How to Tackle SQL NULLs: The NULLIF Function

NULLIF Function

Usually we try to avoid NULLs, but sometimes we need to have a NULL instead of an actual value. When this need pops up, a helpful but little-known SQL function called NULLIF makes it possible. This post will introduce you to NULLIF and demonstrate its use in two example cases.

In the article How to Tackle NULLS: The COALESCE Function , we got to know the COALESCE function. It returns the first not-null expression. The NULLIF function works differently. It accepts two parameters and returns a NULL if value1 equals value2. Otherwise, the function returns value1.

The syntax for the NULLIF function is:

NULLIF(value1, value2)

For example:

SELECT NULLIF(1, 1);
NULL 

and

SELECT NULLIF(1, 0);
1

This is sugar syntax for the following CASE statement:

CASE 
WHEN value1 = value2
THEN NULL
ELSE value1
END

Now, let’s take a look at some usage examples for the NULLIF function.

sql cheat sheet

Use Case 1: Avoiding “Division by Zero” Errors

Business rules should never be faced with an illegal math function, but it does sometimes happen. For example, we may unintentionally try to divide a value by zero. Suppose we try to calculate the male-female ratio between students registered for a course. What problematic thing might happen here?

Baner_ogolny_akademia

Let’s delve into this, starting with the database model shown below. For our query we will need the student and student_courses tables.

student table

pic-3

student_courses table

pic-7

So, let’s do some SQL:

 WITH 
  male_student AS (
    SELECT COUNT(*) AS male_count
    FROM student s
    JOIN student_courses c
      ON s.id = c.student_id
    WHERE gender = 'm'
      AND course_instance_id = 1),
  female_student AS (
    SELECT COUNT(*) AS female_count
    FROM student s
    JOIN student_courses c
      ON s.id = c.student_id
    WHERE gender = 'w'
      AND course_instance_id = 1)

SELECT male_count/female_count
FROM male_student, female_student

Result: ERROR

Upon execution, the above query fails and issues a “division by zero” error when it tries to calculate the male-to-female ratio for the course_instance_id = 1.

It happened that the female_count in the chosen course equals “0”, so the SELECT statement will return a “division by zero” error. To avoid this, we can replace the zero with another value. The best solution is to paste in a NULL. There are two ways to do this:  via the CASE WHEN THEN expression or the NULLIF function.

For the NULLIF function, the construction is:

NULLIF(female_count, 0)

This will return a NULL if the female_count equals “0”. If the values aren’t equal, the female_count will be returned. In our case, the modification in the SELECT statement looks as follows:

...
...
SELECT male_count/NULLIF(female_count, 0)
FROM male_student, female_student

Result: NULL

The result will be a NULL. Now we can use the COALESCE function to replace the final value of the ration (that is currently NULL) with a more precise value like “0”.

...
...
SELECT COALESCE(male_count/NULLIF(female_count, 0), 0)
FROM male_student, female_student

Result: 0

Use Case 2: Using NULLIF with Aggregate Functions

The NULLIF function can be used when a database contains special non-NULL values that we want to treat as NULLs. This is particularly useful for aggregate functions. For example, suppose you need to get the number of records where grades don’t equal zero (the total number of final grades for all courses). Take a look at the table below. In this case, the students that don’t have a final grade yet have a “0” in the grade column.

student_courses table

pic-12

The query for this is:

SELECT COUNT(*)
FROM student_courses
WHERE grade != 0

Or we can use the NULLIF function and skip the WHERE clause:

SELECT COUNT(NULLIF(grade, 0))
FROM student_courses

Result: 1

So there are times when we actually want NULLs? It seems extraordinary, but when you use NULLIF it can be quite convenient to have them. With this handy SQL function, we can treat certain values as NULL for the sake of better computation.

Junior Java Developer @ e-point

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