Question

Hi experts: I have a table Called tblAlarm and it has some records like this:

I have another table for determine what user see what message:

Now I want to write a query to show Messages that user has not seen if message didinot expired.(for example it’s year between BeginYear and EndYear and so on …). I write this query:

SELECT  *
FROM
tblAlarms LEFT OUTER JOIN tblUsersAlarms tua ON tblAlarms.Id=tua.MessageID
WHERE @CurrentYear BETWEEN  tblAlarms.BeginYear AND tblAlarms.EndYear 
  AND @CurrentMonth BETWEEN  tblAlarms.BeginMonth AND tblAlarms.EndMonth 
  AND @CurrentDay BETWEEN tblAlarms.BeginDay AND tblAlarms.EndDay
  AND @CurrentHour * 60 + @CurrentMinute BETWEEN tblAlarms.BeginHour*60 + tblAlarms.BeginMinute AND tblAlarms.EndHour*60 + tblAlarms.EndMinute
  --AND (tua.UserID <> 128  AND tua.UserID IS NULL)

and it returns :

but if I unComment last line it does not return any record.How I can determine what messages that users has not been seen?

thanks

Answer 1

The result of comparing a NULL with a value is UNKNOWN. A filter condition only selects rows where the conditions evaluate to TRUE, so rows where the condition evaluates to FALSE or UNKNOWN are not selected.

The IS NULL and IS NOT NULL tests are different and always produce a TRUE or FALSE answer. You also cannot write column = NULL or column != NULL (at least, the SQL standard does not allow that; some SQL dialects may).

Your condition is:

AND (tua.UserID <> 128 AND tua.UserID IS NULL)

• If the value of tua.UserID is NULL, then the second term evaluates to TRUE, but the first will then evaluate to UNKNOWN, and UNKNOWN and TRUE is UNKNOWN, which is not TRUE, so the row is not selected.

• If the value of tua.UserID is not NULL, then the second term evaluates to FALSE, so the overall condition evaluates to FALSE, and so the row is not selected.

Hence, when you add the condition, nothing is selected, as you observed.

More typically, your condition would be either of these two:

AND (tua.UserID <> 128 AND tua.UserID IS NOT NULL)

AND (tua.UserID <> 128 OR tua.UserID IS NULL)

The double condition in the first alternative is in fact redundant. If tua.UserID is NULL, the first term will evaluate to UNKNOWN, so the overall condition would not be TRUE and the row would not be selected. The second alternative is quite useful.