UNION and UNION ALL

The UNION and UNION ALL operators in SQL are used to combine the results of two or more SELECT statements. However, they have some key differences:

UNION

1. Removes Duplicates: The UNION operator removes duplicate rows from the result set.

2. Syntax:

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

3. Performance: Since UNION removes duplicates, it requires additional processing, which can make it slower than UNION ALL.

Example:

SELECT FirstName, LastName FROM Employees
UNION
SELECT FirstName, LastName FROM Managers

Explanation: This query combines the results from the Employees and Managers tables, removing any duplicate rows.

UNION ALL

1. Includes Duplicates: The UNION ALL operator includes all rows from the result sets, including duplicates.

2. Syntax:

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

3. Performance: UNION ALL is generally faster than UNION because it does not remove duplicates.

Example:

SELECT FirstName, LastName FROM Employees
UNION ALL
SELECT FirstName, LastName FROM Managers;

Explanation: This query combines the results from the Employees and Managers tables, including all duplicate rows.

Key Differences

• Duplicates: UNION removes duplicates, while UNION ALL includes all duplicates.

• Performance: UNION ALL is faster because it does not perform the duplicate removal step.

• Use Case: Use UNION when you need a distinct set of results. Use UNION ALL when you need all results, including duplicates, and performance is a concern.

Visual Representation

• UNION:

  Result Set 1: A, B, C
  Result Set 2: B, C, D
  UNION Result: A, B, C, D

• UNION ALL:

  Result Set 1: A, B, C
  Result Set 2: B, C, D
  UNION ALL Result: A, B, C, B, C, D

Choosing between UNION and UNION ALL depends on your specific requirements and the nature of your data. Here are some guidelines to help you decide when to use UNION over UNION ALL:

When to Use UNION

1. Need for Unique Results:

   • Use UNION when you need to ensure that the result set contains only unique rows. This is particularly important when duplicates would cause issues in your application or analysis.

   SELECT FirstName, LastName FROM Employees
   UNION
   SELECT FirstName, LastName FROM Managers;

Example: Combining employee and manager names where you want each name to appear only once.

2. Data Integrity:

   • Use UNION when data integrity is crucial, and you need to eliminate any potential duplicates that might arise from combining multiple result sets.

   SELECT ProductID FROM Orders
   UNION
   SELECT ProductID FROM Returns;

Example: Ensuring a list of unique product IDs from orders and returns.

3. Small Data Sets:

   • Use UNION when working with small data sets where the performance overhead of removing duplicates is negligible.

   SELECT City FROM Customers
   UNION
   SELECT City FROM Suppliers;

Example: Combining city names from customers and suppliers where the data set is small.

When to Use UNION ALL

1. Performance Considerations:

   • Use UNION ALL when performance is a concern, especially with large data sets. UNION ALL is faster because it does not perform the duplicate removal step.

   SELECT FirstName, LastName FROM Employees
   UNION ALL
   SELECT FirstName, LastName FROM Managers;

Example: Combining employee and manager names where performance is critical, and duplicates are acceptable.

2. Preserving Duplicates:

   • Use UNION ALL when you need to preserve duplicates in the result set. This is useful when duplicates have meaning in your analysis or reporting.

   SELECT ProductID FROM Orders
   UNION ALL
   SELECT ProductID FROM Returns;

Example: Combining product IDs from orders and returns where you need to see all occurrences, including duplicates.

3. Aggregating Data:

   • Use UNION ALL when you are aggregating data and need to include all rows in the aggregation.

   SELECT COUNT(*) AS TotalOrders FROM Orders
   UNION ALL
   SELECT COUNT(*) AS TotalReturns FROM Returns;

Example: Counting total orders and returns separately and combining the results.

Summary

• Use UNION: When you need unique results and data integrity is important.

• Use UNION ALL: When performance is critical, and you need to preserve duplicates.

When working with UNION and UNION ALL in SQL, handling NULL values is an important consideration. Here’s how they work with NULL values:

UNION with NULL Values

• Duplicates Removal: UNION treats NULL values as equal. If there are duplicate NULL values in the result sets, UNION will remove the duplicates and only include one NULL value in the final result.

• Example:

  SELECT FirstName, LastName FROM Employees
  UNION
  SELECT FirstName, LastName FROM Managers;

If both Employees and Managers tables have NULL values in the LastName column, the final result will include only one NULL for LastName.

UNION ALL with NULL Values

• Includes All Duplicates: UNION ALL includes all NULL values from the result sets. It does not remove duplicates, so if there are multiple NULL values, all of them will be included in the final result.

• Example:

  SELECT FirstName, LastName FROM Employees
  UNION ALL
  SELECT FirstName, LastName FROM Managers;

If both Employees and Managers tables have NULL values in the LastName column, the final result will include all NULL values from both tables.

Example with Data

Consider the following example with sample data:

Employees Table:

Managers Table:

Using UNION:

SELECT FirstName, LastName FROM Employees
UNION
SELECT FirstName, LastName FROM Managers;

Result:

Using UNION ALL:

SELECT FirstName, LastName FROM Employees
UNION ALL
SELECT FirstName, LastName FROM Managers;

Result:

Key Points

• UNION: Removes duplicate NULL values, treating them as equal.

• UNION ALL: Includes all NULL values, preserving duplicates.