Null

Agnieszka Kozubek-Krycuń

NULLs and Handling Missing Data in SQL

Handling missing data (i.e. NULLs) in SQL can be challenging. NULLs can pose a lot of traps, especially if you don’t understand how they work. In this article, we’ll talk about dealing with NULL in SQL. We’ll also explain how to avoid common mistakes when working with NULLs. Having missing data in your database is an unavoidable reality of life. There are many reasons why you may have missing data in your database: the complete data isn’t yet available, users provide incomplete information, changes to the database schema, database malfunctions, human error, and more.

How to Select NULL Values in SQL

Problem: You want to select rows with the NULL value in a given column. Example: Let's see a table named Employees with the following columns: EmployeeID, EmployeeName, Dept, and Salary. Assume the Salary column allows NULL values. EmployeeIDEmployeeNameDeptSalary 1Jack RusselManager5600 2Jan KowalskiHRnull 3John DoeHR4500 4Mark RusselSales9000 5Jane DoeSalesnull Now, let’s retrieve all records where the Salary column contains NULL values.

Ignacio L. Bisso

How to Use the COALESCE() Function in SQL

SQL users are often faced with NULL values in their queries and need to process them properly. The COALESCE() function helps handle NULL values. Read this article to learn how to use COALESCE() in your queries. SQL tables store data in records, and records are composed of fields. There can be situations where we don’t know the value for a specific field. For example, when we don’t know the marital status of a person, SQL allows us to assign a NULL for this field.

James Wondrasek

How to Use Comparison Operators with NULLs in SQL

The SQL NULL value serves a special purpose. It also comes with counterintuitive behaviors that can trip up SQL beginners or even experienced programmers. Learn how to avoid these problems when you use NULL with comparison operators. This article is going to help you master best practices for crafting SQL queries that work with NULL values and use comparison operators ( =, <>, <, > ) – which, if you have written any SQL queries before, you know is just about every query you will ever write!

Tihomir Babic

How to Find the Next Non-NULL Value in SQL

You need to find the next non-NULL value in a time series, but you don’t know how. Can you even do that in SQL? Yes, you can! This article will show you what to do. If you work with SQL, you will sooner or later confront NULL values. Having NULLs in a database is almost unavoidable. However, sometimes you want to avoid them in your reports. This is quite often true when you’re analyzing time series data; NULL values mean there’s no data available.

Ignacio L. Bisso

What Is a NOT NULL Constraint in SQL?

SQL has several ways of dealing with NULLs. In this article, we’ll focus on the NOT NULL constraint and other NULL-related clauses. Understand this and you’ll take an important step in your SQL growth! Before we get straight into the NOT NULL constraint, let’s quickly define what NULLs and constraints are. NULL values are a central concept in SQL databases. The idea behind a NULL value is simple: a NULL value means that we do not have a known value for that field.

Gustavo du Mortier

What Is a NULL in SQL?

SQL uses NULLs as a special flag that signals the lack of a value for a field or a variable. NULLs should be used wisely so the database gives a faithful picture of the reality it represents. Some people associate NULLs with zeros; others think that NULL means “none”. In SQL, neither of these are true. Simply put, an SQL NULL means that we don’t have a value for that particular field.

Kateryna Koidan

How ORDER BY and NULL Work Together in SQL

Do NULL values come first or last when you use ORDER BY? Are they considered higher or lower than non-NULL values? In this article, I’ll explain how different relational databases treat NULL values when sorting output and how to change the default behavior of the ORDER BY clause. When LearnSQL users practice the ORDER BY clause in our SQL Basics course, they often ask why NULL values appear first in the output and how they can change this behavior.

How to Find Records with NULL in a Column

Problem You want to find records with NULL in a column. Example Our database has a table named children with data in four columns: id, first_name, middle_name, and last_name. idfirst_namemiddle_namelast_name 1JohnCarlJackson 2TomNULLThomson 3LisaAliceNULL 4AnneNULLSmith Let’s find the id, first name, and last name of children without a middle name (NULL in column middle_name). We use the IS NULL operator for this. Solution SELECT id, first_name, last_name FROM children WHERE middle_name IS NULL; Here’s the result:

How to Filter Rows without NULL in a column

Problem You want to find records without a NULL in a column. Example Our database has a table named product with data in three columns: id, name, and price. idnameprice 1butterNULL 2milk2.35 3bread3.25 4cheeseNULL Let’s find the names and prices of products that have a price (without a NULL). Do this by using the IS NOT NULL operator.

Ignacio L. Bisso

The SQL Coalesce Function: Handling Null Values

You may already know how to return null values in SQL. Now, we’re going to learn how to do the opposite. Though the SQL COALESCE function may seem complex, it’s actually very straightforward and useful. Let’s look at several examples of how the SQL COALESCE function can be used to work with NULL values in SQL. The Need for Coalesce in SQL Before we dive into the SQL COALESCE function in detail, you should understand how NULL values behave in expressions.

Aldo Zelen

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 related to SQL data matching.

Maria Alcaraz

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. The best way to master GROUP BY and NULL in SQL is through practice. I recommend the SQL Practice track at LearnSQL.

Maria Alcaraz

Understanding the Use of NULL in SQL Three-Valued Logic

NULLs are necessary in relational databases, and learning to use them is fundamental to SQL success. However, NULLs should also be handled with care, as we explain in this post. In relational databases, we don’t always have a value to put in a column. For example, suppose we have a table called “persons” that has “first_name”, “last_name”, “birth_date” and “marriage_date” columns. What value will we store in the “marriage_date” column for single persons?

Patrycja Dybka

How to Tackle SQL NULLs: COALESCE function

It's inevitable that some data in the database has no value what in SQL is represented by the NULL keyword. "No value" here is different from zero, false, or an empty string (but with exceptions! In Oracle database, NULL is the same as string of zero length). During playing with the database, treating NULLs are becoming more and more troublesome, therefore MySQL standard provides help with some of the functions, like COALESCE.