Question

I am designing a SQL query to extract all records from a given table. But the trick here is that this logic is based on a numeric database field. So there are 4 choices: 0,1,2,3. If user selects 0,1, or 2, then my query returns rows with the specified value. But if they choose 3, it should return all of the rows. How do I do this in SQL? I know if this was a string, I could do something like:
WHERE = CASE WHEN = 3 THEN ‘%’ ELSE END

But in this case, is an integer. Sounds relatively simple but I’m getting errors.

Answer 1

The simplest way is to do this:

select MyColumn
from   MyTable
where  ( MyValue = @MyParameter or @MyParameter = 3)

If your interested in better optimization, then you can do this, but it is less maintainable:

if (@MyParameter = 3)
    select MyColumn
    from MyTable
else
    select MyColumn
    from MyTable
    where MyValue = @MyParameter

If I were forced to implement this functionality, then I would probably do this, just to make things clear:

declare @AllRecords nchar(1)
if (@MyParameter = 3)
   set @AllRecords = N'Y'
else
   set @AllRecords = N'N'

select MyColumn
from MyTable
where (MyValue = @MyParameter or @AllRecords = N'Y')

Hopefully, I won’t ever have to implement a system that mixes flags and data value in this way.

UPDATED

Here is a version that should work with your expanded requirements (this requires one of the newer versions of SQL Server, I think):

declare @SelectedLevels table (LevelId int not null primary key)
if @LevelId = 3
    insert into @SelectedLevels (LevelId) values (1), (2)
else if @LevelId = 5
    insert into @SelectedLevels (LevelId) values (0), (1), (2)
else
    insert into @SelectedLevels (LevelId) values (@LevelId)

select mt.MyColumn
from   MyTable mt
       inner join @SelectedLevels sl on sl.LevelId = MyTable.LevelId