UNION AND UNION ALL

The UNION vs UNION ALL in MS SQL Server: A Deep Dive

When working with SQL Server, SQL statements like UNION and UNION ALL are essential for combining the result sets of two or more SELECT statements into a single result set. While they might seem similar at first glance, they serve different purposes and have distinct behaviors. Here’s a detailed look at their differences:

1. Duplicate Rows

• UNION: This operation removes duplicates from the result set. When you perform a UNION, SQL Server compares each row from both SELECT statements for uniqueness. If a row appears in both results, it will only appear once in the final result set. This operation can be more resource-intensive, especially with large datasets, because it involves sorting and comparing each row.
  
  SELECT column1 FROM TableA
UNION
SELECT column1 FROM TableB;
  
• UNION ALL: In contrast, UNION ALL includes all rows from both SELECT statements, including duplicates. This operation does not perform any duplicate removal, making it generally faster than UNION since it simply concatenates the results.
  
  SELECT column1 FROM TableA
UNION ALL
SELECT column1 FROM TableB;

2. Performance Considerations

• UNION:
  • Slower: Due to the overhead of sorting and eliminating duplicate rows, UNION can be slower, especially when dealing with large datasets or when the uniqueness of rows is not guaranteed.
  • Memory Usage: Might require more memory because of the sorting operation.
    
• UNION ALL:
  • Faster: Since there’s no need to check for duplicates, UNION ALL generally performs better.
  • Less Resource Intensive: It doesn’t require additional memory for sorting.

3. Use Cases

• Use UNION when:
  • You need to combine result sets but want to ensure all rows are unique. For example, if you’re pulling data from multiple tables where there might be overlapping data, and you want a distinct list.
• Use UNION ALL when:
  • You are certain that there will be no duplicates or when duplicates are acceptable or even desired. This is common in scenarios like log data analysis where each entry is unique or when you’re combining results from different but non-overlapping queries.

4. Syntax and Compatibility

• Both UNION and UNION ALL follow the same syntax:
  
  SELECT column1, column2 FROM TableA
UNION/UNION ALL
SELECT column1, column2 FROM TableB;
  
• They must adhere to:
  • The number of columns in each SELECT statement must match.
  • The data types of the columns at each position must be compatible.

5. Sorting Results

If you need to sort the result of a UNION or UNION ALL, you apply the ORDER BY clause outside of the individual SELECT statements:

SELECT column1 FROM TableA
UNION ALL
SELECT column1 FROM TableB
ORDER BY column1;

Choosing between UNION and UNION ALL depends largely on your specific needs regarding performance and data uniqueness:

• UNION for when you need distinct results without duplicates, accepting potential performance costs.
• UNION ALL for when performance is critical, and you either know there are no duplicates or the duplicates are part of your result requirement.

Understanding these differences allows for more efficient SQL query design in Microsoft SQL Server, optimizing both for the result set you need and the resources your queries consume. Remember, always consider the context of your data and what you aim to achieve with your query when deciding which operation to use.