Master SQL: The Differences Between UNION vs UNION ALL

Hi there! Do you use SQL in your work and want to really master set operators like UNION and UNION ALL? As an experienced data analyst, I can tell you these are incredibly useful tools for combining and transforming data sets.

But UNION and UNION ALL have some key differences you need to understand to use them properly. Don’t worry – I’m going to explain everything in a simple, easy-to-understand way so you can become an expert on UNION vs UNION ALL!

Let’s start with the basics…

What Are UNION and UNION ALL in SQL?

UNION and UNION ALL allow you to combine multiple result sets in SQL into one unified result.

For example, say you have two tables, Products and Categories. You can write SELECT statements to return data from each one:

SELECT ProductName FROM Products

SELECT CategoryName FROM Categories

But what if you need all that data together? That’s where UNION comes in handy!

The UNION operator lets you take those two SELECT statements and stack their results into one result set like this:

SELECT ProductName FROM Products
UNION 
SELECT CategoryName FROM Categories

Now you have one set of results containing the unique ProductName and CategoryName values!

UNION ALL does the same thing, except it keeps all rows instead of removing duplicates.

So that’s the basics – UNION gives you unique rows while UNION ALL keeps duplicates. But there’s more you need to know…

Key Differences Between UNION and UNION ALL

While UNION and UNION ALL serve the same general purpose, there are some notable differences:

Handling of Duplicate Rows

This is the big one you need to understand:

• UNION will remove any duplicate rows across the SELECT statements. Only unique data is returned.

• UNION ALL keeps all rows, including any duplicates.

Here‘s a quick example to illustrate:

SELECT City FROM Cities
UNION
SELECT City FROM Offices

If the Cities table has "Boston" in it twice, and Offices also has "Boston" once, the UNION above would only return "Boston" a single time.

But with UNION ALL, you‘d see "Boston" listed 3 times since all duplicates are preserved.

Performance

• UNION is slower because it does extra work behind the scenes to identify and discard duplicate rows.

• UNION ALL is faster since it simply stacks the output of all the SELECT statements together.

This SQL performance tip is good to keep in mind if speed is important for your queries.

Order of Results

• Both operators will order the final results based on an ORDER BY clause in the last SELECT statement, if included.

• Otherwise, the result order is database-dependent.

So if controlling order is important, add an ORDER BY to the last SELECT. Don’t rely on implicit ordering!

Let‘s summarize the key points:

• Use UNION when you only want distinct rows combined across multiple SELECT statements.

• Use UNION ALL when you don‘t mind keeping duplicate rows from all SELECTS.

Starting to make sense? Let‘s move on to see when to apply these in practice…

When to Use UNION vs UNION ALL

Deciding between UNION or UNION ALL depends on your specific data and goals:

Use UNION:

• When you need to query multiple tables for related data but want unique/distinct rows overall. Removing duplicates is mandatory.

• When performance is not the top priority and you can accept slower query execution.

Use UNION ALL:

• When you simply want to stack results from multiple queries and don‘t care about duplicates. Allowing duplicates is fine.

• When query performance is critical since UNION ALL is faster.

• When you need to retain original row ordering across SELECT statements.

For example, UNION makes sense when querying two inventory tables to get one consolidated product list. UNION ALL could show the same product multiple times incorrectly.

However, for a sales data report across regions, using UNION ALL ensures you don’t lose any transactions. UNION would mistakenly remove valid duplicate sales!

To summarize based on a 2018 SQL performance comparison:

So think about your data requirements and performance needs when deciding between UNION and UNION ALL in practice.

Requirements for UNION and UNION ALL

Both UNION and UNION ALL have two key requirements:

Matching Number of Columns

Every SELECT statement must return the same number of columns. These need to align 1:1 across all statements.

For example:

SELECT City, Population FROM Cities
UNION
SELECT FirstName, LastName FROM People

This would fail because the first SELECT returns 2 columns while the second returns 1. All SELECTS must have equal column counts.

Matching Data Types

The corresponding columns across SELECT statements must have compatible data types. For example, the first column should be a numeric field in all statements.

This example would fail because data types misalign:

SELECT Name, RegistrationDate FROM Devices
UNION 
SELECT Name, IsActive FROM Users

RegistrationDate is a date while IsActive is a boolean. Even in the same column position, data types must match up.

Adhering to these requirements ensures your UNION or UNION ALL query doesn‘t fail or return incorrect results.

Performance Comparisons: UNION vs. UNION ALL

To understand when performance differs between UNION and UNION ALL, you have to grasp how they handle duplicate values.

UNION has to do extra work behind the scenes to identify and remove any duplicate rows across the combined result set. This involves sorting values and checking every row for duplicates – an expensive operation!

UNION ALL skips this step completely since it doesn’t remove duplicates. It just stacks one result set on top of the other as-is.

As this chart illustrates, UNION ALL achieved ~3x faster performance on average compared to UNION in one comprehensive SQL query benchmark:

The more duplicate values there are, the bigger the performance gap. With no duplicates, UNION and UNION ALL would be nearly equivalent.

For best performance:

• Use UNION ALL when order doesn‘t matter and duplicates won‘t distort your analysis.

• Only use UNION when removing duplicates is absolutely required.

Now let‘s look at some examples of UNION and UNION ALL in action…

UNION and UNION ALL Query Examples

Here are some examples of how UNION and UNION ALL work:

UNION

Returns only distinct rows across all SELECTS:

SELECT City FROM Cities 
UNION
SELECT City FROM Offices

UNION ALL

Preserves duplicate rows:

SELECT ProductID FROM Products
UNION ALL  
SELECT ProductID FROM Inventory

UNION With ORDER BY

Sorts final results globally:

SELECT City FROM Cities
UNION
SELECT City FROM Offices 
ORDER BY City DESC

UNION ALL With Multiple SELECTS

Combines any number of SELECT results:

SELECT City FROM Cities
UNION ALL
SELECT City FROM Offices
UNION ALL
SELECT City FROM Warehouses 

Hopefully seeing some concrete examples helps explain how to apply UNION and UNION ALL in practice!

5 Key Facts About UNION and UNION ALL

Here are 5 key facts to remember about UNION and UNION ALL:

1. UNION removes duplicates while UNION ALL keeps duplicates.

2. UNION ALL has faster performance than UNION in most cases.

3. Both operators have the same column number and data type requirements.

4. An ORDER BY after the last SELECT globally sorts the final results.

5. UNION and UNION ALL only combine result sets, not entire tables.

These key points really crystallize the vital differences between UNION and UNION ALL that you need to understand.

Should You Use UNION or UNION ALL?

There‘s no universally better or preferred operator between UNION and UNION ALL.

It depends completely on your specific data and goals:

• If you need to combine results from multiple queries and require only distinct rows, use UNION.

• If you want to retain all duplicate rows from multiple SELECT statements, use UNION ALL.

• When fast query performance is critical, lean towards UNION ALL where possible.

• To globally order your final results, use an ORDER BY clause on the last SELECT.

Here‘s a simple decision flow you can follow:

UNION ALL is a good default in most cases, but UNION still serves an important purpose when removing duplicates is mandatory.

I hope these explanations and examples help you master UNION and UNION ALL in SQL. Using these set operators properly will give you a powerful toolbox for wrangling data! Let me know if you have any other questions.