Working with NULL Values

Kedeisha June 16, 2023

In this lesson, we will explore how to handle NULL values in SQL. NULL represents the absence of a value or an unknown value in a column. Understanding how to work with NULL values is crucial for accurate data retrieval, filtering, and calculations in SQL queries.

Introduction to NULL Values

NULL is a special value in SQL that represents the absence of a value or an unknown value. It is different from an empty string (”) or zero (0). NULL can occur in columns where the value is missing, unknown, or not applicable. It is important to handle NULL values properly to ensure accurate data analysis and calculations.

Filtering NULL Values using IS NULL

The IS NULL operator is used to check if a column contains a NULL value. It returns true if the column value is NULL and false otherwise. Here’s an example:

				
					SELECT 
    column1, 
    column2
FROM 
    table_name
WHERE 
    column1 IS NULL;

This query retrieves rows where the value in column1 is NULL.

Filtering Non-NULL Values using IS NOT NULL

The IS NOT NULL operator is used to check if a column does not contain a NULL value. It returns true if the column value is not NULL and false if it is NULL. Here’s an example:

				
					SELECT 
    column1, 
    column2
FROM 
    table_name
WHERE 
    column1 IS NOT NULL;

This query retrieves rows where the value in column1 is not NULL.

Handling NULL Values with COALESCE

The COALESCE function is used to handle NULL values in SQL. It returns the first non-NULL value from a list of expressions. If all expressions evaluate to NULL, it returns NULL. Here’s an example:

				
					SELECT 
    COALESCE(column1, 'Default Value') AS column_alias
FROM 
    table_name;

In this query, if column1 is NULL, the COALESCE function substitutes it with the ‘Default Value’. Otherwise, it returns the value of column1. The COALESCE function allows us to handle NULL values gracefully and provide alternative values.

Handling NULL Values in Calculations

When performing calculations involving columns with NULL values, the result will also be NULL. It is important to consider NULL values and handle them appropriately in calculations. You can use functions like COALESCE or ISNULL to replace NULL values with appropriate defaults or zero, depending on the context.

Best Practices for Handling NULL Values

Use NULL values when the actual value is unknown, missing, or not applicable.
Use proper constraints to enforce NULL or NOT NULL values based on business rules.
Handle NULL values explicitly in your queries to avoid unexpected results.
Use COALESCE, IS NULL, or IS NOT NULL to handle NULL values in a controlled manner.
Consider the implications of NULL values when performing calculations or aggregations.

Understanding how to work with NULL values allows us to handle missing or unknown data effectively in SQL queries. It helps ensure accurate data retrieval, filtering, and calculations