Why Does My MySQL CASE WHEN Statement Choose the Wrong Condition When `base_price` is 0?

Using the CASE..WHEN Statement Correctly in MySQL

In MySQL, the CASE..WHEN statement allows you to evaluate different conditions and return a specific result based on each condition. However, it's important to use it properly to avoid unexpected results.

Question:

When executing a CASE..WHEN statement to fetch records where the base_price is 0, why is the condition with a value of 3 being chosen instead of the first condition, even though the specified value is 0?

Answer:

The problem lies in the syntax of the CASE statement. In the provided query:

SELECT
   CASE course_enrollment_settings.base_price
    WHEN course_enrollment_settings.base_price = 0      THEN 1
    WHEN course_enrollment_settings.base_price < 101      THEN 2
    WHEN course_enrollment_settings.base_price > 100 AND
                      course_enrollment_settings.base_price < 201 THEN 3
        ELSE 6
   END AS 'calc_base_price',
   course_enrollment_settings.base_price
FROM
    course_enrollment_settings
WHERE course_enrollment_settings.base_price = 0

the statement evaluates the conditions and returns the first matching condition. However, the syntax is incorrect because the column name course_enrollment_settings.base_price is repeated immediately after the CASE statement. This leads to the statement comparing its own result with the rest of the conditions, resulting in the 3 condition being always chosen.

To fix the issue, remove the column name from that point:

SELECT
   CASE
    WHEN course_enrollment_settings.base_price = 0      THEN 1
    ...
    END

The CASE statement now correctly evaluates the conditions without unnecessary repetition, ensuring the expected result.

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