Oracle SQL CASE WHEN IS NULL

Question

I am learning Oracle SQL:)

1.

 CASE s.COURSE_SCHEDULED_ID WHEN IS NULL THEN 'false' ELSE 'true' END AS "Is scheduled?"

 CASE WHEN s.COURSE_SCHEDULED_ID IS NULL THEN 'false' ELSE 'true' END AS "Is scheduled?"

Why 1 is not working?

Answer 1

Your first CASE expression won't even compile , but if we make a small change then it will:

CASE s.COURSE_SCHEDULED_ID
    WHEN NULL THEN 'false'
    ELSE 'true'
END AS "Is scheduled?"

This will be evaluated as this:

CASE WHEN s.COURSE_SCHEDULED_ID = NULL
     THEN 'false'
     ELSE 'true'
END AS "Is scheduled?"

Note very carefully that the COURSE_SCHEDULED_ID is being compared to NULL using the equality operator. This won't work as expected with NULL . Instead, for NULL checks the second verbose version always must be used as it allows IS NULL and IS NOT NULL checks.

Answer 2

From the documentaion :

In a simple CASE expression, Oracle Database searches for the first WHEN... THEN pair for which expr is equal to comparison_expr and returns return_expr . If none of the WHEN... THEN pairs meet this condition, and an ELSE clause exists, then Oracle returns else_expr . Otherwise, Oracle returns null.

In you first version you have

CASE s.COURSE_SCHEDULED_ID WHEN IS NULL THEN

which will throw "ORA-00936: missing expression because IS NULL` is a condition, not a value or expression. So then you might think you could do:

CASE s.COURSE_SCHEDULED_ID WHEN NULL THEN

but that will go to the else value because the comparison is " expr is equal to comparison_expr ", and nothing is equal to (or not equal to) null ( documentation ).

In your second version you now have a searched case expression:

In a searched CASE expression, Oracle searches from left to right until it finds an occurrence of condition that is true, ...

Now it is expecting a condition (rather than a value or expression), so s.COURSE_SCHEDULED_ID IS NULL can be evaluated and is true.

Answer 3

you can check multiple column situations in case when statements. for example

case 
     when s.COURSE_SCHEDULED_ID is null and s.COURSE_ID = 4 
     then true 
     else false 
 end as is_scheduled 

if you try to show in the 1st way, which value will asign to which columns? Or you can write different cases:

case 
     when s.COURSE_SCHEDULED_ID is null  
     then true 
     when s.COURSE_ID = 4
     then null 
     else false 
 end as is_scheduled 

Answer 4

just write:

CASE
    WHEN s.COURSE_SCHEDULED_ID IS NULL THEN 'false'
    ELSE 'true'
END AS "Is scheduled?"

It works anyway.