SQLite Join

Summary: in this tutorial, you will learn about various kinds of SQLite joins to query data from two or more tables.

For the demonstration, we will use the artists and albums tables from the sample database.

An artist can have zero or many albums while an album belongs to one artist.

To query data from both artists and albums tables, you use can use an INNER JOIN, LEFT JOIN, or CROSS JOIN clause. Each join clause determines how SQLite uses data from one table to match with rows in another table.

Note that SQLite doesn’t directly support the RIGHT JOIN and FULL OUTER JOIN.

SQLite INNER JOIN

The following statement returns the album titles and their artist names:

SELECT 
    Title,
    Name
FROM 
    albums
INNER JOIN artists 
    ON artists.ArtistId = albums.ArtistId;

Here is the partial output:

In this example, the INNER JOIN clause matches each row from the albums table with every row from the artists table based on the join condition (artists.ArtistId = albums.ArtistId) specified after the ON keyword.

If the join condition evaluates to true (or 1), the columns of rows from both albums and artists tables are included in the result set.

This query uses table aliases (l for the albums table and r for artists table) to shorten the query:

SELECT
    l.Title, 
    r.Name
FROM
    albums l
INNER JOIN artists r ON
    r.ArtistId = l.ArtistId;

In case the column names of joined tables are the same e.g., ArtistId, you can use the USING syntax as follows:

SELECT
   Title, 
   Name
FROM
   albums
INNER JOIN artists USING(ArtistId);

The clause USING(ArtistId) is equipvalent to the clause ON artists.ArtistId = albums.ArtistId.

SQLite LEFT JOIN

This statement selects the artist names and album titles from the artists and albums tables using the LEFT JOIN clause:

SELECT
    Name, 
    Title
FROM
    artists
LEFT JOIN albums ON
    artists.ArtistId = albums.ArtistId
ORDER BY Name;

Here is the output:

The LEFT JOIN clause selects data starting from the left table (artists) and matching rows in the right table (albums) based on the join condition (artists.ArtistId = albums.ArtistId) .

The left join returns all rows from the artists table (or left table) and the matching rows from the albums table (or right table).

If a row from the left table doesn’t have a matching row in the right table, SQLite includes columns of the rows in the left table and NULL for the columns of the right table.

Similar to the INNER JOIN clause, you can use the USING syntax for the join condition as follows:

SELECT
   Name, 
   Title
FROM
   artists
LEFT JOIN albums USING (ArtistId)
ORDER BY
   Name;

If you want to find artists who don’t have any albums, you can add a WHERE clause as shown in the following query:

SELECT
    Name,
    Title
FROM
    artists
LEFT JOIN albums ON
    artists.ArtistId = albums.ArtistId
WHERE Title IS NULL   
ORDER BY Name;

This picture shows the partial output:

Generally, this type of query allows you to find rows that are available in the left table but don’t have corresponding rows in the right table.

Note that LEFT JOIN and LEFT OUTER JOIN are synonyms.

SQLite CROSS JOIN

The CROSS JOIN clause creates a Cartesian product of rows from the joined tables.

Unlike the INNER JOIN and LEFT JOIN clauses, a CROSS JOIN doesn’t have a join condition. Here is the basic syntax of the CROSS JOIN clause:

SELECT
    select_list
FROM table1
CROSS JOIN table2;

The CROSS JOIN combines every row from the first table (table1) with every row from the second table (table2) to form the result set.

If the first table has N rows, the second table has M rows, the final result will have NxM rows.

A practical example of the CROSS JOIN clause is to combine two sets of data for forming an initial data set for further processing. For example, you have a list of products and months, and you want to make a plan when you can sell which products.

The following script creates the products and calendars tables:

CREATE TABLE products(
    product text NOT null
);

INSERT INTO products(product)
VALUES('P1'),('P2'),('P3');



CREATE TABLE calendars(
    y int NOT NULL,
    m int NOT NULL
);

INSERT INTO calendars(y,m)
VALUES 
    (2019,1),
    (2019,2),
    (2019,3),
    (2019,4),
    (2019,5),
    (2019,6),
    (2019,7),
    (2019,8),
    (2019,9),
    (2019,10),
    (2019,11),
    (2019,12);

This query uses the CROSS JOIN clause to combine the products with the months:

SELECT * 
FROM products
CROSS JOIN calendars;

Here is the output:

In this tutorial, you have learned various kind of SQLite joins that allow you to query from multiple tables.