In SQL, what is the main difference between a JOIN and a UNION when combining data from multiple tables or queries?

Introduction / Context:
Structured Query Language offers several ways to combine data from multiple tables or queries. Two of the most commonly confused constructs are JOIN and UNION. Although both involve multiple sources, they serve very different purposes. Correctly distinguishing between them is essential for writing clear queries and explaining query behaviour in interviews. This question asks you to identify the main difference between a JOIN and a UNION when combining data.

Given Data / Assumptions:

• We have two or more tables or queries that we wish to combine.
• JOINs operate within a single SELECT statement, relating tables by specified conditions.
• UNION operates between the result sets of separate SELECT statements.
• The question is conceptual and does not focus on all variations such as UNION ALL or different join types.

Concept / Approach:
A JOIN combines columns from two or more tables into single rows based on a join condition, such as matching keys. The result has more columns per row and a number of rows determined by matches between tables. For example, joining Customers and Orders on CustomerId produces one row per matching customer order pair with columns from both tables. A UNION takes the result sets of two separate queries that have the same number and type of columns and stacks them on top of each other, effectively combining rows vertically. UNION by default removes duplicate rows, while UNION ALL keeps them. Thus, JOINs extend rows horizontally by adding columns, and UNIONs extend result sets vertically by adding rows.

Step-by-Step Solution:
Step 1: Recall that JOIN clauses appear in the FROM or JOIN portion of a query to link tables based on a condition. Step 2: Remember that the output of a JOIN has columns from all joined tables and that rows represent combinations of matching records. Step 3: Recognise that UNION appears between two complete SELECT statements and requires that each SELECT return the same number of columns with compatible data types. Step 4: Understand that UNION combines the results by stacking them vertically, optionally removing duplicates. Step 5: Evaluate option a, which clearly contrasts JOIN as a column combining operation and UNION as a row stacking operation.

Verification / Alternative check:
As a concrete example, SELECT Name FROM Customers UNION SELECT Name FROM Suppliers returns a single column with names from both tables. This is a UNION that stacks rows. In contrast, SELECT Customers.Name, Orders.OrderId FROM Customers INNER JOIN Orders ON Customers.Id = Orders.CustomerId produces rows with columns from both Customers and Orders, illustrating a JOIN. These simple examples confirm the conceptual distinction described in option a.

Why Other Options Are Wrong:
Option b incorrectly associates JOIN with delete operations and UNION with updates, which is not how Structured Query Language works; both constructs are used primarily in SELECT queries. Option c claims that JOIN always sorts data and UNION can never be sorted, but ordering is controlled by ORDER BY and is independent of whether JOIN or UNION is used. Option d states that JOIN and UNION are interchangeable, which is false because they have different syntax, semantics, and use cases.

Common Pitfalls:
A common pitfall is trying to use UNION when a JOIN is needed and vice versa, leading to incorrect or inefficient queries. Another mistake is forgetting that UNION requires compatible column structures, while JOIN can combine tables with different columns as long as a join condition exists. For exam purposes, remember the short summary: JOIN adds columns side by side, while UNION adds rows on top of each other, as explained in option a.

Final Answer:
The main difference is that a JOIN combines columns from two or more tables based on a related column, producing wider rows, while a UNION combines the result sets of two queries with the same structure by stacking rows vertically.