SQL UNION Operator

A SQL UNION operator is a set operator for joining the result of two or more SELECT statements into a single result set. There might be situations in which you need to draw data from

several tables or queries and then express everything as one consistent dataset. By default, UNION eliminates any duplicate values in the results, whereas its counterpart, UNION ALL, returns all results, including duplicates. In this article, we’ll explore how the UNION operator works, its syntax, and practical examples to help you understand its usage in SQL queries.

What is the SQL UNION Operator?

The SQL UNION operator merges the result of two or more SELECT statements. All SELECT statements combined with UNION must have the same number of columns. Corresponding columns must also have compatible data types. Using a UNION operator ensures unique values within a dataset returned; in the case of the UNION ALL operator, all of them are returned, including duplicates.

Key Features:

• Combining Results: Merges the result sets of multiple SELECT queries.
• Distinct Values: Automatically removes duplicate records.
• Data Aggregation: Useful for gathering data from different sources and presenting it together.
• UNION ALL: Includes duplicate records when combining results.
• Data Filtering: Applies to results after combining them.

Syntax of the UNION Operator

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

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

• The number of columns and their data types must match across all SELECT statements.
• By default, UNION returns distinct values from both result sets.

Example of SQL UNION

Consider the following two tables, customers_A and customers_B, which contain customer data from two different regions:

Table: customers_A

customer_id	customer_name
1	Alice
2	Bob
3	Carol

Table: customers_B

customer_id	customer_name
3	Carol
4	Dave
5	Eve

To combine the customer data from both tables without duplicates, we use the UNION operator:

SELECT customer_name FROM customers_A
UNION
SELECT customer_name FROM customers_B;

This query will return the following result, with distinct values:

customer_name
Alice
Bob
Carol
Dave
Eve

SQL UNION ALL Operator

If you want to include all the results, including duplicates, you can use the UNION ALL operator:

SELECT customer_name FROM customers_A
UNION ALL
SELECT customer_name FROM customers_B;

This query will return:

customer_name
Alice
Bob
Carol
Carol
Dave
Eve

Using UNION with Data Filtering

The UNION operator can also be used in combination with conditions in the WHERE clause to filter data before combining the results. Let’s look at an example:

SELECT customer_name FROM customers_A
WHERE customer_id < 3
UNION
SELECT customer_name FROM customers_B
WHERE customer_id > 3;

This query will return customers from the first table with customer_id less than 3 and from the second table with customer_id greater than 3:

customer_name
Alice
Dave
Eve

Combining Results with Different Conditions

You can apply SQL UNION when you want to combine results based on different conditions from the same or different tables.

For example, let’s say you want to retrieve a list of customers from the orders table who have placed either small orders (less than 100) or large orders (more than 500):

SELECT customer_id, order_amount FROM orders
WHERE order_amount < 100
UNION
SELECT customer_id, order_amount FROM orders
WHERE order_amount > 500;

This will return the customer IDs and order amounts where the amounts are either less than 100 or greater than 500.

Example Table: orders

order_id	customer_id	order_amount
1	1	50
2	2	250
3	3	600
4	4	30
5	5	700

The result will be:

customer_id	order_amount
1	50
3	600
4	30
5	700

Syntax Examples

Here are a few more syntax examples to illustrate how the SQL UNION operator works:

Combining Results from Different Tables

SELECT product_name FROM products_A
UNION
SELECT product_name FROM products_B;

This query combines product names from two different tables.

UNION ALL with Filtering

SELECT customer_name FROM customers_A
WHERE customer_id > 2
UNION ALL
SELECT customer_name FROM customers_B
WHERE customer_id < 5;

This query will include all values (even duplicates) from both result sets while applying conditions to filter the data.

Handling Duplicates with UNION and UNION ALL

• UNION: Removes duplicates and only returns distinct rows.
• UNION ALL: Returns all rows, including duplicates.

Understanding the difference between these two operators is crucial when you’re combining data sets that may contain overlapping information.

Duplicate Removal Example

Duplicate removal in SQL is essential for ensuring data integrity, as it eliminates repeated records from query results, allowing for more accurate analysis and reporting. Let’s say we have two SELECT statements that return some overlapping data. The UNION operator automatically removes duplicates:

SELECT employee_id, employee_name FROM employees
WHERE department = 'HR'
UNION
SELECT employee_id, employee_name FROM employees
WHERE department = 'Sales';

This query will return distinct employees who work in either HR or Sales departments, ensuring no duplicates in the final result.

UNION with Set Operators in SQL

Set operators in SQL, such as UNION, INTERSECT, and EXCEPT, allow users to combine the results of multiple SELECT statements, enabling efficient data retrieval and comparison across different datasets. The UNION operator is part of a family of SQL set operators that are used to combine result sets from different queries. Other operators in this group include:

• INTERSECT: Returns only the common rows between the two result sets.
• EXCEPT (or MINUS): Returns rows from the first result set that are not found in the second result set.

Example with INTERSECT:

SELECT employee_id FROM employees
WHERE department = 'HR'
INTERSECT
SELECT employee_id FROM employees
WHERE department = 'Sales';

This query will return only the employees who work in both the HR and Sales departments.

Advantages of SQL UNION Operator

The SQL UNION operator is a powerful tool used to combine the results of two or more SELECT queries into a single result set. It provides several advantages when working with data from multiple tables or queries, helping streamline and simplify complex query processes. Below are the key advantages of the SQL UNION operator:

1. Combines Multiple Result Sets

The biggest strength of the UNION operator is that it will enable one to union multiple result sets into an all-inclusive single output. This is especially helpful when data is spread across multiple tables or when multiple queries generate complementary results that need to be read together in an integrated format.

2. Removes Duplicate Records

By default, the UNION operator excludes any duplicate rows which may seem in the result set. It returns only unique results. This is beneficial if you need to remove redundant data and, therefore, have unique entries in your final output. Avoid extra filtering or de-duping operations.

3. Enhances Query Efficiency

The UNION operator can make complex queries easier to deal with. In this regard, a query with several SELECT statements can be written as one statement, which reduces the requirement of having multiple queries in individual statements. This also enhances readability and maintainability of queries since bringing data together from varied sources is only handled in one structure of the query.

4. Simplifies Merging Data from Multiple Sources

The UNION operator becomes convenient when you are needed to merge more than one source’s data, and that includes different tables, views, or even different databases. In UNION, it is very easy to combine related data sets that may not stay at a single place, thus making it an extremely important tool for working with distributed or federated data systems.

5. Promotes Flexibility in Query Design

The UNION operator promotes flexibility in designing queries. It can be used to combine results from queries that filter data in different ways or that apply different conditions. It thus makes easier the construction of dynamic and modular queries that aggregate data from various perspectives, based on varying criteria.

6. Supports Different Query Structures

Another nice perk of the UNION operator is it can sometimes take queries with fundamentally different structures, provided that they have the same number of columns and the same data types for those columns. It then lets you combine those datasets in ways that are not necessarily identical, while still related, without having to restructure or rewrite large parts of your queries in significant ways.

7. Data Integration Across Multiple Tables

The UNION operator allows you to combine data from other tables, which is specifically helpful in cases where your data spreads across several tables of a relational database. It simplifies your ability to condense that information into one result set without performing JOIN operations, making your data easier to manage and analyze.

8. Improved Data Representation

Using the UNION operator allows you to represent a broad range of data in a consolidated form. By combining results from different queries, you can offer a more comprehensive view of the data, especially when analyzing data that comes from various tables or queries with different filtering conditions.

9. Minimizes Code Duplication

The UNION operator can help minimize code duplication by reducing the need for repeating similar queries separately. Instead of running multiple queries and manually combining their results, the UNION operator automates this process, reducing the amount of repetitive query code.

Disadvantages of SQL UNION Operator

The SQL UNION operator is a very powerful operator that allows combining result sets produced by multiple queries. However, several disadvantages are associated with it. These limitations tend to affect performance, complexity, and flexibility in designing queries. Here are some of the major disadvantages of the SQL UNION operator:

1. Performance Overhead

Another one of the most common disadvantages of the UNION operator is that it may cause considerable overhead in performance, especially when dealing with large data sets. Since UNION always removes duplicate rows, the database must compare each record from each of the result sets and delete any duplicates. Deleting duplicate records is very time-consuming, particularly when you are working with large tables or queries that return a lot of data.

2. Requirement for Matching Column Structures

The UNION operator requires that all queries being joined should be of the same number of columns, with corresponding data types in each position. This sometimes imposes inflexibility because the queries to be joined must be aligned correctly on structural considerations irrespective of the differences among column layouts of the source tables. It may be tough to align queries coming from different table schemas.

3. Sorting and Efficiency Issues

This is because UNION automatically orders the result set which can make any query executed much slower than it ought to if the data combined is large or contains a lot of things that need complex algorithms in sorting.

4. No Control Over Duplicate Elimination

The UNION operator automatically removes duplicate rows, which is not always what you want. When you want to keep the duplicates around for reporting and analysis, then automatic de-duplication is actually an unwanted behavior. Sometimes, if you need to have duplicates in the result set, you would have to use UNION ALL, which does not remove duplicates.

5. Limited Error Handling

This is because sometimes error handling becomes very cumbersome when one uses UNION. A simple error in syntax or non-alignment of column structure in any query in the SELECT queries of the UNION operation makes the entire query fail to execute. It becomes even more difficult to debug and troubleshoot because one needs to be sure that each individual query of a UNION statement is free of errors and aligned with the column structure.

6. Complex Query Design

The UNION operator results in a query design that is difficult to read and understand, especially when there are more than one SELECT statements are joined. It also becomes challenging to handle and maintain the code when multiple result sets are combined, hence several result sets are to be joined in a query. If filtering conditions and/or aggregate functions are different in the combined queries, it will be worse.

7. Reduced Optimization Opportunities

Often, database query optimizers have problems optimizing UNION queries very well. Since the challenge posed by UNION queries is that of merging resulting sets and eliminating redundancy, it becomes difficult for the optimizer to take advantage of any indexes or any other performance-enhancing techniques in this regard. This means that the query might run slower than other methods, such as JOINs or UNION ALL because the database can use indexing strategies more effectively in these methods .

8. Difficulties in Combining Different Data Types

Further, with UNION, queries should have compatible data types in corresponding columns. This may be cumbersome if you are dealing with tables where the used data types are mismatched; in such cases you would have to cope with type conversion functions and thus add complexity and whose usage can lead to performance degradation.

9. Potential for Incorrect Results

Hence, if queries in the UNION statement are not designed properly, it may result in the failure of results to reflect the intended logic. Automatic removal of duplicate records may create a misleading notion about the data without explicit control over those to be included or those to be excluded.