Articles Cookbook

Author: Agnieszka Kozubek-Krycuń

Agnieszka is a Chief Content Officer at Vertabelo. Before coming to Vertabelo, she worked as a Java programmer. She has a PhD in mathematics and over 10 years of experience in teaching mathematics and computer science at the University of Warsaw. In her free time, she enjoys reading a good book, going mountain hiking and practicing yoga.

Why Take the “SQL Basics” Course at’s interactive “SQL Basics” course teaches the foundations of SQL. Discover why we built this online course, our philosophy behind it, and what it contains! What Is SQL, and Why Should You Learn It? SQL is a language used to talk to databases, computer programs that hold data and can process it efficiently. SQL can select data from a database and perform various computations on the data, be they simple or complex.

How to Get the First Day of the Week in SQL Server

This is additional content for the course Customer Behavior Analysis in SQL Server. In the previous article, we talked about how you can use the SQL Server DATEPART() function with week or iso_week to group events by week. The query might look like this: SELECT DATEPART(week, RegistrationDate) AS Week, COUNT(CustomerID) AS Registrations FROM Customers WHERE '20180101' The result looks something like this: WeekRegistrations 1 58 2 123 .

How to Group Data by Week in SQL Server

This is additional content for the course Customer Behavior Analysis in SQL Server. In this course, we showed you how you can analyze the customer lifecycle (customer acquisition, conversion, activity, retention, and churn) with SQL. We discussed customer registration cohorts, or groups of customers who registered during the same period (e.g. same week, same month). Analyzing customer registration cohorts lets you see registration trends and relate registration cohorts with marketing campaigns.

Why Window Functions Are Not Allowed in WHERE Clauses

Window functions are extremely powerful, but there are a lot of nuances and little details you need to learn about using window functions in order to use them correctly and efficiently. A fairly common question SQL users have is why window functions are not allowed in WHERE. TL;DR: You can use window functions in SELECT and ORDER BY. You cannot use window functions in WHERE, GROUP BY, or HAVING.

GROUP BY Clause: How Well Do You Know It?

The database can do different kinds of computations: it can add and subtract, multiply and divide, it can do computations on dates, it can count rows and sum row values, and so on. It can also do quite sophisticated statistics. The GROUP BY clause is the most basic way to compute statistics in SQL. It can be quite tough for beginners but it is really powerful. Let's look at the details of the GROUP BY clause, starting with the basics.