Question

I have a table that I wish to select a subset of columns from but also add on the end a computed column based upon where you are located in a queue. There are the following fields (that are pertinent):

id: int, auto increment, primary key
answertime: datetime, nullable

By default, when something is submitted to the queue, its answertime is NULL. So, I wish to select the ID of the thing in the queue as well as its rank in the queue (i.e. rank 1 is the next item that is unanswered, etc). Here’s what I was thinking:

rank – id – COUNT(ids below my id where answertime is not null). However, I’m having an issue with the syntax of this query:

SELECT id AS outerid, COUNT(
    SELECT * FROM tablename WHERE id<outerid AND answertime IS NOT NULL
)
FROM tablename 
WHERE answertime IS NULL;

Now, obviously, this is wrong because I’m fairly confident you can’t embed a select inside of an aggregate function, likewise flipping the SELECT and COUNT doesn’t work as you can’t embed a SELECT at that point in the code (it can only be used in a WHERE clause).

Is this even possible to do with just SQL or do I need to add some logic on the program end?

If it helps, I’m doing this on SQL Server 2008, although I doubt that would add any value.

Answer 1

You can do that, you just can’t use SELECT * in an aggregate sub-query. Try this, which gets the COUNT value as a scalar result:

SELECT
   id AS outerid,
   (SELECT COUNT(Id) FROM tablename
    WHERE id<outie.id AND answertime IS NOT NULL)
FROM tablename outie
WHERE answertime IS NULL;

You may need to choose for yourself between using COUNT(*), COUNT(Id) or some other column depending on what you’re really after.