SQL Union

In this tutorial you will learn how to combine the results of two or more SQL queries.

## The UNION Operator
The `UNION` operator is used to combine the results of two or more `SELECT` queries into a single result set. The union operation is different from using joins that combine columns from two tables. The union operation creates a new table by placing all rows from two source tables into a single result table, placing the rows on top of one another.

These are basic rules for combining the result sets of two `SELECT` queries by using `UNION`:

- The number and the order of the columns must be the same in all queries.
- The data types of the corresponding columns must be compatible.

When these criteria are met, the tables are union-compatible:

## Syntax
The basic syntax of UNION can be given with:

```sql
SELECT column_list FROM table1_name
UNION SELECT column_list FROM table2_name;
```

To understand the union operation in a better way, let's assume that some hypothetical fields, like first_name and last_name exists in our employees and customers tables. Please note that these fields do not actually exist in our demo database tables.

Let's perform a union operation to combine the results of two queries.

The following statement returns the first and last names of all the customers and employees:

```sql
SELECT first_name, last_name FROM employees 
UNION
SELECT first_name, last_name FROM customers;
```

After executing the above statement, the result set will look something like this:

However, if you want to keep the duplicate rows you can use the `ALL` keyword, as follow:

```sql
SELECT first_name, last_name FROM employees 
UNION ALL
SELECT first_name, last_name FROM customers;
```