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PostgreSQL Joins Explained with Practical Examples

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Dinesh Nair
·

4 min read

This guide explores the world of PostgreSQL joins, covering inner, left, right, and full outer joins through practical examples. We'll start by setting up sample tables in the Windows environment and inserting data into them, and then we'll walk through each type of join with an example and explanation.

Setting Up Tables with Data

When you start psql and connect to your PostgreSQL server, you're generally connecting to an existing database, usually the postgres database with the postgres username by default. To work through the examples, we’ll create a separate database.

Step 1: Create the database:

CREATE DATABASE dinesh_sql_guide;

Step 2: Connect to the new database:

\c dinesh_sql_guide

Here’s my psql window where I am creating and connecting to the new database:

Step 3: Create the tables to store skills and training programs.

CREATE TABLE Skills (
    SkillID SERIAL PRIMARY KEY,
    SkillName VARCHAR(255) NOT NULL
);

CREATE TABLE TrainingPrograms (
    ProgramID SERIAL PRIMARY KEY,
    SkillID INT,
    ProgramName VARCHAR(255) NOT NULL,
    FOREIGN KEY (SkillID) REFERENCES Skills(SkillID)
);

Step 4: Add the data to the tables:

-- Inserting Skills into the Skills table
INSERT INTO Skills (SkillName) VALUES 
    ('Business'),
    ('Personal Development'),
    ('Web Development');

-- Inserting Training Programs into the TrainingPrograms table
INSERT INTO TrainingPrograms (SkillID, ProgramName) VALUES 
    (1, 'Basics of Entrepreneurship'),
    (1, 'Business Analysis'),
    (2, 'Problem Solving'),
    (NULL, 'General Skills Workshop'); -- This entry has no matching SkillID

Inner Join

An inner join is an SQL operation that combines rows from two or more tables based on a related column. The operation will return only the rows that have matching related column values in both tables.

Here’s an inner join:

SELECT Skills.SkillName, TrainingPrograms.ProgramName
FROM Skills
INNER JOIN TrainingPrograms ON Skills.SkillID = TrainingPrograms.SkillID;

This query will return skill names and corresponding training programs. Any training programs that don’t have a SkillID will be filtered out. The programs Basics of Entrepreneurship, Business Analysis, and Problem Solving will be returned because they all have SkillID values that match existing skills in the Skills table. The General Skills Workshop program is filtered out because it has a NULL SkillID, which doesn't match any SkillID in the Skills table.

Left Join

A left join returns all rows from the left table and the matched rows from the right table. When there's no match, the result is NULL on the side of the right table.

Here’s a left join:

SELECT Skills.SkillName, TrainingPrograms.ProgramName
FROM Skills
LEFT JOIN TrainingPrograms ON Skills.SkillID = TrainingPrograms.SkillID;

This query will return skill names and corresponding training programs. All skill names from the left table (Skills) are returned, including those that don’t have corresponding entries in the TrainingPrograms table.

For example, the Web Development skill will be included in the results with a NULL value for ProgramName, indicating that there are no training programs associated with that skill.

Right Join

A right join returns all rows from the right table and the matching rows from the left table. If there’s no match, you’ll see NULLs for columns from the left table.

Here’s a right join:

SELECT Skills.SkillName, TrainingPrograms.ProgramName
FROM Skills
RIGHT JOIN TrainingPrograms ON Skills.SkillID = TrainingPrograms.SkillID;

This query will return skill names and corresponding training programs. All training programs from the right table (TrainingPrograms) are returned, including those that don’t have corresponding entries in the Skills table.

For example, the General Skills Workshop program will be included in the results with a NULL value for SkillName, indicating that there are no skills associated with that program.

Full Outer Join

A full outer join returns all rows from both tables, inserting NULLs in places where there are no matches on either side. This way, no data from either table is excluded from the result.

Here’s a full outer join:

SELECT Skills.SkillName, TrainingPrograms.ProgramName
FROM Skills
FULL OUTER JOIN TrainingPrograms ON Skills.SkillID = TrainingPrograms.SkillID;

The query will return every skill name and training program from both the Skills and TrainingPrograms tables, with NULLs where there are no matching entries.

Congratulations! We’ve covered how SQL joins, such as left join and full outer join, can be used for querying and integrating data. With this understanding, you'll find it easier to handle basic data tasks.

PostgreSQL