Understanding the SQL UNION Operator

SQL, or Structured Query Language, is a vital tool in handling all sorts of data. Amongst its many operators is the UNION, a set operation that combines the result sets of two or more SELECT statements into a single one. It's important to emphasize that UNION operator only allows distinct values to be selected.

How Does UNION Work?

UNION operates over two SQL statements each yielding its own result set. It takes these two (or more) result sets and essentially merges them into a single one. To use UNION, each SELECT statement within the UNION must have the same number of columns, the columns must also have similar data types, and they must also be in the same order.

Here is an example:

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

This command will execute each SELECT statement to get its result set and then combine these into a single output.

Practical Example

To illustrate with a practical example, imagine we have two tables, Orders_East and Orders_West, which record orders from the East and West branches of a company, respectively. If we wanted to create a list of all unique clients from both branches, we would use the UNION operator:

SELECT client_name FROM Orders_East
UNION
SELECT client_name FROM Orders_West;

This command will create a list of all client names, from both orders_east and orders_west, without any duplicates as UNION automatically eliminates any duplicate rows.

Additional Insights

Keep in mind that while UNION can be very useful, there is a variant command, UNION ALL, which does not eliminate duplicate rows. Depending on the circumstances and what you need to achieve with your data, UNION ALL may be a better fit. Proficiency in SQL involves not only knowing these commands but also knowing when best to use them.

Bear in mind, the UNION operator is somewhat resource-intensive, especially when working with large databases, as it needs to eliminate duplicates. Thus, if your specific case does not require eliminating duplicates, using UNION ALL instead of UNION can be more efficient.