they were collecting factual information about real world things, like baseball teams and tourist destinations and celebrities. Once they came to Google, it was an amazing match, because we have all this data, including Maps, Business Photos, Google Books – gigantic data sets that let us identify real world things.”
  >
  > Fast forward to the present, and from the **12 million entities** Metaweb brought with it to Google, the Knowledge Graph has ballooned to include **500 million items and 3.5 billion facts**. As for the related search results that are algorithmically displayed, Google recognizes patterns in search data no competitor can see due to its massive scale.
  
  
  ---
  
  