testing inequality with columns that can be null

Question

So, I asked a question this morning, which I did not phrase correctly, so I got a lot of responses as to why NULL compared to anything will give NULL/FALSE.

My actual q

Answer 1

IBM Informix Dynamic Server has a somewhat peculiar view of booleans for a variety of historical (aka 'bad') reasons. Adapting the idea suggested by @astander, this CASE expression 'works', but I'd be the first to say 'not obvious' (see - I said it before you did!). The setup phase:

create table x(a int, b int);
insert into x values(null, null);
insert into x values(null, 1);
insert into x values(1, null);
insert into x values(1, 1);
insert into x values(1, 2);

The SELECT statement:

SELECT *
  FROM x
  WHERE   CASE
          WHEN a IS NULL AND b IS NULL THEN 'f'::BOOLEAN
          WHEN a IS NULL OR  b IS NULL THEN 't'::BOOLEAN
          WHEN a != b                  THEN 't'::BOOLEAN
          ELSE                              'f'::BOOLEAN
          END
;

The result from this query is:

                 1
      1           
      1          2

Issues:

• IDS does not recognize FALSE or TRUE or UNKNOWN as keywords.
• IDS does not recognize boolean expressions such as 'a != b' (or 'a <> b') as such.

Yes, it pains me greatly to have to state this.

Answer 2

can you try something like this in informix?

CASE
    WHEN a IS NULL AND B IS NULL THEN false 
    WHEN a IS NULL OR B IS NULL THEN true
    ELSE a <> B
END

from IBM Informix Guide to SQL: Syntax , CASE Expressions

Answer 3

I would personally write out the expression you came up with, especially if the table is expected to grow large. Wrapping the columns in function calls hurts performance by making it so the engine can't use any indexes you have on those columns. Of course, in a small table, this may not be any sort of issue, but I still like to do it the explicit way just in case a table ends up growing.

Answer 4

Depending on the data type and possible values for the columns:

COALESCE(A, -1) <> COALESCE(B, -1)

The trick is finding a value (here I used -1) that will NEVER appear in your data.

The other way would be:

(A <> B) OR (A IS NOT NULL AND B IS NULL) OR (A IS NULL AND B IS NOT NULL)

This can be a problem depending on how your particular RDBMS handles NULLs. By the ANSI standard, this should give you what you want, but who follows standards anyway. :)

P.S. - I should also point out that using the COALESCE function may invalidate the use of indexes in comparing the columns. Check your query plan and performance of the query to see if that's a problem.

P.P.S. - I just noticed that OMG Ponies mentioned that Informix doesn't support COALESCE. It's an ANSI standard function I believe, but see what I said above about standards...

Answer 5

For SQL Server, use:

WHERE ISNULL(A, '') <> ISNULL(B, '')

Answer 6

If you want to be sure about how NULLs are handled, you'll have to use whatever Informix supports for null checking. I haven't turned up much, other than the SE version doesn't support COALESCE, but it does support DECODE and possibly CASE.

WHERE COALESCE(t.a, 0) != COALESCE(t.b, 0)
WHERE DECODE(NULL, 0, t.a) != DECODE(NULL, 0, t.b)