Question

I’m trying to figure out why I’m not getting a result from a MySQL Query I’m running.

I’m trying to replace a NULL value with a number with in a query, but I can’t figure out what I’m doing wrong.

Here’s my query:

UPDATE Details
SET HowHear_ID = CASE HowHear_ID
  WHEN '' THEN 25   
  WHEN NULL THEN 25                         
  WHEN 7  THEN 25
  WHEN 8  THEN 5
  WHEN 16 THEN 25
  WHEN 17 THEN 16
END
WHERE HowHear_ID IN ('',NULL,7,8,16,17)

This Query will effect all but the NULL values.

What am I doing wrong??

Answer 1

No value will ever equal (or “unequal”) NULL in SQL. Understand the following truth table:

NULL = NULL yields NULL   -- not FALSE!
NULL != NULL yields NULL  -- not TRUE!
[ANY] = NULL yields NULL  -- not FALSE!
[ANY] != NULL yields NULL -- not TRUE!

Since the following are equivalent…

[expression] IN (a, b, c)
[expression] = ANY (a, b, c)
[expression] = a OR [expression] = b OR [expression] = c

… you cannot put NULL on the right hand side of an IN predicate. Interestingly, things get even worse when you put NULL on the right hand side of a NOT IN predicate:

[expression] NOT IN (a, b, c)
[expression] != ANY (a, b, c)
[expression] != a AND [expression] != b AND [expression] != c

If b were NULL, the whole expression will become NULL (or maybe FALSE), but never TRUE. This is also the case for NOT IN (subselect) predicates! So, never do this:

[expression] NOT IN (NULL, 1, 2)

The correct solution in your case uses a NULL predicate instead. Do this:

UPDATE Details
SET HowHear_ID = CASE 
  WHEN HowHear_ID = ''    THEN 25   
  WHEN HowHear_ID IS NULL THEN 25  -- Use a NULL predicate here
  WHEN HowHear_ID = 7     THEN 25
  WHEN HowHear_ID = 8     THEN 5
  WHEN HowHear_ID = 16    THEN 25
  WHEN HowHear_ID = 17    THEN 16
END
WHERE HowHear_ID IN ('',7,8,16,17)
   OR HowHear_ID IS NULL           -- Use a NULL predicate here

Or this:

WHERE COALESCE(HowHear_ID, '') IN ('',7,8,16,17)