What is SQL NULL Value?

Imagine you are in charge of distributing food packets for an office party. Each person will get one packet. You have made a list of all the employees’ names with a place against each of them for them to sign. In case of somebody is absent, you put a dot against his name.Now imagine you are handling the same situation again but this time electronically. You have created a small application having a small database containing the employee names attached to it. Whenever an employee takes a food packet, you mark him as present in the system, how the system handles a situation where a particular employee is absent. It puts a null value against his or her name.

Null is a way for the database to mark an entry as blank.Over the next few paragraphs, we will learn about SQL NULL Values. Or to know in-depth about SQL, else you can also enroll in an online SQL server training course and shape your ever-growing SQL career.

What is Null in SQL 

A field without a value is one with the value NULL. It is possible to insert a new record or update an existing record without adding a value to a field in an optional table. The value of the field will then be saved as NULL. Note: A NULL value is distinct from a zero or space-filled field.

Different Types of SQL Server Null Functions

Following are the different types of SQL Server Null Functions

ISNULL():

The ISNULL function has different uses in SQL Server and MySQL. In SQL Server, the ISNULL () function replaces NULL values.

Syntax:

SELECT column(s), ISNULL(column_name, value_to_replace)
FROM table_name;
Let us take the case of [dbo].[Employee_Null_Test]

The SQL query looks like below

Query: Find the sum of the salary of all Employees; if the Salary of any employee is not available (or NULL value), use salary as 10000. 

SELECT SUM(ISNULL(Salary, 10000) AS Salary

FROM Employee:

The output looks like below

IFNULL():

This function is available in MySQL, not SQL Server or Oracle. This function takes two arguments. If the first argument is not NULL, the function returns the first argument. Otherwise, the second argument is returned. This function is commonly used to replace a NULL value with another value.

Syntax:

SELECT column(s), IFNULL(column_name, value_to_replace)
FROM table_name;
Let us again take the case of [dbo].[Employee_Null_Test].

Query: Find the sum of the salary of all Employees; if the Salary of any employee is not available (or NULL value), use salary as 10000.

SELECT SUM(IFNULL(Salary, 10000)) AS Salary
FROM [dbo].[Employee_Null_Test];

The output looks like below

COALESCE():

COALESCE function in SQL returns the first non-NULL expression among its arguments. The COALESCE function will return null if all the expressions evaluate to null.

Syntax:

SELECT column(s), COALESCE(expression_1,....,expression_n)
FROM table_name;

Example:

Consider the following Contact_info table,

Query: Fetch the name and contact number of each employee.

SELECT Name, COALESCE(Phone1, Phone2) AS Contact

FROM Contact_info;

The output looks like below

NULLIF():

The NULLIF function takes two arguments. If the two arguments are equal, then NULL is returned. Otherwise, the first argument is returned.

Syntax:

SELECT column(s), NULLIF(expression1, expression2)

FROM table_name;

Example:

Consider the following [dbo].[Employee_Null_Test];

Query

SELECT Store, NULLIF(Actual, Goal)

FROM [dbo].[Employee_Null_Test];;

The output looks like below

Importance of SQL Null Value

The importance of NULL in SQL Server is as follows.

• It is essential to comprehend the distinction between a NULL value and a zero value.
• A NULL value is used to indicate a missing value, but it can typically be interpreted in one of three ways:
• The value is unknown (a value exists but is unknown);
• The value is not available (a value exists but is intentionally withheld); and
• The attribute is not applicable (undefined for this tuple).
• It is frequently impossible to ascertain which of the meanings is intended. As a result, SQL does not differentiate between the various meanings of NULL.

Principles of NULL Values in SQL

The properties of Null values in SQL are as follows.

• When the actual value is unknown or would not be meaningful, setting a NULL value is appropriate.
• If the data type is a number, a NULL value is not equivalent to zero; if the data type is a character, a NULL value is not equivalent to spaces.
• Any data type's columns can contain a NULL value.
• In any expression, a NULL value will evaluate as NULL.
• SQL will ignore UNIQUE, FOREIGN key, and CHECK constraints if any column contains a NULL value.

How to Implement Null in Sql Server

The following SQL ensures that the "ID," "LastName," and "FirstName" columns will NOT accept NULL values, and Age can accept the null value when the "Persons" table is created:

CREATE TABLE Persons (
ID int NOT NULL,
LastName varchar(255) NOT NULL,
FirstName varchar(255) NOT NULL,
Age int NULL
);

The output looks like the one below.

The database's NULL values are generally regarded as distinct from one another. The outcome of a comparison operation is deemed to be UNKNOWN when it involves a NULL. As a result, SQL makes use of logic with three values: True, False, and Unknown. When the logical connectives AND, OR, and NOT are used, defining the results of three-valued logical expressions is necessary.

How to Test NULL Values in SQL

SQL allows queries to determine whether a value in an attribute is NULL. SQL uses IS and IS NOT rather than = or NULL to compare an attribute value to NULL. This is because equality comparison is inappropriate. After all, SQL treats each NULL value as distinct from all other NULL values. Do you love working with data? Or want to pursue a career in the Microsoft SQL Server Database domain? But feel stuck with doubts? A comprehensive SQL career path will help you explore all the career options.

Now let us create an Employee table with the following SQL Statement.

create table Employee_Null_Test
(
Fname varchar(max) NOT NULL,
Lname varchar(max) NOT NULL,
SSN integer Not Null,
Salary integer Not Null,
Super_ssn integer Null
)

The structure looks like the one below.

Let us now insert some data. The script to insert data in the table is as below.

INSERT [dbo].[Employee_Null_Test] ([Fname], [Lname], [SSN], [Salary], [Super_ssn]) VALUES (N'John', N'Smith', 123456789, 300000, 3334)
INSERT [dbo].[Employee_Null_Test] ([Fname], [Lname], [SSN], [Salary], [Super_ssn]) VALUES (N'Fraklin', N'Wong', 33445566, 40000, 7788)
INSERT [dbo].[Employee_Null_Test] ([Fname], [Lname], [SSN], [Salary], [Super_ssn]) VALUES (N'James', N'Borg', 88995566, 55000, NULL)

The output looks like the below.

Suppose we find the Fname and Lname of the Employee having no Super_ssn then the query will be:

SELECT Fname, Lname FROM  [dbo].[Employee_Null_Test] WHERE Super_ssn IS NULL;

The output looks like below

Now if we find the Count of the number of Employees having Super_ssn, the query looks like the one below.

SELECT COUNT(*) AS Count FROM [dbo].[Employee_Null_Test] WHERE Super_ssn IS NOT NULL;

The output looks like the one below.

Advantages of SQL Null Values

The Advantages of Null Constraints in SQL Server

• When using internal functions, these values are automatically excluded from the databases.
• Having NULLs in your database improves logic when programming. There is no need to consider multiple options for placeholder values. Utilizing your logic, you simply display content to your users and test for NULL. You would need to write code for each of your different placeholders if you had multiple of them.
• Databases make it possible to enter NULL into a field without value automatically. Because of this, you won't have to create a value for each placeholder. Any database programmer will recognize NULL as the default placeholder if you just use it.

Disadvantage of SQL Null Constraint

The disadvantages of Null Constraint are as follows.

• The first drawback of using null constraints in SQL Server is that NULL is treated as a variable-length value. This indicates that it could be a few or many bytes. The database allows for additional bytes if the value is larger than what is stored in the field. As a result, your database may require more storage space on your hard drive than if you used regular values.
• Database administrators also argue that a record should not be created if all values cannot be filled. The idea is that a record should only be created if all fields have actual values and no placeholders. However, this argument is not always applicable to all environments. For example, you wouldn't let a bank transaction take place if you didn't know how much it would cost. This standard is effective in the financial sector but ineffective in other sectors, such as e-commerce and websites that collect user data.
•  Your database programming procedures are another drawback. Custom functions must be written to eliminate NULLs, but functions that automatically detect NULL values can be used. This indicates that your SQL procedures may be too complicated to read and significantly longer than necessary. A database administrator will reject the code changes if the procedures are too complicated and muddled.

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Conclusion

In this write-up, we discussed SQL Server Null constraint. We learned how to implement Null constraint and its advantages and disadvantages. We also learned about different SQL Server Null functions. This will become a starting reference for people wanting to study Null Constraints further. Still, have doubts regarding career benefits? Go through the SQL DBA career path and the top companies hiring SQL developers worldwide. Get in touch with our consultant today!