Add JOINs piece to relational databases section

authors.json

@@ -23,5 +23,10 @@
       "name": "Dian Fay",
       "avatar": "/authors/dmfay.jpg",
       "bio": "Dian didn't exactly plan to drop out of college to specialize in SQL and backend development, but that’s how it happened. Fifteen years later, she's designed databases supporting everything from industrial logistics and traceability systems to million-plus user social media games. She is the current maintainer of MassiveJS, an open source data mapper for Node.js focused on using PostgreSQL to the fullest."
+    },
+    "grishmapatel": {
+      "name": "Grishma Patel",
+      "avatar": "https://github.com/grpatel.png",
+      "bio": "Grishma is a recent Computer Science graduate and worked as a Developer Advocate intern at Prisma."
     }
 }

content/04-postgresql/13-reading-and-querying-data/03-joining-tables.mdx

@@ -10,7 +10,7 @@ authors: ['justinellingwood']
 
 Splitting related data into separate tables can be beneficial from the standpoint of consistency, flexibility, and certain types of performance. However, you still need a reasonable way of reintegrating records when the relevant information spans multiple tables.
 
-In relational databases, _joins_ offer a way to combine the records in two or more tables based on common field values. Different types of joins can achieve different results depending on how unmatched rows should be handled. In this guide, we'll discuss the various types of joins that PostgreSQL offers and how you can use them to combine table data from multiple sources.
+In relational databases, [_joins_](/types/relational/what-are-joins-in-sql) offer a way to combine the records in two or more tables based on common field values. Different types of joins can achieve different results depending on how unmatched rows should be handled. In this guide, we'll discuss the various types of joins that PostgreSQL offers and how you can use them to combine table data from multiple sources.
 
 ## What are joins?
 

content/05-mysql/10-reading-and-querying-data/03-joining-tables.mdx

@@ -10,7 +10,7 @@ authors: ['justinellingwood']
 
 Though it's often useful to separate data into discrete tables for performance and consistency purposes, you often need to consult data from multiple tables to answer certain requests. _Joining_ tables is a way of combining the data from various tables by matching each record based on common field values.
 
-There are a few different types of joins, which offer various ways of combining table records. In this article, we'll cover how MySQL implements joins and discuss the scenarios in which each is most useful.
+There are a few different types of [joins](/types/relational/what-are-joins-in-sql), which offer various ways of combining table records. In this article, we'll cover how MySQL implements joins and discuss the scenarios in which each is most useful.
 
 ## What are joins?