How do I use properly CASE..WHEN in MySQL

Question

Here is a demo query, notice it is very simple, Fetches only where base_price is 0, And still, it chooses the condition 3:

SELECT
   CASE course_enrollment_s         


        

Answer 1

CASE course_enrollment_settings.base_price is wrong here, it should be just CASE

SELECT 
CASE 
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 

Some explanations. Your original query will be executed as :

SELECT 
CASE 0
WHEN 0=0 THEN 1 -- condition evaluates to 1, then 0 (from CASE 0)compares to 1 - false
WHEN 0<1 THEN 2 -- condition evaluates to 1,then 0 (from CASE 0)compares to 1 - false
WHEN 0>100 and 0<201 THEN 3 -- evaluates to 0 ,then 0 (from CASE 0)compares to 0 - true
ELSE 6, ...

it's why you always get 3