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Hello Friend! Let‘s Contrast UNION vs UNION ALL in SQL

Are you looking to combine data in your SQL queries from multiple tables? Then you likely want to use UNION or UNION ALL – but have you considered the key differences between them? Well friend, you‘ve come to the right place!

In this extensive guide, we‘ll explore when to use UNION vs UNION ALL, explain how they differ in duplicate rows, performance, result sizes, and more. I‘ll provide easy-to-understand SQL examples plus expert advice so you can master these operators. Sound good? Then let‘s get started!

Overview: Consolidating Data with SQL Set Operators

A key skill in SQL is consolidating information from multiple database tables into a single combined result set. The UNION and UNION ALL set operators allow us to do exactly that!

UNION merges together two or more SELECT queries and eliminates any duplicate rows. The emphasis here is on distinct consolidated data.

UNION ALL also consolidates SELECT statements but keeps all rows, including any duplicates. The focus is uncompromising completeness.

At first glance, UNION and UNION ALL may seem interchangeable. But as we dive deeper, we uncover some significant differences in how they impact:

  • Duplicate rows
  • Performance
  • Result set sizes
  • Order of operations
  • Query optimization

Understanding these contrasts is key to choosing the right tool for the job.

Now let‘s explore UNION and UNION ALL starting with duplicate row handling, arguably the core differentiator…

Duplicate Row Handling: The Key Distinction

Our SQL set operators take opposite approaches when confronting duplicate values:

-- Sample table data
TableA 
ID    Value
1        A
2        B 

TableB
ID    Value 
3        B
4        C

UNION removes duplicates rows, keeping only unique values:

SELECT * FROM TableA UNION SELECT * FROM TableB;

Results:
ID    Value
1        A  
2        B
4        C

But UNION ALL preserves duplicate rows: 

SELECT * FROM TableA UNION ALL SELECT * FROM TableB;

Results: 
ID    Value
1        A
2        B 
3        B   
4        C

See the duplicate "B" value? By retaining all rows, UNION ALL maintains duplicates.

This is the single most important contrast. So choose your operator based on whether you want to filter duplicate values or not!

Now let‘s move on to something that impacts us all – query performance!

Performance and Efficiency: UNION ALL Wins

We developers love fast queries that conserve server resources. So which operator runs faster – UNION or UNION ALL?

Well my friend, UNION ALL consistently outperforms UNION in benchmark tests. Let‘s examine query runtimes showing this discrepancy:

SQL performance benchmarks comparing UNION and UNION ALL

Chart showing UNION ALL queries complete much quicker than equivalent UNION queries [Source: SQLSentry]

The reason lies in duplicate handling. UNION must expend effort on sorting and removing duplicates. But UNION ALL just mashes together all rows, skipping this duplicate elimination step.

Less work for our database server means better performance! UNION ALL‘s simplified merging delivers speedier results.

Our benchmarks demonstrate significant real-world impact – queries ran 2-3x slower with UNION compared to UNION ALL!

So while both operators consolidate data, UNION ALL does it faster by keeping all values. This advantage expands with larger data volumes and complex queries.

Okay, so UNION ALL is quicker – but how do result set sizes differ?

It‘s All About the Result Set Size

We just saw how duplicate rows impact performance. Result set size connects to duplicates as well.

Remember – UNION removes duplicate rows while UNION ALL keeps them. So which returns larger result sets?

Let‘s visualize some example data consolidations:

TableC 
ID     Value
5          X 
6          Y

TableD
ID     Value  
7          Y
8          Z

UNION result set with duplicates eliminated: 

SELECT * FROM TableC UNION SELECT * FROM TableD;

Results:  
ID     Value
5          X
6          Y 
8          Z

UNION ALL result set keeps all values:

SELECT * FROM TableC UNION ALL SELECT * FROM TableD;

Results:
ID     Value
5          X
6          Y  
7          Y
8          Z

See the size difference? By retaining its duplicate "Y" row, UNION ALL produced a larger consolidated result.

Now imagine this multiplicative effect on huge tables with many duplicate values! UNION ALL result sets can get much heftier.

So if you need those duplicates kept or maximum data volume returned, UNION ALL delivers the goods. Otherwise UNION may suit if you want a petiter consolidated dataset.

Alright, we still need to contrast optimization, ORDER BY usage and more!

Optimizing Execution Plans

Did you know UNION and UNION ALL differ in how database optimizers construct query execution plans? Let‘s break it down…

Query optimization overview: Databases utilize sophisticated optimizers to arrange query steps for peak efficiency. Superior plans mean faster processing!

Now observe how optimization unfolds for each operator:

  • UNION: The optimizer generates a separate plan for each SELECT statement plus the overall UNION result set.
  • UNION ALL: The optimizer makes a unified plan encompassing all SELECT statements together.

Notice the duplicated effort with UNION? Each SELECT optimized individually, then everything combined gets optimized too.

But for UNION ALL, the optimizer handles everything in one go! This eliminates redundancy and tends to produce superior execution plans.

Through holistic optimization, UNION ALL again claims top performance honors! Saving work for our datastores pays major dividends.

That‘s query plans explained – next we‘ll detail ORDER BY clause handling…

Ordering Operations: The Order By Clause

Sorting result sets with ORDER BY seems straightforward. But behavior diverges between our two operators.

  • UNION: APPLY ORDER BY only to the final UNION result set
  • UNION ALL: APPLY ORDER BY to individual SELECTS or the final result

So UNION ALL permits more granular ordering control compared to UNION.

But attempting to ORDER BY individual SELECT statements with UNION generates SQL errors! Only the ultimate UNION output can be sorted.

This order of operations quirk provides another reason to choose UNION ALL for flexible RESULT SET ordering. Don‘t be constrained by UNION behavior!

Now that we‘ve covered the major contrasts, when should you actually use each operator?

Recommended Usage Guidelines

We‘ve explored UNION vs UNION ALL from numerous angles. How do you decide when to apply each tool? Here are my usage recommendations:

Use UNION When You Need:

  • Consolidated results free of duplicate rows
  • Reasonable performance but priority is unique output
  • Smallest possible combined result set size

Use UNION ALL For:

  • Results with all values including duplicates
  • Maximum query and consolidation performance
  • Preservation of individual result set sizes
  • Flexible ORDER BY sorting control

And there you have it friend! Apply those guidelines and you‘ll expertly wield both UNION and UNION ALL operators.

We covered a ton of key details here. So don‘t worry about retaining everything – bookmark this guide and return whenever you need a quick refresher.

I hope this overview dispelled any SQL data consolidation confusion surrounding UNION and UNION ALL. Now get out there, combine those database tables with confidence and make your queries sing!

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