SQL UNION Operator

The SQL UNION Operator is a powerful tool used in SQL to combine the results of two or more SELECT queries into a single result set. It allows developers to retrieve related data from different tables while maintaining the structure and simplicity of SQL. In this article, we will explore the SQL UNION operator in detail, including its syntax, usage, examples, and a comparison with UNION ALL.

What is the SQL UNION Operator?

The SQL UNION operator is designed to merge the results of multiple SELECT statements. It combines rows from different tables that have the same number of columns and compatible data types. The result of a UNION operation is a single unified result set, which eliminates duplicate entries by default.

Why Use UNION?

Here are a few reasons why the UNION operator is useful:

• Combining data: It enables you to combine results from different tables that may have similar data.
• Data analysis: You can perform analyses across multiple datasets without needing complex joins.
• Simplicity: The operator simplifies queries when dealing with similar structured datasets.

How to Use UNION

To use the UNION operator effectively, follow these steps:

1. Ensure that each SELECT statement has the same number of columns.
2. Make sure that the columns have compatible data types.
3. Use the keyword UNION between the SELECT statements.
4. Optionally, you can use the ORDER BY clause at the end of the final result set.

SQL UNION Syntax

The basic syntax for the UNION operator is as follows:

SELECT column1, column2, ...
FROM table1
UNION
SELECT column1, column2, ...
FROM table2;

It is important to note that if you want to include all rows, including duplicates, you will use the UNION ALL operator instead, which we will discuss later.

Example of SQL UNION

Let’s consider two tables: employees and contractors.

employees	contractors
John Doe	Jane Smith
Mary Johnson	Mike Lee

We can use the UNION operator to retrieve a list of names from both tables:

SELECT name FROM employees
UNION
SELECT name FROM contractors;

The result set will be:

Name
John Doe
Mary Johnson
Jane Smith
Mike Lee

As we see above, the SQL UNION operator combines the names from both tables into a single result set, eliminating any duplicates.

SQL UNION ALL

While the UNION operator removes duplicates, the UNION ALL operator includes all rows from the combined datasets, keeping duplicates intact. This can be useful when you need a complete view of the data.

Example of SQL UNION ALL

Using the same tables, let’s see how UNION ALL works:

SELECT name FROM employees
UNION ALL
SELECT name FROM contractors;

The result set will be:

Name
John Doe
Mary Johnson
Jane Smith
Mike Lee

As shown, the UNION ALL operator includes all names, including duplicates, which may exist if an employee is also a contractor.

Summary

In summary, the SQL UNION Operator is an essential component of SQL that allows developers to retrieve and manipulate data across multiple tables efficiently. By understanding the differences between UNION and UNION ALL, you can decide which to use based on whether you want to include or exclude duplicate entries in your result set. Using these operators can significantly simplify data retrieval and analysis processes.

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