MySQL LEFT JOIN

In databases, data is often stored in multiple tables, making it necessary to combine them to fetch required information. MySQL JOIN statements enable merging tables based on common columns.

In this article, we'll explore the MySQL LEFT JOIN keyword, a type of outer join that returns all records from the left table and matched records from the right table. We'll cover its syntax, examples, and use cases to understand its functionality better.

MySQL LEFT JOIN

MySQL LEFT JOIN is a type of outer join that returns all records from the left table and matches records from the right table. If there is no match, the result is NULL from the right table. This join is also known as Left Outer Join.

Syntax:

SELECT column1, column2, ...
FROM table1
LEFT JOIN table2 
ON table1.column_name = table2.column_name;

LEFT JOIN Examples

To understand the LEFT JOIN in a more depth manner, we need a table on which we will perform various operations. So here we take 3 tables called books, authors, and book_authors.

Query to create authors Table:

CREATE TABLE authors (
    author_id INT PRIMARY KEY,
    author_name VARCHAR(255)
);

Query to create books Table:

CREATE TABLE books (
    book_id INT PRIMARY KEY,
    title VARCHAR(255),
    publication_year INT
);

Query to create book_authors Table:

CREATE TABLE book_authors (
    book_id INT,
    author_id INT,
    FOREIGN KEY (book_id) REFERENCES books(book_id),
    FOREIGN KEY (author_id) REFERENCES authors(author_id)
);

Here, book_id and author_id which are the PRIMARY KEY of books and authors table acts as a FOREIGN KEY for book_authors table.

After inserting some data into tables, tables look like:

Table: books

Table: authors

Table: book authors

Example 1: MySQL LEFT JOIN with USING Clause

We can use the USING clause, to specify the column/field on which the JOIN operation should be performed.

Query:

SELECT *
FROM books
LEFT JOIN book_authors USING (book_id);

Output:

Explanation: The following query gets all the records from the left table books and the matching records from the right table book_authors. The query uses the USING Clause which performs the left join based on a column that is common to both the tables and has matching records.

Example 2: MySQL LEFT JOIN with GROUP BY Clause

We can use the GROUP BY clause with the LEFT JOIN keyword to view organized results based on specific columns.

Query:

SELECT authors.author_name, COUNT(books.book_id) as book_count
FROM authors
LEFT JOIN book_authors ON authors.author_id = book_authors.author_id
LEFT JOIN books ON book_authors.book_id = books.book_id
GROUP BY authors.author_name;

Output:

Explanation: The query uses LEFT JOIN to combine the authors, book_authors, and books tables. It groups results by author name using the GROUP BY clause, and the COUNT function calculates the number of books each author has written, providing the frequency of books per author.

Example 3: LEFT JOIN with WHERE Clause

We can use the WHERE clause with LEFT JOIN to view the records that fulfill a specific request. this allows the filtering of data from tables.

SELECT authors.author_name, books.title
FROM authors
LEFT JOIN book_authors ON authors.author_id = book_authors.author_id
LEFT JOIN books ON book_authors.book_id = books.book_id
WHERE books.publication_year < 2015;

Output:

Explanation: WHERE Clause is used in MySQL to filter the result set based on a specific condition. This query performs a LEFT JOIN between the table authors and books to return the name of the author and the book he/she has written but only those authors who have published a book before 2015.

Example 4: MySQL LEFT JOIN to Get Unmatched Records

Query:

SELECT authors.author_name
FROM authors
LEFT JOIN book_authors ON authors.author_id = book_authors.author_id
WHERE book_authors.author_id IS NULL;

Output:

Explanation: The query finds unmatched records between tables using a LEFT JOIN and a WHERE clause. For instance, it identifies customers who never placed an order. In this example, it finds authors with no books. No resultant rows indicate every author has published at least one book.

Difference Between WHERE & ON Clause in MySQL LEFT JOIN

Although we can achieve the same results using the WHERE and ON Clause in MySQL, but we should use them differently as they are designed for different purposes.

• The main purpose of the ON Clause is to specify the join conditions. The ON Clause defines how two or more tables should be joined. The ON Clause defines the common attributes between the two tables which must be matched to perform a join.
  
• On the other hand, the main purpose of the WHERE Clause is to filter the resultant table. It defines which rows must be included in the resultant set of the query. It is used to fetch only those records which fulfill the specified condition by the user.

Conclusion

In this article, we have learned about MySQL LEFT JOIN, its syntax, and various Clauses to apply with LEFT JOIN. You can use LEFT JOIN whenever the result of the first table is a topmost priority along with the matching records of both the tables. However, always use Clauses carefully to filter out the results when applying the LEFT JOIN between the tables. Always use the ON Clause to specify join conditions and the WHERE Clause to filter out the result.

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Article Tags :

• Geeks Premier League
• Databases
• MySQL
• Geeks Premier League 2023