Sometimes it is convenient to combine the result of several queries into one continuous result. For this purpose, SQL has the `UNION` operator.

But we have two different UNION:
1. `UNION ALL` - duplicates are included.
2. `UNION` - duplicates are excluded.

Let's take a look at the syntax of using this operator.

SELECT columns FROM table
UNION
SELECT columns FROM table

In this section, you will learn how to group and work with such data. For this, we will get acquainted with such an operator as GROUP BY. You will also learn how to filter grouped data using the operator HAVING.

In this section, you will learn what nested subqueries are, how to create them, and how to work with them.

In this section, you will learn how you can combine several tables, thus working with one common table, which will simplify the creation of queries.

In this section, you will learn how to combine several tables in different ways, in particular: INNER JOIN, LEFT JOIN, RIGHT JOIN.