difference between union and union all

The Difference Between UNION and UNION ALL:

When it comes to data manipulation language (DML) in SQL, the UNION and UNION ALL operators are widely used for combining data from multiple tables. However, both operators have distinct differences that can affect the outcome of your SQL queries. In this article, we’ll explore the differences between UNION and UNION ALL, and when to use each one.

UNION Operator:

The UNION operator combines the result sets of two or more SELECT statements into a single distinct result set. It eliminates duplicate rows from the result set so that each row appears only once.

Let’s see an example:

SELECT employee_id, first_name, last_name, department_id FROM employees
UNION
SELECT employee_id, first_name, last_name, department_id FROM departments;

In this example, the UNION operation combines the data from two tables, employees and departments, into a single result set. The resulting table will contain unique rows based on the combination of the columns specified in the SELECT statements.

While UNION is a useful operator for ensuring distinct rows in your result set, it can also be slower than UNION ALL because it requires the extra overhead of sorting the result set to eliminate duplicates.

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UNION ALL Operator:

The UNION ALL operator also combines the result sets of two or more SELECT statements into a single result set, but it does not eliminate duplicate rows. In other words, it retains all rows returned by the individual SELECT statements.

Let’s see an example:

SELECT employee_id, first_name, last_name, department_id FROM employees
UNION ALL
SELECT employee_id, first_name, last_name, department_id FROM departments;

In this example, the UNION ALL operation combines the data from two tables, employees and departments, into a single result set. Unlike UNION, the resulting table will contain all rows from the individual tables.

UNION ALL is faster than UNION because it does not need to eliminate duplicates. It is useful if you know that the result sets do not contain any duplicates or if you do not care about duplicates in the final result set.

When to use UNION and UNION ALL:

The choice between UNION and UNION ALL depends on your specific needs. If you want to combine the results of two or more SELECT statements while avoiding duplicates, use UNION. This is useful in scenarios where you need the output to be distinct and free of repetition.

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If your SELECT statements return distinct results or you do not care about duplicates, use UNION ALL. This is useful in scenarios where performance is a concern and you do not need to remove duplicate rows.

In summary, UNION and UNION ALL are powerful operators in SQL that allow you to combine data from multiple tables into a single result set. While UNION eliminates duplicate rows, it can be slower than UNION ALL due to the extra overhead of sorting the result set. Use UNION when you need a distinct result set and use UNION ALL when you do not care about duplicates or when performance is a concern.

Table difference between union and union all

Union Union All
Removes duplicates from the result set Does not remove duplicates from the result set
Combines two or more SELECT statements into a single result set Combines two or more SELECT statements into a single result set with all records including duplicates
Slower than UNION ALL since it has to perform the additional step of deduplication Faster than UNION due to the absence of deduplication