How to check null values in SQL

Overview

In SQL, NULL represents a missing or unknown value.

To check for missing values in SQL, we use IS NULL expression.

The IS NULL operator, when used in conjunction with WHERE, helps us figure out what data is missing.

Syntax

SELECT column_name(s)
FROM table_name
WHERE column_name IS NULL;

Code

Let’s assume we have a table called Person with the columns ID, name, age, state, and gender.

Now, we want to get the names of all people whose ages are missing from the Person table.

How do we get this information from our table?

The following code shows how to do this using the IS NULL operator in SQL.

CREATE TABLE Person (
    ID int,
    name varchar(100),
    age int,
    gender varchar(10),
    state varchar(15)
);
-- Insert data
INSERT INTO Person
VALUES (1,'Sharon Peller','16','Female','Kogi');
INSERT INTO Person
VALUES (2,'Paul Dons', NULL,'Male','Lagos');
INSERT INTO Person
VALUES (3,'Ameera Abedayo','28','Female','Imo');
INSERT INTO Person
VALUES (4,'Maria Elijah',NULL,'Female','Lagos');
INSERT INTO Person
VALUES (5,'David Hassan',NULL,'Male','Abuja');
INSERT INTO Person
VALUES (6,'Niniola Disu','28','Female','Lagos');
INSERT INTO Person
VALUES (7,'Praise Dominion','16','Female','Ibadan');
INSERT INTO Person
VALUES (8,'Joe Smith','16','Male','Lagos');
-- Query 
SELECT name
FROM Person
WHERE age IS NULL;

Explanation

  • Lines 1–7: We create a table called Person with the columns ID, name, age, gender, and state.
  • Lines 10–25: We insert data into the Person table.
  • Lines 28–30: We filter the missing data using the IS NULL operator with the WHERE keyword.

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