Union vs. Inner Join in SQL: Combining Data the Right Way

In the world of SQL, where data manipulation reigns supreme, understanding how to combine information from different tables is crucial. Two prominent methods for achieving this are UNION and INNER JOIN. While they might seem similar at first glance, these operators serve distinct purposes with significant differences in their functionalities and outcomes.

UNION: Merging Result Sets

Imagine you have two separate lists: one containing book titles and another containing movie titles. UNION allows you to combine these lists into a single, unified list. Here's how it works:

1. Selects Data: You use separate SELECT statements to retrieve data from different tables or queries.
2. Combines Results: UNION merges the results of these SELECT statements into a single table.
3. Handles Duplicates (UNION ALL): By default, UNION removes duplicate rows from the combined result set. However, if you use UNION ALL, all rows, including duplicates, will be included.

Here's an example:

SQL

SELECT title FROM books;
UNION
SELECT title FROM movies;        

This query would combine book titles and movie titles into a single list, eliminating duplicates (using regular UNION).

INNER JOIN: Finding Relationships

Now, imagine you have a library database with tables for books and authors. An INNER JOIN helps you identify relationships between these tables. For instance, you might want to find all books written by a specific author. Here's the process:

1. Selects Data: Similar to UNION, you use SELECT statements to choose columns from participating tables.
2. Matches Rows: The core difference lies in the JOIN clause. An INNER JOIN only includes rows where there's a match between the specified columns in both tables.
3. Creates New Columns: The joined table typically combines columns from both tables, providing a more comprehensive view of the data.

Here's an example:

SQL

SELECT b.title, a.name
FROM books AS b
INNER JOIN authors AS a ON b.author_id = a.id;        

Use code with caution.

This query will return a list of book titles along with the corresponding author names, based on the matching author_id between the books and authors tables.

Key Differences in a Nutshell

Choosing the Right Tool

Selecting between UNION and INNER JOIN depends on your specific data manipulation goals:

• Use UNION when you want to combine separate result sets, potentially from different queries, into a single list. This is useful for creating merged reports or views.
• Use INNER JOIN when you need to identify relationships between data points in different tables based on a matching condition. It's perfect for tasks like finding customers with specific orders or products associated with specific categories.

By understanding the distinct functionalities of UNION and INNER JOIN, you'll be well-equipped to tackle data manipulation tasks in SQL effectively, extracting the insights you need from your databases.