SQL UNION vs UNION ALL

SQL UNION and SQL UNION ALL operators are among the SQL set operators that combine two or more SELECT queries into one result set. Both have a similar purpose but differ when it comes

to how they work with duplicates and even performance; here’s how it differs between SQL UNION and SQL UNION ALL, which we will expand with examples to show how this works.

What is the SQL UNION Operator?

The SQL UNION operator makes a result set from multiple queries where the column names are similar but may be different on the two or more queries. It removes duplicates by default. That is if both return identical rows, only distinct rows will be included in the final result.

Key Features of SQL UNION:

• Combining Results: Merges data from multiple queries.
• Removing Duplicates: Automatically removes any duplicate records.
• Data Aggregation: Data Aggregation Useful for consolidating data from different sources.
• Distinct Records: Ensures only unique rows appear in the final result.

Syntax of SQL UNION

The general syntax for using the SQL UNION operator is as follows:

SELECT column_name(s) FROM table1
UNION
SELECT column_name(s) FROM table2;

The SELECT statements must have the same number of columns and compatible data types. By default, the UNION operator removes duplicates.

Example of SQL UNION

Consider two tables, employees_A and employees_B, containing employee data:

Table: employees_A

employee_id	employee_name
1	Alice
2	Bob
3	Carol

Table: employees_B

employee_id	employee_name
3	Carol
4	Dave
5	Eve

To combine employee names from both tables and remove duplicates, we use the UNION operator:

SELECT employee_name FROM employees_A
UNION
SELECT employee_name FROM employees_B;

Result:

employee_name
Alice
Bob
Carol
Dave
Eve

In this case, the name “Carol” appeared in both tables, but the UNION operator removed the duplicate and returned only distinct records.

What is the SQL UNION ALL Operator?

SQL UNION ALL combines results of multiple SELECT queries similar to SQL UNION, but SQL UNION ALL does not eliminate any duplicates from the resulting dataset. All records are included in the final result set without regard to whether they are a duplicate or not. That means SQL UNION ALL could be more efficient simply because there is no overhead of checking and then removing duplicates.

Key Features of SQL UNION ALL:

• Combining Results: Similar to SQL UNION, but includes all records.
• No Duplicate Removal: Every result is included, even if duplicates exist.
• Better Performance: Since no duplicate checking occurs, it’s faster for large datasets.
• Data Aggregation: Ideal for combining large datasets without the need for distinct records.

Syntax of SQL UNION ALL

The syntax for SQL UNION ALL is the same as SQL UNION, but with the ALL keyword:

SELECT column_name(s) FROM table1
UNION ALL
SELECT column_name(s) FROM table2;

Example of SQL UNION ALL

Using the same employees_A and employees_B tables, let’s apply the SQL UNION ALL operator:

SELECT employee_name FROM employees_A
UNION ALL
SELECT employee_name FROM employees_B;

Result:

employee_name
Alice
Bob
Carol
Carol
Dave
Eve

In this case, UNION ALL keeps both instances of “Carol,” as it doesn’t remove duplicates.

Performance Comparison: UNION vs UNION ALL

One of the major differences between SQL UNION and SQL UNION ALL is their impact on performance:

• SQL UNION removes duplicates, which requires extra processing time. It first combines the result sets and then checks for duplicate rows, which can slow down the query, especially with large datasets.
• SQL UNION ALL is faster because it skips the duplicate-checking process, returning results immediately. If you’re certain that there are no duplicates or if you don’t care about duplicates, UNION ALL is a more efficient option.

When to Use SQL UNION:

• When you need distinct records.
• When the datasets might contain duplicate values that you want to exclude from the result.
• When accuracy and uniqueness are more important than speed.

When to Use SQL UNION ALL:

• When performance is critical, especially with large datasets.
• When you’re sure there are no duplicates in the result sets.
• When you don’t mind having duplicate records in the final result.

Removing Duplicates with SQL UNION

To further clarify the difference, let’s look at an example where removing duplicates is necessary. Suppose we have two queries that return overlapping results:

SELECT product_name FROM products_A
UNION
SELECT product_name FROM products_B;

With the UNION operator, you eliminate duplicated product names that appear in both tables. This may be important to scenarios that require unique records-for example, trying to create a list of different products from several suppliers.

Performance Consideration

In scenarios where the data is large and no duplicate checking is required, SQL UNION ALL will be more efficient:

SELECT product_name FROM products_A
UNION ALL
SELECT product_name FROM products_B;

This query will perform better as it avoids the overhead of comparing rows to remove duplicates, especially useful when combining large datasets.

Syntax Examples of UNION and UNION ALL

Here are a few syntax examples that demonstrate the practical use of SQL UNION and SQL UNION ALL in real-world queries.

Example 1: UNION with Data Filtering

SELECT customer_name FROM customers_A
WHERE city = 'New York'
UNION
SELECT customer_name FROM customers_B
WHERE city = 'New York';

This query merges names of customers from two tables while considering only names located in New York and avoiding duplicates in the result.

Example 2: UNION ALL with Data Aggregation

SELECT order_id, order_amount FROM orders_2023
UNION ALL
SELECT order_id, order_amount FROM orders_2024;

This query combines all order records from 2023 and 2024, including any possible duplicates, providing a complete view of all orders.

Example 3: SQL UNION with Different Data Sources

SELECT department_name FROM hr_departments
UNION
SELECT department_name FROM finance_departments;

This query combines department names from two different departments—HR and Finance—and returns only distinct department names across the company.

Example 4: SQL UNION ALL for Performance

SELECT transaction_id, transaction_date FROM transactions_2023
UNION ALL
SELECT transaction_id, transaction_date FROM transactions_2024;

In this example, UNION ALL is used to combine transaction data from 2023 and 2024, including duplicates, for faster performance without duplicate checking.

Advantages of SQL UNION vs UNION ALL

SQL query operators UNION and UNION ALL fundamentally combine multiple result sets based on a SELECT query. The difference between these operators is in some key areas, giving different advantages of using one over the other according to the given context. According to those, here are the benefits of UNION to UNION ALL and vice versa:.

Advantages of SQL UNION

1. Removes Duplicate Records

The most significant benefit of UNION over UNION ALL is the elimination of duplicate rows in the result set. This shall be very helpful if you need to ensure that your final result contains only distinct values. It gets rid of duplicate rows, which means UNION provides a cleaner and simpler dataset.

2. Result Set Simplified

But if your question is a union query that provides outcomes from several source tables, UNION enables you to reduce output so you don’t get duplicates in the end. This is very useful when you’ll have to deal with duplicates that could misleadingly represent data or could lead to wrong analyses.

3. Useful When Specific Data Is Needed

UNION is used when the uniqueness of the output is essential, such as while reporting or summarizing, which requires unique values of the data. It automatically removes any duplicate rows; hence it is preferred at the time of dealing with unique records.

4. Enhanced Data Integrity

Since UNION eliminates duplicates, this enhances the integrity of your results from your query. This can truly serve as a real plus when you’re trying to combine data from more than one table or query, with the intent to avoid inconsistent values due to repeated elements.

SQL UNION ALL Advantages

1. Better Performance

Because UNION ALL does not eliminate the duplicates, it is faster and more efficient than UNION because there is some overhead in eliminating the uniqueness using large datasets. With large tables or complex queries, the cost of eliminating some lines because of the elimination of uniqueness could be expensive. With this, this step is not done in UNION ALL, hence offering much better performance.

2. All Records Preserved

UNION ALL has a big advantage: it preserves all the rows, even if it means duplicating records. That comes in handy in cases where rows are duplicated, and you’d like to preserve those rows for analysis, or the duplicates are imbued with some meaning like sales figures or repeated transactions.

3. No Overhead from Sorting

Whereas UNION typically necessitates a sort of the resulting set to remove duplicates, UNION ALL does not resort to such an operation because it reduces the computational overhead of the query, which can therefore result in substantially faster execution times for queries.

4. Good for Merging Data Sets

The UNION ALL statement should be the choice you make if you merge datasets where duplicates are expected or valid. It performs the MERGE of multiple SELECT statements without changing data, meaning that it carries out the merge of all rows without data duplication. A significant usage of this statement will be to keep full datasets, including repetitions where important

5. Massive Data End

UNION ALL is much simpler and faster if you’re dealing with large data sets where you don’t even care about duplicates. It saves you from the resource-intensive steps involved in checking for and removing duplicates, which can slow down queries.

Disadvantages of SQL UNION vs UNION ALL

Both the UNION and UNION ALL operators are used to combine the results of multiple SELECT queries but come with their peculiar disadvantages based on the use case. The main disadvantages of using SQL UNION versus SQL UNION ALL and vice versa are summarized below.

SQL UNION Disadvantages

1. Overhead Performance

The main disadvantage of the UNION operator is performance overhead. As UNION removes duplicate rows from the resultant set, it involves overhead in sorting of data, and then comparing each row, which can be very time-consuming, especially while dealing with large datasets.

2. Unnecessary Duplicate Removal

In certain situations, duplicate removal may not be needed, yet UNION still performs the operation by default. When the data set is inherently unique or when duplicate values do not create an issue to the problem at hand, overhead removal of duplicates might just be squandering system resources and slowing up its performance.

3. Sorting Requirement

Even in the case of record deduplication, the UNION operator has to sort the result set. Sorting is an extra time-consuming process in computation, not good as it would counter the performance when dealing with large data sets or large numbers of tables.

4. Inconsistent Results

If you are combining data from multiple sources where duplicates might indicate meaningful information, then using UNION may produce inconsistent or incomplete results because it automatically removes rows that might otherwise be important for your analysis or reporting.

Disadvantages of SQL UNION ALL

1. No Duplicate Elimination

The main disadvantage of UNION ALL is that it has no duplicate row elimination. Since UNION ALL does not eliminate duplicates, and you are working with datasets for which duplicates are unwanted or would totally distort the result, you will need to remove duplicates manually in your query, which can complicate the query and lead to wrong interpretations if not handled appropriately.

2. Risk of Data Redundancy

UNION ALL will add redundant data in the resulting dataset at the end as it is merging two result sets belonging to two different queries. This may cause bloated result sets and create difficulties in inferring data correctly or take more time and processing for further analysis while there is no automatic de-duplication.

3. Risk of Misleading Results

In many cases where the duplicate records are unacceptable, and only unique records are required, UNION ALL might produce wrong results due to the presence of duplicate rows. This also might mislead the data analysis, reporting, or the appropriate decision to be taken from the result of the query.

4. Greater Storage and Transfer Costs

Since UNION ALL does not eliminate duplicate rows, it increases the size of the result set, thereby increasing cost in storage and transfer, especially when large datasets are involved; this will, in turn influence the performance of the system as well as the capacity-to-capacity resource distribution within capacities that are limited in their capabilities.