What is the difference between UNION and UNION ALL?

Understanding the Difference Between UNION and UNION ALL in SQL

When working with SQL, particularly when dealing with multiple result sets, you might come across two powerful operators: UNION and UNION ALL. Both are used to combine the results of two or more SELECT queries into a single result set, but they behave differently in terms of how they handle duplicate records. Understanding these differences is crucial for writing efficient and accurate SQL queries.

What is UNION?

The UNION operator combines the results of two or more SELECT statements and removes duplicate records from the final result set. This means that if any rows in the result sets being combined are identical, only one instance of that row will be included in the final output.

Syntax:

SELECT column1, column2, ...
FROM table1
UNION
SELECT column1, column2, ...
FROM table2;

Example:

Consider two tables, employees and contractors, both with a column named name. Here’s how you would use UNION to combine their names:

SELECT name FROM employees
UNION
SELECT name FROM contractors;

In this example, if both tables contain the name “Alice,” the result will only show “Alice” once.

Key Characteristics of UNION:

1. Duplicate Removal: UNION automatically eliminates duplicate rows from the final result set.
2. Performance Overhead: Because UNION removes duplicates, it may incur additional processing time, especially with large datasets. The database engine must check for duplicates across all selected records, which can slow down query performance.

What is UNION ALL?

The UNION ALL operator also combines the results of two or more SELECT statements, but it includes all records, regardless of whether they are duplicates. If there are identical rows in the result sets, UNION ALL will include all occurrences.

Syntax:

SELECT column1, column2, ...
FROM table1
UNION ALL
SELECT column1, column2, ...
FROM table2;

Example:

Using the same employees and contractors tables, here’s how you would use UNION ALL to get a combined list of names:

SELECT name FROM employees
UNION ALL
SELECT name FROM contractors;

In this case, if “Alice” appears in both tables, the result will show “Alice” twice, once for each table.

Key Characteristics of UNION ALL:

1. Includes Duplicates: UNION ALL retains all duplicate rows in the final result set.
2. Performance Advantage: Since UNION ALL does not require the database engine to remove duplicates, it typically performs better than UNION, especially with large datasets.

When to Use Each

• Use UNION when you want to ensure that your result set contains unique records and duplicates are not relevant to your analysis.
• Use UNION ALL when duplicates are acceptable or when you need to count occurrences of records. It’s often the preferred option for performance reasons, especially in large datasets where filtering duplicates is unnecessary.

Practical Example

Let’s consider a more practical scenario with a sample dataset:

• Table: Employees ID Name 1 Alice 2 Bob 3 Charlie
• Table: Contractors
  ID Name
  1 Alice
  2 David
  3 Bob Using UNION: SELECT Name FROM Employees UNION SELECT Name FROM Contractors; Result: Name Alice Bob Charlie David Using UNION ALL: SELECT Name FROM Employees UNION ALL SELECT Name FROM Contractors; Result: Name Alice Bob Charlie Alice David Bob As illustrated, the results from UNION yield a unique list of names, while UNION ALL shows every instance of each name. Conclusion The choice between UNION and UNION ALL should be guided by your specific requirements regarding duplicates and performance. Understanding these differences will allow you to craft more efficient and appropriate SQL queries based on your data needs. By choosing the right operator, you can optimize your SQL performance while ensuring the accuracy of your results.