MySQL 8: Sorting rows in table A by column in table B

Introduction

When working with relational databases, you often have the need to sort data in one table based on the values in another. MySQL, one of the most popular open-source relational database management systems, provides several ways to accomplish this task. In this tutorial, we will learn how to sort rows in one table (Table A), according to the values in a column of another table (Table B) using MySQL 8.

Understanding the Basic JOIN Operation

The JOIN operation in SQL is fundamental to combining rows from two or more tables based on a related column between them. Here’s the basic syntax for a simple JOIN:

SELECT
    A.*
FROM
    TableA A
JOIN
    TableB B ON A.SomeKey = B.SomeKey
ORDER BY
    B.SortColumn;

This query will retrieve all rows from TableA and sort them based on the SortColumn in TableB.

Example One

Imagine we have a table of products (products_table) and another table that holds the corresponding categories (categories_table). We want to sort the products table by the category name.

products_table:

product_id | product_name | category_id
-----------+--------------+-------------
         1 | Alpha        |           3
         2 | Beta         |           2
         3 | Gamma        |           1

categories_table:

category_id | category_name
------------+---------------
          1 | Electronics
          2 | Books
          3 | Clothing

We can sort the products by the category name with the following query:

SELECT
    p.*
FROM
    products_table p
JOIN
    categories_table c ON p.category_id = c.category_id
ORDER BY
    c.category_name;

The result will be the list of products, sorted alphabetically by the category_name field:

product_id | product_name | category_id
-----------+--------------+-------------
         3 | Gamma        |           1
         2 | Beta         |           2
         1 | Alpha        |           3

Using Subqueries for Sorting

A subquery can also be used to sort the data from one table based on the values from another table:

SELECT
    A.*
FROM
    TableA A
ORDER BY
    (SELECT B.SortColumn
     FROM TableB B
     WHERE A.SomeKey = B.SomeKey);

This will have a similar result as the JOIN operation but is often less efficient.

Example Two

Let’s look at sorting the previous products_table, but this time by the largest to smallest category_id in the categories_table:

SELECT
    p.*
FROM
    products_table p
ORDER BY
    (SELECT c.category_id
     FROM categories_table c
     WHERE p.category_id = c.category_id
     DESC);

This will display the products sorted from the highest category_id to the lowest.

It is important to note that when using subqueries for sorting, especially in large databases, performance may be impacted. Therefore, JOIN operations are generally preferred.

Advanced Sorting with Multiple Criteria

Sometimes it’s necessary to sort data by multiple columns distributed across different tables:

SELECT
    A.*, B.SortColumn, C.AnotherSortColumn
FROM
    TableA A
JOIN
    TableB B ON A.SomeKey = B.SomeKey
JOIN
    TableC C ON A.AnotherKey = C.AnotherKey
ORDER BY
    B.SortColumn,
    C.AnotherSortColumn;

This will first sort the rows in TableA by SortColumn in TableB and then by AnotherSortColumn in TableC.

Example Three

(Let’s add one more table to the previous example, named brands_table, for the purpose of this tutorial)

brands_table:

brand_id | brand_name
---------+------------
       1 | XYZ
       2 | ABC

Now we want to sort the rows in products_table first by category name alphabetically, and within each category, by brand name alphabetically.

SELECT
    p.*,
    c.category_name,
    b.brand_name
FROM
    products_table p
JOIN
    categories_table c ON p.category_id = c.category_id
JOIN
    brands_table b ON p.brand_id = b.brand_id
ORDER BY
    c.category_name,
    b.brand_name;

Example Four: Sorting with LEFT JOIN to include all rows

What if we want to sort products, but include all products even if they don’t currently have a category? We use a LEFT JOIN.

SELECT
    p.*,
    IFNULL(c.category_name, 'Uncategorized') AS category_name
FROM
    products_table p
LEFT JOIN
    categories_table c ON p.category_id = c.category_id
ORDER BY
    category_name;

This query will show all products and sort them by category_name. Products without a category will show ‘Uncategorized’ and be grouped together.

Handling Null Values

When sorting data that may contain NULL values, you can use the IS NULL or COALESCE functions to handle them.

Example Five

Let’s sort the products table by brand name, noting any NULL values:

SELECT
    p.*,
    coalesce(b.brand_name, 'No Brand') AS sorted_brand
FROM
    products_table p
LEFT JOIN
    brands_table b ON p.brand_id = b.brand_id
ORDER BY
    sorted_brand;

Challenges when Joining Tables for Sorting

Joining large tables or performing complex sorting can lead to performance issues. Here are some tips to optimize your queries:

• Use appropriate indexes on the tables.
• Keep the queries as simple as possible.
• Consider the use of temporary tables to reduce complexity.
• Filter your results as much as possible before sorting.

Conclusion

Sorting data in one table based on the column of another table is a common task in SQL, and with the techniques covered in this article, you should be well-equipped to handle it. Make sure to always consider the implications of complexity and volume on performance when executing such queries.