Select from Nothing

ELSE SELECT nothing/blank

You can assign a variable:

IF EXISTS(SELECT 1 FROM Account WHERE Account.username=@Username)
BEGIN
    SELECT Status 
    FROM   Account 
    WHERE Account.username = @Username;
END;
ELSE
BEGIN
    DECLARE @X int;
    SELECT @X = 0;
END;

sql:using not in and select nothing

Don't use NOT IN with a subquery. As Jarlh says in a comment, when any value is NULL, then no rows are returned at all.

Instead, get used to NOT EXISTS:

select t.id, 'Leaf' as type
from tree t
where not exists (select 1 from tree t2 where t2.p_id = t.id);

This behaves as you would expect.

Although you could fix the problem using WHERE t2.p_id IS NOT NULL in the subquery, you might as well use NOT EXISTS. At some future point, you'll find yourself debugging another NOT IN where you left out the WHERE clause.

SQL select dummy data

You would have to use UNION with two select statements:

 SELECT 'dummy1' AS [Dummies]
 UNION
 SELECT 'dummy2'

This will produce a single column.

Dummies
-------
dummy1
dummy2

SQL - If Select returns nothing then do another Select

You can do this in one statement, assuming the columns are the same:

SELECT *
FROM table
WHERE criteria = criteria 
UNION ALL
SELECT *
FROM table
WHERE criteria2 = criteria2 AND
      NOT EXISTS (SELECT 1 FROM table WHERE criteria = criteria);

Select dummy values if where clause returns zero rows

The problem is, that if the WHERE clause eliminates all rows, you end up with an empty set.

One way around it, is to use an aggregate function to force one row in the result.

Then, with COALESCE (or ISNULL), you can assign your default values.

This solution is only feasible if:

your basic query always returns exactly 0 or 1 row
your columns do not contain NULL
the selected columns can be aggregated (e.g. it will not work for text columns)
your default values have the same data type as the columns

For example:

select COALESCE(MAX(step_id),123456) AS step_id,
       COALESCE(MAX(step_name),'There are no step_id matching the WHERE clause') AS step_name
  from msdb.dbo.sysjobhistory
 where step_id = 9999

Select Null statement, Is it possible?

Is this what you need

SELECT null FROM  table where password= 'abc'