UNION vs. UNION ALL in SQL: Understanding the Difference for Combining Results

2024-04-05

UNION and UNION ALL: Combining Result Sets in SQL

In SQL, UNION and UNION ALL are operators used to combine the results of two or more SELECT statements into a single result set. They are both used for vertical concatenation, meaning they stack the rows of the separate queries on top of each other. However, the key distinction lies in how they handle duplicate rows:

  • UNION: Returns only unique rows. It eliminates duplicate records (rows with identical values in all columns) before combining the results. This is similar to using SELECT DISTINCT on the combined result set.

  • UNION ALL: Returns all rows, including duplicates. It simply merges the results from the individual queries without removing any duplicates.

Here's a table summarizing the key differences:

FeatureUNIONUNION ALL
Duplicate RowsRemoves duplicatesIncludes duplicates
PerformanceGenerally slower (extra step of deduplication)Generally faster (no deduplication)
Use CasesWhen you only need unique resultsWhen duplicates are expected or don't matter

Choosing Between UNION and UNION ALL

  • Use UNION when you want a distinct set of results, ensuring no rows appear multiple times. This is common when combining data from tables that might have overlapping information.
  • Use UNION ALL when you want all rows from the combined queries, including duplicates. This could be useful when you're accumulating data from different sources and need to preserve all records, regardless of redundancy.
  • If you're certain there will be no duplicates in the combined result set, UNION ALL might be slightly faster due to the lack of deduplication processing. However, for most cases, the performance difference is negligible.

Example:

Consider two tables: Customers and Orders, both having columns customer_id and name.

SELECT customer_id, name FROM Customers;

+------------+--------+
| customer_id | name   |
+------------+--------+
| 1           | Alice  |
| 2           | Bob    |
| 3           | Charlie|
+------------+--------+

SELECT customer_id, name FROM Orders;

+------------+--------+
| customer_id | name   |
+------------+--------+
| 2           | Bob    |
| 3           | Charlie|
| 4           | David  |
+------------+--------+
  • Using UNION:
SELECT customer_id, name FROM Customers
UNION
SELECT customer_id, name FROM Orders;

This will return only unique rows, combining customers from both tables without duplicates:

+------------+--------+
| customer_id | name   |
+------------+--------+
| 1           | Alice  |
| 2           | Bob    |
| 3           | Charlie|
| 4           | David  |
+------------+--------+
  • Using UNION ALL:
SELECT customer_id, name FROM Customers
UNION ALL
SELECT customer_id, name FROM Orders;

This will include all rows, even duplicates:

+------------+--------+
| customer_id | name   |
+------------+--------+
| 1           | Alice  |
| 2           | Bob    |
| 2           | Bob    | (duplicate)
| 3           | Charlie|
| 3           | Charlie| (duplicate)
| 4           | David  |
+------------+--------+

I hope this explanation clarifies the concepts of UNION and UNION ALL in SQL!




Example 1: Combining Products from Different Categories

Imagine you have two tables: Products and DiscountedProducts. Both have columns product_id, name, and price. You want to list all products, including those on discount:

-- Select products from the main table
SELECT product_id, name, price FROM Products;

-- Select products on discount (assuming a 'discount' column exists)
SELECT product_id, name, price FROM DiscountedProducts
WHERE discount > 0;

Using UNION to remove duplicates (products listed in both tables):

SELECT product_id, name, price FROM Products
UNION
SELECT product_id, name, price FROM DiscountedProducts
WHERE discount > 0;

This will return a combined list with only unique products (those not on discount won't be duplicated even if they exist in DiscountedProducts).

Using UNION ALL to include duplicates (all products, even if discounted):

SELECT product_id, name, price FROM Products
UNION ALL
SELECT product_id, name, price FROM DiscountedProducts
WHERE discount > 0;

This will list all products, including duplicates if a product appears in both tables and the discounted list.

Suppose you have tables Orders2023 and Orders2024 with columns order_id, customer_name, and order_date. You want to see all orders from both years:

-- Select orders from 2023
SELECT order_id, customer_name, order_date FROM Orders2023;

-- Select orders from 2024
SELECT order_id, customer_name, order_date FROM Orders2024;

Using UNION to remove duplicates (unique orders across both years):

SELECT order_id, customer_name, order_date FROM Orders2023
UNION
SELECT order_id, customer_name, order_date FROM Orders2024;

This will combine orders, ensuring no order appears twice even if placed by the same customer in both years.

Using UNION ALL to include duplicates (all orders, including potentially repeated ones):

SELECT order_id, customer_name, order_date FROM Orders2023
UNION ALL
SELECT order_id, customer_name, order_date FROM Orders2024;

This will list all orders from both years, even if an order appears multiple times (e.g., if a customer placed the same order in both years).

Remember, the choice between UNION and UNION ALL depends on whether you want unique or all rows from the combined result set.




Joins are the most powerful and flexible way to combine data from multiple tables based on relationships between them. They allow you to filter and combine data based on specific conditions. For example, instead of using UNION to combine customers and their orders, you could use an INNER JOIN on customer_id to retrieve orders associated with each customer in a single result set.

Common Table Expressions (CTEs):

CTEs are temporary named result sets defined within a single SQL statement. You can use multiple CTEs to break down complex queries into smaller, more manageable steps. This can help improve readability and maintainability, especially for combining data from multiple sources with complex logic.

Subqueries are nested SELECT statements within another SELECT statement. They can be used to filter or aggregate data based on conditions within the main query. While not always a direct replacement for UNION, subqueries can be used for specific scenarios like conditional inclusion or aggregation across result sets.

Temporary tables are created within a session and hold data temporarily. You can populate them with data from separate queries and then use a single SELECT statement to retrieve the combined results. This approach can be useful when dealing with large datasets or when you need to perform additional transformations on the combined data before presenting it.

Programming Language Integration:

In some cases, depending on your specific environment, you might be able to leverage programming languages like Python or R to process and combine data retrieved from separate SQL queries. This can be helpful for complex operations or when you need to perform advanced data manipulation not readily available in SQL.

Choosing the Best Alternative:

The best alternative depends on the specific requirements of your query and the structure of your data. Here's a general guideline:

  • Use joins for combining data based on relationships between tables.
  • Use CTEs for breaking down complex queries into smaller steps.
  • Use subqueries for filtering or aggregating data based on conditions within the main query.
  • Use temporary tables when you need to perform additional transformations on the combined data before presenting it.
  • Use programming language integration for complex operations or data manipulation beyond SQL capabilities (consider the trade-offs of managing code within the database environment).

sql union union-all


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