Differences Between UNION and UNION ALL

Feature	UNION	UNION ALL
Purpose	Combines the result sets from multiple tables and returns distinct records into a single result set.	Combines the result sets from multiple tables and returns all records into a single result set, including duplicates.
Syntax	SELECT column_list FROM table1 UNION SELECT column_list FROM table2;	SELECT column_list FROM table1 UNION ALL SELECT column_list FROM table2;
Duplicate Handling	Automatically eliminates duplicate rows.	Does not eliminate duplicate rows.
Performance	Slower performance because it requires removal of duplicates.	Faster performance because it does not check for duplicates.
Use Case	Useful when you want unique results and need to remove duplicates.	Useful when you want to retain all rows, including duplicates.
User Preference	Most database users prefer using UNION for clean, distinct results.	UNION ALL is less preferred in many cases due to possible unnecessary duplicates.
Efficiency	Less efficient due to the additional step of eliminating duplicates.	More efficient because no additional operation is performed to eliminate duplicates.
Memory Usage	May use more memory due to duplicate removal process.	Generally uses less memory because no duplicates are checked.

Key Differences:

1. Duplicate Handling:
• UNION: Automatically removes duplicate rows from the combined result set. This is useful when you want a distinct list of results from multiple queries.
• UNION ALL: Includes all rows, even if some of them are duplicates. This operator does not filter out duplicates, making it ideal when you want to retain every row from all queries.
2. Performance:
• UNION: Because it needs to compare and remove duplicates, the performance of UNION is generally slower than UNION ALL. It has to go through the additional step of eliminating duplicates, which can be resource-intensive, especially with large datasets.
• UNION ALL: Since it does not perform any duplicate removal, it is generally faster. It simply combines the results from the multiple queries without any further processing.
3. Use Cases:
• UNION: Best used when you need to ensure the results are distinct. For example, when combining results from multiple tables that may have overlapping data, and you want to avoid repetition.
• UNION ALL: Useful when you need to retain all the rows from the individual queries. For instance, when summing or counting data from multiple sources, where duplicates are meaningful, and you don’t need to filter them out.
4. Examples:
• Using UNION:

SELECT first_name, last_name FROM employees

UNION

SELECT first_name, last_name FROM contractors;

• This query will return only distinct first_name and last_name values from both the employees and contractors tables, removing any duplicates.
• Using UNION ALL:

SELECT first_name, last_name FROM employees

UNION ALL

SELECT first_name, last_name FROM contractors;

• This query will return all rows from both the employees and contractors tables, including duplicates.