Beginners Guide to SQL – SQL Join Fundamentals

(SQL Tutorials for Citizen Data Scientist)

SQL Join Fundamentals

All the queries you’ve seen so far have been concentrated on a single table. But in real life situation you often need to query two or more tables at time and bring a combined result set. This is technically referred to as a join, since it involves joining different tables, based on a common field between them (the foreign key) to create new views of the data.

To understand this easily, let’s look at the following employees and departments tables. Here, the dept_id column of the employees table is the foreign key to the departments table. Therefore, these two tables can be joined to get the combined data.

| emp_id | emp_name     | hire_date  | dept_id |
|      1 | Ethan Hunt   | 2001-05-01 |       4 |
|      2 | Tony Montana | 2002-07-15 |       1 |
|      3 | Sarah Connor | 2005-10-18 |       5 |
|      4 | Rick Deckard | 2007-01-03 |       3 |
|      5 | Martin Blank | 2008-06-24 |    NULL |
| dept_id | dept_name        |
|       1 | Administration   |
|       2 | Customer Service |
|       3 | Finance          |
|       4 | Human Resources  |
|       5 | Sales            |
Table: employees Table: departments

Note: In order to join tables, data of the columns which are used for joining tables should match, not necessarily the column names.

Types of Joins

When you join tables, the type of join that you create in your query affects the rows that appear in the result set. You can create the following types of joins:

Inner join

A join that returns only those rows that have a match in both joined tables. For example, you can join the employees and departments tables to create a result set that shows the department name for each employee. In an inner join, employees for which there is no department information are not included in the result set, nor are departments with no employees.

We will learn more about inner join in next chapter.

Outer join

Outer joins are an extension to inner joins. An outer join returns the rows even if they don’t have related rows in the joined table. There are three types of outer joins: left outer join (or left join), right outer join (or right join), and full outer join (or full join).

We will learn more about these variations of the outer join in later chapters.

Cross join

Cross joins are joins without a join condition. Each row of one table is combined with each row of another table. This type of result set is called a Cartesian product or cross product. For example, a cross join between the employees and departments tables yields a result set with one row for each possible employees/departments combination.

We will learn more about cross join in upcoming chapters.


Beginners Guide to SQL – SQL Join Fundamentals

Personal Career & Learning Guide for Data Analyst, Data Engineer and Data Scientist

Applied Machine Learning & Data Science Projects and Coding Recipes for Beginners

A list of FREE programming examples together with eTutorials & eBooks @ SETScholars

95% Discount on “Projects & Recipes, tutorials, ebooks”

Projects and Coding Recipes, eTutorials and eBooks: The best All-in-One resources for Data Analyst, Data Scientist, Machine Learning Engineer and Software Developer

Topics included: Classification, Clustering, Regression, Forecasting, Algorithms, Data Structures, Data Analytics & Data Science, Deep Learning, Machine Learning, Programming Languages and Software Tools & Packages.
(Discount is valid for limited time only)

Disclaimer: The information and code presented within this recipe/tutorial is only for educational and coaching purposes for beginners and developers. Anyone can practice and apply the recipe/tutorial presented here, but the reader is taking full responsibility for his/her actions. The author (content curator) of this recipe (code / program) has made every effort to ensure the accuracy of the information was correct at time of publication. The author (content curator) does not assume and hereby disclaims any liability to any party for any loss, damage, or disruption caused by errors or omissions, whether such errors or omissions result from accident, negligence, or any other cause. The information presented here could also be found in public knowledge domains.

Learn by Coding: v-Tutorials on Applied Machine Learning and Data Science for Beginners

Please do not waste your valuable time by watching videos, rather use end-to-end (Python and R) recipes from Professional Data Scientists to practice coding, and land the most demandable jobs in the fields of Predictive analytics & AI (Machine Learning and Data Science).

The objective is to guide the developers & analysts to “Learn how to Code” for Applied AI using end-to-end coding solutions, and unlock the world of opportunities!