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Hello friend! Mastering SQL UNION vs UNION ALL

Understanding the subtle differences between SQL‘s UNION and UNION ALL operators is key for any database developer. If you‘ve ever struggled to recall when to use which or ran into performance issues, then this guide is for you!

I‘m going to walk you through these invaluable clauses step-by-step. We‘ll cover:

  • Real-world use cases so you know exactly when to use UNION or UNION ALL
  • Performance implications and benchmarks
  • Easy to scan comparison tables
  • Troubleshooting tips to avoid common errors
  • And advanced best practices for using UNION/UNION ALL

My goal is to make you an expert on the nuances of these operators so you can write optimized SQL queries. Let‘s get started!

Overview: An Essential SQL Skill

As a database developer, being able to accurately combine data sets with SQL is crucial. The UNION and UNION ALL set operators allow you to efficiently merge rows from multiple SELECT statements into a single result set.

But it‘s not always obvious whether UNION or UNION ALL is appropriate. I see many developers struggle with…

  • Removing duplicate rows when needed
  • Allowing duplicates when acceptable
  • Performance issues slowing queries
  • Sorting final combined dataset correctly

Mastering the subtle differences between UNION and UNION ALL will alleviate these issues and unlock faster queries. Let‘s break it down…

Real-World Use Case Examples

Let me provide some realistic examples of when to properly use UNION vs UNION ALL. This will give you an intuitive sense before we dive into the technical nitty-gritty.

Use Case for UNION

You work for an ecommerce company and need to combine product data from the clothing and electronics databases. But having duplicate products show up in searches would create a bad customer experience.

SELECT product_id, name, brand FROM clothing
UNION  
SELECT product_id, name, brand FROM electronics;

Use Case for UNION ALL

You are an analyst gathering quarterly sales data. You expect a lot of duplicate customer IDs as the same person could have multiple sales events across different periods and divisions. Preserving this raw data is critical.

SELECT customer_id, sale_amount, sale_date FROM q1_sales
UNION ALL
SELECT customer_id, sale_amount, sale_date FROM q2_sales;

As you can see, choosing the right operator depends on the context and results you expect. Next let‘s contrast the key differences…

Handling Duplicates: Key Difference

The core distinction that trips many developers up is how UNION vs UNION ALL handles duplicate rows:

UNION UNION ALL
Duplicate Row Handling Removes duplicates Allows and includes duplicates

UNION does additional behind-the-scenes work to eliminate copies between the different SELECT statements. This gives you a compact result set with only distinct values.

In contrast, UNION ALL returns all rows from all SELECT statements. So you could get the same row multiple times if it appears in more than one data set.

This difference affects performance which we‘ll explore next…

Performance Showdown: UNION vs UNION ALL

We database developers love squeezing every ounce of performance out of our SQL queries.

So you‘ll be happy to know UNION ALL generally executes much faster than UNION, often by 10-50%!

Why? Remember that UNION has to store all rows in memory and compare them to find and remove duplicates. That‘s heavy lifting!

With UNION ALL, the database engine can immediately return rows from each SELECT without this duplicate removal step. This reduces overall processing requirements for improved speed.

However, don‘t sacrifice accuracy for the sake of performance alone. If you specifically need uniqueness guarantee, UNION may be perfectly acceptable even with a slower runtime.

Use this cheat sheet I‘ve compiled to inform your decisions:

UNION UNION ALL
Duplicate Row Handling Removes duplicates Allows duplicates
Performance Slower due to duplicate removal Faster as all rows returned instantly
When to Use Only need distinct values Duplicates acceptable or preferred

Let your specific use case and priorities guide which option you choose.

Now let‘s cover proper usage and requirements…

UNION and UNION ALL Requirements

There are two key requirements that apply to both UNION and UNION ALL:

1. Matching Number of Columns

The SELECT statements you attempt to combine must have the same number of columns chosen.

For example:

-- Will NOT work

SELECT col1, col2 FROM table1 
UNION
SELECT col1, col2, col3 FROM table2;

2. Compatible Data Types

The corresponding columns must also have the same data type or types that can be implicitly converted.

For example:

-- Will NOT work

SELECT colA (varchar) FROM tableA
UNION 
SELECT colB (int) FROM tableB;

This includes strings, dates, integers, etc. Failure to abide by these rules will cause errors.

Best Practices for Efficient Usage

Here are a few professional tips I‘ve picked up for using UNION and UNION ALL effectively:

1. Put ORDER BY At The End

Only sort the fully combined dataset instead of individual SELECT statements. This ensures proper ordering:

SELECT col FROM t1
UNION 
SELECT col FROM t2
ORDER BY col DESC;

2. Use CASE Statements For Conditional Logic

For context-based filtering, use CASE within the total WHERE versus trying to filter individual SELECT statements. This is more efficient:

SELECT col,
  CASE WHEN col > 100 THEN ‘High‘ ELSE ‘Low‘ END AS status
FROM t1
UNION 
SELECT col,
  CASE WHEN col > 100 THEN ‘High‘ ELSE ‘Low‘ END  
FROM t2;

That wraps up the core differences and usage guidance around these clauses! Let‘s recap…

Key Takeaways: Applying Your Knowledge

We‘ve covered a ton of ground here. Use this condensed cheat sheet to master when and how to use SQL‘s UNION vs UNION ALL operators confidently:

UNION UNION ALL
Removes duplicate rows Includes duplicate rows
Use when uniqueness needed Use when duplicates acceptable
Slower performance Faster performance
Sort combined set with ORDER BY Sort combined set with ORDER BY
Match # columns selected Match # columns selected

Bookmark this for easy reference later.

You‘re now a UNION vs UNION ALL expert! Use this knowledge to optimize queries and take your SQL skills to the next level!

Let me know if you have any other database or SQL questions!

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