An Illustrated Guide to Multiple Join

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So far, our articles in the “An Illustrated Guide” series have explained several join types: INNER JOINs, OUTER JOINs (LEFT JOIN, RIGHT JOIN, FULL JOIN), CROSS JOIN, self-join and non-equi join. In this final article of the series, we show you how to create SQL queries that match data from multiple tables using one or more join types. Join Types in SQL Queries Before we start discussing example SQL queries that use multiple join types, let’s do a short recap of the join types we’ve covered so far, just to be sure you understand the differences.

An Illustrated Guide to the SQL Non Equi Join

Did you know that in SQL, a join doesn’t have to be based on identical matches? In this post, we look at the SQL non equi join, which uses ‘non-equal’ operators to match records. We’ve already discussed several types of joins, including self joins and CROSS JOIN, INNER JOIN and OUTER JOIN. These types of joins typically appear with the equals sign (=). However, some joins use conditions other than the equals (=) sign.

An Illustrated Guide to the SQL Self Join

What is an SQL self join and how does it work? When should it be used? We’ll provide answers to those questions! In SQL, we can combine data from multiple tables by using a JOIN operator. JOIN has several variants; we’ve already discussed CROSS JOIN, INNER JOIN, and OUTER JOIN. Most of the time, these operators join data from two or more different tables. In this article, however, we will explain how to join records from the same table.

So You Want to Be a Data Scientist?

A career in data science is hot right now. What is a data scientist, and how can you become one? Very few of us said “I want to grow up to be a data scientist” when we were kids. But now, in the age of Big Data and economic uncertainty, a career in data science is looking mighty attractive. If you like the idea of working with information and earning a good paycheck, read on.

What Is Vertabelo’s SQL Cheat Sheet?

Everyone, from rookie programmers to SQL ninjas, needs a bit of help now and again. Enter Vertabelo's SQL Cheat Sheet! Working with databases is hard. There's a lot of data to manage, which can be daunting. But the main challenge many database users face is the SQL commands themselves. It's all right when you're using the same familiar ones day after day, but what about the commands you dust off once or twice a year?

Data Science: Market Demand or Just Hype?

Earth’s 7.5 billion people together use several billion different devices, generating an annual global IP traffic of more than one zettabyte. Out of these impressive numbers rises a new field above the others — “data science”. Is data science an inevitable reality, or will it yet be dismissed as just another “wave of the future” that never came to be? A few days ago I signed a new employment contract with my company.

The 5 Highest Paying Jobs That Use SQL

IT provides a wide and growing job market, offering many excellent opportunities in both career development and earning potential. Today we’ll take a look at several very interesting types of database jobs that use SQL. Every year, many in the work force set their sights and aim for new and better jobs. When it comes to jobs, “better” means different things to different people, but earning potential is almost always a top priority.

How to Track Down Duplicate Values in a Table

When it comes to information management, duplicates present one of the most common challenges to data quality. In this article, I’ll explain how it is possible to find and distinguish duplicate names with the help of the SQL data programming language. I really like my maiden name. The reason I like it so much is because it’s rare. My maiden name (first with last) provided a unique identifier on platforms such as LinkedIn, Facebook, Twitter and similar.

Common SQL Window Functions: Positional Functions

Positional SQL window functions deal with data’s location in the set. In this post, we explain LEAD, LAG, and other positional functions. SQL window functions allow us to aggregate data while still using individual row values. We’ve already dealt with ranking functions and the use of partitions. In this post, we’ll examine positional window functions, which are extremely helpful in reporting and summarizing data. Specifically, we’ll look at LAG, LEAD, FIRST_VALUE and LAST_VALUE.

Four Reasons Aspiring Data Scientists Must Learn SQL

Isn’t data science way too advanced for simple SQL? Nope! SQL can help you build a foundation for your data science career. Let’s see how. Data science is hot right now. What if you could predict the next market crash? Or contain the spread of Ebola? Or accurately predict a health crisis months or even years before it happens? Data scientists are working hard on these kinds of projects, and they are earning healthy salaries in the process.

Kill Online Distractions: 5 Tools to Help You Stay Focused

Phones, emails, social media accounts, and about a hundred other things are constantly competing for our attention. Can tech tools help us stay focused? A lot of apps say that they help humans work better and faster. But the truth is that the more interactive tools we use, the more focus-shattering notifications we get. In many ways, technology has gotten us into this problem. Can it help us get out of it?

Kill Online Distractions: 5 Tools to Help You Stay Focused

Phones, emails, social media accounts, and about a hundred other things are constantly competing for our attention. Can tech tools help us stay focused? A lot of apps say that they help humans work better and faster. But the truth is that the more interactive tools we use, the more focus-shattering notifications we get. In many ways, technology has gotten us into this problem. Can it help us get out of it?

SQL Mythbuster – 5 Reasons Why No One Should Be Afraid of SQL

Anyone can learn SQL. It’s not as hard as you think! In today’s world, even those in non-technical jobs need some technical skills. And you don’t have to be a hard-core nerd to get these skills. Let me tell you my own story. A few years ago, I wasn’t considering learning SQL or anything else that I labelled “technical”. My background is in sociology, journalism, and scriptwriting, and I thought computer languages were pretty sci-fi.

Referential Constraints and Foreign Keys in MySQL

Foreign keys and referential constraints allow you to set relationships between tables and modify some of the database engine’s actions. This beginner’s guide explains referential integrity and foreign key use in MySQL. One of the most important aspects of database usage is being able to trust the information you store. Database engines provide several features that help you maintain the quality of your data, like defining required columns as NOT NULL and setting an exact data type for each column.

An Illustrated Guide to the SQL OUTER JOIN

We’ve already discussed the SQL CROSS JOIN and INNER JOIN statements. It’s time to explore another: OUTER JOIN. What is it? How does it work? Let’s find out! If you’ve read our other posts, you know that you can link the data in two or more database tables using one of the many types of SQL join operator. Today, we’ll discuss the three kinds of OUTER JOIN: LEFT OUTER JOIN, RIGHT OUTER JOIN, and FULL OUTER JOIN.

Jane Solves Her Business Analysis Problem

Jane has a data problem and an overdue report. Will SQL save the day? Jane works as a business analyst for a large London bank. As she walks to the station on Monday morning, she’s thinking about last week’s unfinished work. She couldn’t get the data she needed from IT, so she could not deliver her report on a financial product’s non-performing assets. She absolutely must submit the report to her manager today; she’s already delayed it for more than a week.

Common SQL Window Functions: Using Partitions With Ranking Functions

You’ve started your mastery of SQL window functions by learning RANK, NTILE, and other basic functions. In this article, we will explain how to use SQL partitions with ranking functions. Mastering SQL window functions (AKA analytical functions) is a bumpy road, but it helps to break the journey into logical stages that build on each other. In the previous Common SQL Functions article, you learned about the various rank functions, which are the most basic form of window functions.

Common SQL Window Functions: Using Partitions with Ranking Functions

You’ve started your mastery of SQL window functions by learning RANK, NTILE, and other basic functions. In this article, we will explain how to use SQL partitions with ranking functions. Mastering SQL window (or analytical) functions is a bumpy road, but it helps to break the journey into logical stages that build on each other. In the previous Common SQL Functions article, you learned about the various rank functions, which are the most basic form of window functions.

SQL Window Functions by Example

Interested in how SQL window functions work? We use some simple examples to get you started. SQL window functions are a bit different; they compute their result based on a set of rowsrather than on a single row. In fact, the “window” in “window function” refers to that set of rows. Window functions are similar to aggregate functions, but there is one important difference. When we use aggregate functions with the GROUP BYclause, we “lose” the individual rows.

How Recursive Common Table Expressions Work

Recursive Common Table Expressions are immensely useful when you’re querying hierarchical data. Let’s explore what makes them work. Common Table Expressions (CTEs) are some of the most useful constructions in SQL. Their main purpose is improving query design, which makes queries easier to read. One of the reasons CTEs are so popular is that they let you divide longer queries into shorter subqueries. These are easier to read and edit.

Opening the Door to SQL Window Functions

When you want to analyze data in a database, you need SQL window functions. What are they? How do they work? Read on to find out. There are two ways people use databases. One way is for the creation, modification and deletion of data. The other is analyzing the data, which means getting answers to specific questions. The more precise the questions you ask, the more tools you’ll need to answer them efficiently.

High Performance Statistical Queries –Skewness and Kurtosis

In descriptive statistics, the first four population moments include center, spread, skewness, and kurtosis or peakedness of a distribution. In this article, I am explaining the third and fourth population moments, the skewness and the kurtosis, and how to calculate them. Mean uses the values on the first degree in the calculation; therefore, it is the first population moment. Standard deviation uses the squared values and is therefore the second population moment.

An Illustrated Guide to the SQL INNER JOIN

What is an SQL INNER JOIN, and how does it work? Let’s find out! In my last article, I discussed the CROSS JOIN operation in SQL. Today, we’ll look at INNER JOIN and how to use it. Is it the same as a JOIN? How many tables can you link with an INNER JOIN? These are all good questions. Let’s look at the answers! What is an INNER JOIN? INNER JOIN combines data from multiple tables by joining them based on a matching record.

Common SQL Window Functions: Ranking Functions

Want to learn how to use SQL window functions? Ranking functions are a good place to start! Learning about SQL window functions usually comes after you’ve built a foundation in the language, but these powerful functions take your skills up a level. As you master them, you’ll find better ways to solve query problems. When used for business intelligence applications, SQL queries combine data retrieval and advanced computations. These operations are more complex than those used in OLTP systems.

5 Documentaries for People Who Love the Internet

Here’s a shock: not everything in the movies is true. But these five documentaries offer interesting insights into the Internet and its people. Non-tech people tend to consider coding, hacking, and anything remotely dev-y as mysterious and magical processes. This myth is helped along by the movie industry, and there’s nothing wrong with that. Movie watchers generally like thrilling, action-packed stories, and coding can be anything but exciting. But not every good movie needs to be filled with action; some get our brains going by presenting new ideas or interesting viewpoints.

High Performance Statistical Queries in SQL: Part 3 – Measuring the Spread of a Distribution

Besides knowing the centers of a distribution in your data, you need to know how varied the observations are. In this article, we’ll explain how to find the spread of a distribution. Are you dealing with a very uniform or a very spread population? To really understand what the numbers are saying, you must know the answer to this question. In the second part of this series, we discussed how to calculate centers of distribution.

An Illustrated Guide to the SQL CROSS JOIN

What is an SQL CROSS JOIN statement? When should you use it? When shouldn’t you use it? This post will tell you what you need to know about CROSS JOIN. You already know that you can use the SQL JOIN statement to join one or more tables that share a matching record. And if you’re read the Vertabelo Academy post Learning SQL JOINs Using Real Life Situations, you know that there are many types of JOINs.

An Introduction to Using SQL Aggregate Functions with JOINs

-- -- Previously, we've discussed the use of SQL aggregate functions with the GROUP BY statement. Regular readers of the Vertabelo Academy blog will also remember our recent tutorial about JOINs. If you're a bit rusty on either subject, I encourage you to review them before continuing this article. That's because we will dig further into aggregate functions by pairing them with JOINs.

Why I Choose Learning Apps … And You Should Too

Learning from books is so 1999. Can app-based learning do any better? Do you like wasting your time? I don’t. Any time I start something, I find out how to do it efficiently. For me, this means learning new skills using apps and online learning rather than books. Online and app-based learning focuses on you, the student. This is the exact opposite of traditional learning, which is teacher-driven. There is a physical classroom and students, but the teacher plays the primary role.

A Beginner’s Guide to SQL Aggregate Functions

Aggregate functions are powerful SQL tools that compute numerical calculations on data, allowing the query to return summarized information about a given column or result set. These functions can be used in conjunction with the GROUP BY statement. Let’s see how they work using some easy examples. SQL Aggregate Functions Suppose we have users residing in a city, and we store their information in two tables. These tables and their relationship are shown below:

Introducing SQL Set Operators: Union, Union All, Minus, and Intersect

-- -- -- -- -- When you need to combine information from multiple tables or queries, SQL set operators are a useful tool. SQL queries let us choose the most important bits from large amounts of information. Of course, we can’t expect that all necessary data will be stored in one table. Let’s say we want to present every aspect of some key data group in one results table (e.

Useful SQL Patterns: Date Generator

As you start coding in SQL, you will use some statements and techniques over and over again. We call these “SQL patterns”. This series will look at the most common SQL patterns and consider how to use them. SQL patterns, such as the pivot pattern we discussed last week, can save you a lot of time and effort. Suppose you are asked to get a range of days in a financial quarter, but the only records you have are for the start and end dates of each quarter.

Useful SQL Patterns: Pivoting

As you start coding in SQL, you will use some statements and techniques over and over again. We call these “SQL patterns”. This series will look at the most common SQL patterns and consider how to use them. Pivoting in SQL refers to taking the data in table rows and making that data into columns. This is very important in reporting, and it’s easy to do when you use the CASE statement.

Learning SQL JOINs Using Real-Life Situations

-- The JOIN statement lets you work with data stored in multiple tables. This article is a practical introduction to the SQL JOIN. Imagine if you could only work with one database table at a time. Fortunately, this isn’t anything we have to worry about. Once you learn the JOIN statement, you can start linking data together. This article will give you examples that illustrate how we use JOINs, how each type of JOIN works, and when to use each type.

Useful SQL Patterns: Conditional Summarization with CASE

As you start coding in SQL, you will use some statements and techniques over and over again. We call these “SQL patterns”. This series will look at the most common SQL patterns and consider how to use them. Previously, we looked at the SQL pattern of matching NULLs. This is important when you are comparing columns containing NULL values. Today, we’re going to consider another SQL practice: conditional summarization with CASE operator.

Using CASE with Data Modifying Statements

What happens when you combine CASE with SQL’s data modifying statements? Find out in this article. The CASE expression is a very useful part of SQL and one that you’ll employ frequently. We’ve already covered what the CASE expression does, how to format it, and how to use it in a SELECT statement in “Using CASE to Add Logic to a SELECT”. Another article, “How to Sort Records with the ORDER BY Clause” demonstrated how to use CASE in an ORDER BY clause.

FROM: A Clause with Plenty of Possibilities

Think that FROM only specifies tables in an SQL statement? Think again … When I first began to work with SQL, I saw the FROM clause as the simplest part of any query. You’d put only one table there, or may be two or more tables in the case of a join. That was all I thought about the FROM clause in those days. As I began to learn more about SQL, I discovered that there are a lot of techniques that can be used in a FROM.

Useful SQL Patterns: Matching Nulls by Masking Nulls

As you start coding in SQL, you will use some statements and techniques over and over again. We call these “SQL patterns”. This series will look at the most common SQL patterns and consider how to use them. In database development, SQL developers often find themselves returning to the same SQL statements. Learning about these now, early in your SQL journey, will help you work more efficiently. Today, in the first post of this series, we will consider the match by null SQL pattern.

Using Transactions to Prevent Database Errors

What happens when a database receives commands from two different users? We look at the problems that can arise and how to avoid them. Every time you execute a statement in your database, you change the database’s state. When working in a multi-user environment with asynchronous database access, it will sometimes happen that two users are trying to change the same record at the same time. If both of their statements is an operation that changes the database’s state, this can create erroneous results.

How to Sort Records with the ORDER BY Clause

Relational databases don’t store records in alphabetical, numerical, ascending, or in any particular order. The only way to order records in the result set is to use the ORDER BY clause. You can use this clause to order rows by a column, a list of columns, or an expression. You can also order rows using the CASE expression. In this post, we’ll take a look at the ORDER BY clause – how to write it, how it works, and what it does.

Oracle Top-N Queries for Absolute Beginners

It’s common to run a query using only part of a dataset – for example, the top 100 salespeople in a company. In this article, we’ll see how to use Oracle’s Top-N query method to query just these rows. Top-N queries retrieve a defined number of rows (top or bottom) from a result set. In other words, they find the best or worst of something – the ten best selling cars in a certain region, the five most popular routers, the 20 worst-performing stores, etc.

High-Performance Statistical Queries in SQL: Part 2 – Calculating Centers of Distribution

My previous article explained how to calculate frequencies using T-SQL queries. Frequencies are used to analyze the distribution of discrete variables. Today, we’ll continue learning about statistics and SQL. In particular, we’ll focus on calculating centers of distribution. In statistics, certain measurements are known as moments. You can describe continuous variables (i.e. a variable that has a large range of possible numbers, such as household incomes in a country) with population moments.

Using CASE to Add Logic to a SELECT

As you write an SQL query, you may need to get values from multiple columns and change values from one form to another. The simple way to achieve this goal is to add a CASE expression to your SELECT statement. In this article, we’ll introduce you to the syntax, formats, and uses of the CASE expression. The CASE expression is a conditional expression: it evaluates data and returns a result.

Book Review: “Learn SQL the Hard Way” by Zed A. Shaw

“Learn SQL the Hard Way” by Zed A. Shaw is a great ebook for those who want to learn SQL essentials. At $19.99 USD, it may seem a bit expensive for an ebook, but you’re getting a DRM-free PDF, plus a whole bunch of explanatory videos and additional files. Who should read “Learn SQL the Hard Way”? Anyone interested in SQL basics – modifying databases or selecting information. Make no mistake about it: this book focuses heavily on the practical side of database language.

NULL Values and the GROUP BY Clause

We’ve already covered how to use the GROUP BY clause and some aggregation functions like SUM(), AVG(), MAX(), MIN(), COUNT(). In this article, we will explain how the GROUP BY clause works when NULL values are involved. We will also explain about using NULLs with the ORDER BY clause. In SQL, NULL is a special marker used to indicate that a data value does not exist in the database. For more details, check out Wikipedia’s explanation of NULL in SQL.

Vertabelo Academy Good Book Review: “SQL Cookbook” by Anthony Molinaro

Recipes that make querying easy, even for the novice. In today’s information age, we have at our fingertips almost an overabundance of teaching and information on any and every subject. In the midst of this glut of information, it can be hard to decide which is the best source for our needs. But, fear not; we are here to assist you as you navigate through decisions about books on SQL topics!

Using LIKE to Match Patterns in SQL

Matching patterns is a very important and useful ability. In this article, we look at how you can match patterns using the SQL LIKE operator. Pattern matching is a very simple concept. It allows you to search strings and substrings and find certain characters or groups of characters. You can find pattern matching mechanisms in SQL and in other programming languages. In this article, we’ll examine how the LIKE operator can be used to search substrings.

Understanding Numerical Data Types in SQL

-- -- -- Working with databases of any kind means working with data. This data can take a couple of predefined formats. As you start on your learning path with Vertabelo Academy, you will start to understand SQL’s different data types. In this article, we will cover the numeric data types of ANSI SQL. We’ll also examine some functions that convert data from one type to another.

5 Functions for Manipulating SQL Strings

-- -- -- -- SQL string functions are among most important SQL’s tools. In this post, we’ll look at five ways you can perform various operations on strings. There are many SQL functions that let you “edit” string data. Some sql trim off unneeded spaces or characters; others tell you how long a string is. These functions give you a lot of opportunities to transform and work with strings, which makes your code more effective.

Preventing Common SQL Mistakes

Regardless of the engine you are using (SQL Server, mySQL, Oracle, etc), you can prevent common errors and simplify the debugging process. This article will discuss some of the issues you’ll face and will help you tackle them successfully. One of the best ways to prevent SQL errors is to keep your queries clear and readable. It’s very easy to forget the ideas behind your code! If you have to revisit it (and you will), messy code can be hard to understand.

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