But what if we want to calculate the number of unique site visitors? Sounds like a combination of selecting unique and then counting...

Yes, that's the point! To calculate the number of unique observations over a certain column use `COUNT(DISTINCT( ... ))` statement. For example,

SELECT COUNT(DISTINCT(user_id))
FROM visitors

This query will return a single value - the number of unique `user_id` in the `visitors` table.

SQL is a domain-specific programming language widely used by data scientists, analysts, etc for manipulating and extracting necessary data. Nowadays, SQL is a must-have point in the CV for every potential analyst, data scientist. In this course, you will be introduced to the very basics of SQL.

In this section, you will be introduced to the very basics of SQL: extracting one or multiple columns from a database.

In this section, you will learn how to filter observations so only necessary rows will be extracted.

In this section, you will learn about the usage of different aggregating functions for your table, such as minimum, maximum, average... 

In this section, you will learn how to sort your table, even having filters and aggregate functions applied to a table.

In this section, you will learn about grouping observations and usage of this principle.

This chapter is designed for users who have completed the previous sections and want to consolidate knowledge.

How many unique?

Solution