UNION, INTERSECT, and MINUS SQL Operations

Question: How do I combine the results of two separate queries using the UNION command in Oracle?
To combine the results of two separate queries using the UNION command in Oracle, you need to follow these steps:

1. Write the first query, making sure to select the desired columns. The number of columns, their data types, and their order should be the same in both queries.
2. Use the 'UNION' keyword to combine the results of the two queries. This keyword will only return distinct rows from both queries (i.e., it will remove duplicates).
3. Write the second query, making sure to select the same number of columns, with the same data types and in the same order as the first query.

Here's an example:
Assuming you have two tables 'employees' and 'contractors' with the same columns: 'id', 'first_name', 'last_name', and 'email', and you want to create a list of all employees and contractors with their full names and email addresses, you would write the following query:

SELECT first_name || ' ' || last_name AS full_name, email
FROM employees
UNION
SELECT first_name || ' ' || last_name AS full_name, email
FROM contractors;

In this example, the '||' operator is used to concatenate the first_name and last_name columns to create a full_name column.
The 'UNION' keyword combines the results of the two SELECT statements, and it will only return distinct rows. If you want to include duplicate rows in the result, you can use 'UNION ALL' instead:

SELECT first_name || ' ' || last_name AS full_name, email
FROM employees
UNION ALL
SELECT first_name || ' ' || last_name AS full_name, email
FROM contractors;

Remember to make sure the number of columns, their data types, and their order are the same in both queries for the UNION command to work correctly.