Hi there! Welcome to the Intermediate SQL course! In the first section, we will work with a Monreal metro database containing three tables: `station`,  `line` and `relation`. The first table contains the names of metro stations. The second table contains the lines of the Monreal metro. The last table contains the time for each station which you need to get there.

Let's look at the first table - `station`.

Let's look at the third table - `relation`.

Let's look at the connection between these three tables.

For instance, you want to get information not about all subway stations separately but for groups of subway stations by lines. Consider an example of finding each subway line's total number of subway stations. To do it, we have to use the `COUNT()` function.

SELECT line_name, COUNT(station_name) AS number_of_station
FROM relation
GROUP BY line_name

If you run this query, you will get the following result:

**Analysis:** In the above query, we grouped our data by lines using the `GROUP BY` clause, then we counted the numbers of rows in each obtained group using the `COUNT()` function and created a new column for these values named `number_station`.

**Syntax**: To group your data, you just have to use the `GROUP BY` clause in the following way:

SELECT column1, AGGREGATE_FUNCTION(column2) AS alias
FROM table
GROUP BY column1

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.