Difference Between JOIN and UNION in SQL

JOIN and UNION are two important operations with distinct purposes. While both are used to combine data from multiple tables, they serve different functions and are applied in different scenarios. Understanding the difference between JOIN and UNION is crucial for writing efficient and accurate database queries. Let's delve into the details of these operations to grasp their unique characteristics and applications.

Understanding the difference between JOIN and UNION in SQL is essential for anyone working with databases. This article breaks down the distinct purposes of these operations and provides clear examples to illustrate their usage.

Table of Content

• Difference Between JOIN and UNION in SQL: JOIN vs UNION
• What is JOIN in SQL?
• What is UNION in SQL?
• Key Difference Between JOIN and UNION in SQL

What is the Difference Between JOIN and UNION in SQL?

Parameter	JOIN	UNION
Purpose	Used to combine rows from multiple tables based on a related column(s) between them	Used to combine the result sets of two or more SELECT statements
Number of Input Tables	Requires at least two tables	Can combine result sets from any number of SELECT statements
Row Combining Method	Combines rows horizontally by matching rows based on the join condition	Combines rows vertically by appending rows from one result set to another
Duplicate Rows	Retains duplicate rows unless specified otherwise	Removes duplicate rows by default, unless UNION ALL is used
Column Structure	Columns in the result set are from the tables involved in the JOIN	Columns in the result set are from the first SELECT statement, and all other SELECT statements must have the same number of columns
Null Handling	Handles NULL values based on the join type (INNER, LEFT, RIGHT, FULL)	Preserves NULL values from the input result sets
Performance	Generally faster for combining related data from multiple tables	Generally slower for combining unrelated data from multiple tables
Data Integrity	Maintains referential integrity between related tables	Does not maintain referential integrity
Data Transformation	Allows data transformation during join operation	Data transformation must be done before combining result sets
Use Case	Commonly used for querying related data across multiple tables	Commonly used for combining similar data from different sources or queries

What is JOIN in SQL?

A JOIN in SQL is a way to combine rows from two or more tables based on a related column(s) between them. The resulting table contains data from both tables, with rows combined horizontally based on the join condition.

For example, consider two tables: Employees and Departments:

Employees
+----+----------+-------------+
| ID | Name     | DepartmentID|
+----+----------+-------------+
| 1  | John Doe | 1           |
| 2  | Jane Smith| 2          |
| 3  | Bob Johnson| 1         |
+----+----------+-------------+

Departments
+----+----------------+
| ID | Name           |
+----+----------------+
| 1  | IT             |
| 2  | Sales          |
| 3  | Marketing      |
+----+----------------+

To get the employee names along with their department names, we can use an INNER JOIN like this:

Output

+----------+------------+
| Name     | Department |
+----------+------------+
| John Doe | IT         |
| Jane Smith| Sales     |
| Bob Johnson| IT       |
+----------+------------+

Here, the INNER JOIN combines rows from the Employees and Departments tables where the DepartmentID in Employees matches the ID in Departments.

What is UNION in SQL?
The UNION operator in SQL combines the result sets of two or more SELECT statements. It vertically appends the rows from one result set to another and removes any duplicate rows by default.

For example, consider two tables: Customers and Suppliers:

Customers
+----+----------+
| ID | Name     |
+----+----------+
| 1  | John Doe |
| 2  | Jane Smith|
| 3  | Bob Johnson|
+----+----------+

Suppliers
+----+----------+
| ID | Name     |
+----+----------+
| 4  | Acme Corp|
| 5  | XYZ Inc. |
+----+----------+

To get a combined list of names from both tables, we can use the UNION operator like this:

Output

+----------+
| Name     |
+----------+
| John Doe |
| Jane Smith|
| Bob Johnson|
| Acme Corp|
| XYZ Inc. |
+----------+

Here, the UNION operator combines the result sets of the two SELECT statements, removing any duplicate rows.

If you want to include duplicate rows, you can use the UNION ALL operator instead:

Key Difference Between JOIN and UNION in SQL

1. Purpose: JOIN is used to combine rows from multiple tables based on a related column(s) between them, while UNION combines the result sets of two or more SELECT statements.
2. Data Combination Method: JOIN combines rows horizontally by matching rows based on a join condition, while UNION combines rows vertically by appending rows from one result set to another.
3. Duplicate Rows: JOIN retains duplicate rows unless specified otherwise, while UNION removes duplicate rows by default (unless you use UNION ALL).
4. Referential Integrity: JOIN maintains referential integrity between related tables, while UNION does not maintain referential integrity as it combines unrelated data.
5. Performance: JOIN is generally faster for combining related data from multiple tables, while UNION is generally slower for combining unrelated data from multiple tables, as it involves separate queries and then combining the result sets.

Conclusion

the difference between JOIN and UNION in SQL lies in their fundamental purposes and the way they merge data. JOIN is used to combine rows from two or more tables based on a related column, while UNION is used to combine the results of two or more SELECT statements into a single result set. By comprehending the disparities between these operations, SQL practitioners can leverage them effectively to retrieve and manipulate data according to their specific requirements.

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