MySQL CASE WHEN NULL

MySQL's CASE statement is a powerful tool that allows users to perform conditional logic within their queries. However, there are some quirks to its behavior with null values that developers need to be aware of.

The Problem

Suppose we have a table called employees with three columns: id, name, and salary. Some of the employees don't have a salary value, and are represented as null in the salary column. We want to return a list of all employees, but for those without a salary, we want to display a message instead. Here's a sample query:

SELECT id, name, 
  CASE 
    WHEN salary IS NULL THEN 'No salary information available.' 
    ELSE salary 
  END AS salary 
FROM employees;

This seems like it should work, and for the most part, it does! When we run this query, we see that all employees with a non-null salary have that salary displayed correctly, and all employees without a salary have the message 'No salary information available.' displayed instead.

The Catch

However, there's one gotcha: the salary column is returned as a string! For employees with a salary, this isn't a problem, since the value is already a string. But for employees without a salary, we'd really prefer to see null displayed – after all, null is the canonical representation of "no value".

Unfortunately, it's not as simple as changing the case statement to return null:

SELECT id, name, 
  CASE 
    WHEN salary IS NULL THEN NULL 
    ELSE salary 
  END AS salary 
FROM employees;

This returns the same string value for employees without a salary: 'No salary information available.'.

The Solution

To return null, we need to explicitly cast it to the appropriate type. In this case, we want the salary column to be numeric, so we can cast null to a numeric value using the CAST() function. Here's the correct query:

SELECT id, name, 
  CASE 
    WHEN salary IS NULL THEN CAST(NULL AS DECIMAL(10,2)) 
    ELSE salary 
  END AS salary 
FROM employees;

This query returns numeric values for employees with a salary, and null values (in numeric format) for employees without a salary.

Conclusion

MySQL's CASE statement is a powerful tool for performing conditional logic in our queries. However, we need to be aware of its behavior with null values. By explicitly casting null to the appropriate data type, we can ensure that our queries return the expected results, even when dealing with null values.