NULL question

Hi,

This is a kind of stupid question. I created a new field in one of the table, ActivityStatus. The value in that field is NULL.

When I do the SELECT * FROM MyTable WHERE ActivityStatus <> 'CLOSED'

It returns 0. Why is that? I though it should have returned everything? thanks
mcrmgAsked:
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ste5anSenior DeveloperCommented:
Cause in (ANSI) SQL NULL means no value entered by user, not value unknown by user. This results in a tri-state logic:

1. Comparisions with NULL on normal operators are evaluated to UNKOWN internally. But WHERE conditions require a TRUE to return a row.
2. Comparisions to return rows where a column is NULL requires explicit, different syntax: column IS [NOT] NULL. column = NULL follows the first rule and evaluates to UNKOWN.

Your problem is normally tackled by set theory: your activity status must be a mandatory column. And it consists of the states (OPEN, CLOSED, UNKOWN) as materialized values.

Then your query works as intended. Otherwise you need to implement the tri-state logic:

SELECT * 
FROM MyTable 
WHERE ActivityStatus <> 'CLOSED'
  AND NOT ActivityStatus IS NULL;

Open in new window


btw, using states (OPEN, CLOSED, UNKOWN) is different to states (OPEN, CLOSED, NULL). Cause in the first case you know that the users set this value intentionally. In the second case, you don't know, what the user means. It's

"I checked it, But I don't know it."  vs.  the user was lazy and forgot to enter it.
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mcrmgAuthor Commented:
thank you very much
0
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