Articles Cookbook
4 Feb 2021
Martyna Sławińska

What Is a Unique Constraint in SQL?

To ensure the uniqueness of data in a database, we use the SQL UNIQUE constraint. In this article, we’ll discuss how, when, and why to implement it. The idea of a unique constraint is not unique to SQL; it’s a familiar concept in real life, although we probably call it something else! For example, think of ID numbers. There can be only one social security number (SSN) per person and each one must be unique.
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3 Feb 2021
Rebecca McKeown

How Much SQL Do I Need to Know?

Learning a new skill or starting a new profession can be a real challenge, especially when you’re unsure of how much you really need to know before taking the plunge. Imposter syndrome is also a real thing – how could you possibly catch up with all those SQL ‘experts’ who seem like they’ve been doing this since they were in diapers? The good news is you can learn SQL quickly and easily, and you don’t need to know nearly as much as you think you do.
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2 Feb 2021
LearnSQL.com Team

Course of the Month: SQL JOINs

If you want to really learn SQL, you can’t skip SQL JOINs. They are some of the most fundamental and commonly used features of the SQL language. That’s why SQL JOINs is our February SQL Course of the Month. What makes SQL JOINs so important? I asked Agnieszka Kozubek-Krycuń, Vertabelo's Chief Content Officer, about it. Here's what she said. Is it true that SQL JOINs are one of the foundations of SQL?
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29 Jan 2021
Ignacio L. Bisso

Data Types in SQL

Read this article and learn the ABC of SQL data types. Data types are used to define the type of data stored in a database. However, there are related concepts that a good SQL developer should know: data type conversion, what operations are possible between different data types, date arithmetic, etc. SQL also provides functions that manipulate values of different data types (i.e. functions for string data, date and time data, etc.
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28 Jan 2021
Andrew Bone

Using AND, OR, and NOT Operators in SQL

AND, OR, and NOT are important logical operators in SQL. They help you combine the conditions used to filter records. They are most commonly used in conjunction with the WHERE or HAVING clauses. Furthermore, this topic is a pretty common SQL job interview question. To help you completely understand these operators, I’ll take you through some examples, fully explaining the syntax and purpose of each. Practicing using examples like these is one of the best methods for learning SQL.
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27 Jan 2021
Agnieszka Kozubek-Krycuń

Why Aren’t SQL Window Functions Allowed in GROUP BY?

It’s true – the SQL order of operations means window functions aren’t allowed in GROUP BY. But you can use them anyway with this special trick. SQL window functions are one of the language’s most powerful features. However, the syntax of window functions is not easy to master. It has lots of details that can cause beginners to stumble. One of the most common traps is trying to use window functions in GROUP BY.
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26 Jan 2021
Marija Ilic

7 Common GROUP BY Errors

Are you making these errors with GROUP BY in SQL? Find out what they are, how to avoid them, and how to fix them. SQL’s GROUP BY statement can quickly uncover powerful data insights. At first, using GROUP BY can seem easy – e.g. when creating basic SQL reports that you’ll present to business decision-makers. But while learning this powerful feature, you could become trapped in weird errors or get incorrect results caused by improperly written GROUP BY statements.
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22 Jan 2021
Zahin Rahman

Why Learn SQL Window Functions in 2025?

SQL window functions provide some extremely powerful and convenient features for data practitioners. This article explains why you should learn window functions and when you should use them. I’ll also provide some examples to help you visualize the concepts. Window functions are part of advanced modern SQL – knowledge of which will definitely enrich the repertoire of any SQL user. Window functions have a broad range of uses and can be applied to almost all facets of business.
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21 Jan 2021
Zahin Rahman

Where Can I Find Good SQL CTE Exercises?

Find the top online resources for SQL Common Table Expression (CTE) exercises! CTEs allow you to structure and organize SQL queries efficiently, which is essential if you want to advance your SQL knowledge. A Common Table Expression, or CTE, is a SQL syntax that creates a temporary data set. This set contains separate data than the main query, which can typically be referenced or reused in a subsequent query. A CTE is considered temporary because the result is not permanently stored anywhere and only exists for the duration of the query.
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20 Jan 2021
Jakub Romanowski

Stories of the LearnSQL Scholarship Program

2020 is behind us. It was an extremely difficult year: We’ve struggled with the Covid-19 pandemic, a worldwide economic crisis, and constant anxiety. Many people have lost their jobs and their prospects for a better tomorrow. That's why we decided to do something positive: establishing a SQL scholarship program. We decided to find people in difficult situations and try to help them. For many, learning SQL can be their chance to get a job and develop their career.
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