Question Re: CASE Expression with Null Values in Technology Q&A

CASE Expression with Null Values

former_SQLA_member1694868 — Wed, 07 Aug 2013 15:38:50 GMT

Why does the case expression ignore the condition I have to catch null values?

With this test table:

create table AATest
(ID integer,
Code varchar (5));
insert into AATest values (1, 'AAAA');
insert into AATest values (2, 'BBBB');
insert into AATest values (3, 'CCCC');
insert into AATest values (4, NULL);
insert into AATest values (5, NULL);

and this select statement:

SELECT ID , 
case AATest.Code
    when 'AAAA' then 'A'
    when 'BBBB' then 'A'
    when 'CCCC' then 'A'
    when NULL then 'X'
else 'U'
end case
as Status
FROM AATest;

I would expect to get 3 rows of status 'A' and two rows of status 'X'. Instead I get 3 rows of status 'A' and two rows of status 'U'.

Why doesn't the When Null line match the null values?

This format does give my expected results:

SELECT ID , 
case 
    when AATest.Code = 'AAAA' then 'A'
    when AATest.Code = 'BBBB' then 'A'
    when AATest.Code = 'CCCC' then 'A'
    when AATest.Code is NULL then 'X'
else 'U'
end case
as Status
 FROM AATest;

SQL Anywhere version 12.0.1 build 3436

Re: CASE Expression with Null Values

Former Member — Wed, 07 Aug 2013 16:16:27 GMT

The first SELECT statement is an example of a 'simple case' statement where AATest.Code is being compared to NULL using the equality comparison (AATest.Code = NULL).

The second SELECT statement is an example of a 'searched case' statement where each condition is specified individually. This allows you to specify AATest.Code is NULL instead of AATest.Code = NULL

The expression AATest.Code = NULL will always evaluate to 'unknown' and never 'true', therefore the condition will never be satisfied. This is standard SQL behavior.

Re: CASE Expression with Null Values

former_SQLA_member1694868 — Wed, 07 Aug 2013 16:39:31 GMT

Got it. Thanks for the clarification. When handling this situation is it more common to use the searched case (which seems like quite a bit more typing) or use the simple case with a more filtered initial comparison like this:

SELECT ID , 
case coalesce(AATest.Code,'X')
    when 'AAAA' then 'A'
    when 'BBBB' then 'A'
    when 'CCCC' then 'A'
    when 'X' then 'X'
else 'U'
end case
as Status
FROM AATest;

Re: CASE Expression with Null Values

VolkerBarth — Thu, 08 Aug 2013 03:58:53 GMT

Note the different behaviour when using an IF expression, as documented here...

Re: CASE Expression with Null Values

Former Member — Thu, 08 Aug 2013 08:47:11 GMT

If you want accurate results, use the searched case. Who cares if it's more typing? You want accurate results, don't you?

Re: CASE Expression with Null Values

VolkerBarth — Thu, 08 Aug 2013 09:23:06 GMT

Hm, besides the particular NULL comparison effect, I'd usually prefer the simple form if it's exactly "that simple case": i.e. comparing one expression to a set of constant values... - here, using the simple form is more comprehensible IMHO.

So I would not generally recommend the "searched case" - and unless one is aware of the "= NULL isn't IS NULL" problem, the searched case will tend to be a potential pitfall, as well...

NULLs are difficult, but usable:)

Re: CASE Expression with Null Values

Breck_Carter — Thu, 08 Aug 2013 16:31:01 GMT

I agree: code the simple case if you can get away with it (you can't, here).

Re: "NULLs are difficult" - if "difficult" is German for "despicable" then I agree with that too 🙂