Counting DISTINCT over multiple columns

Question

Is there a better way of doing a query like this:

SELECT COUNT(*) 
FROM (SELECT DISTINCT DocumentId, DocumentSessionId
      FROM DocumentOutputItems) AS internalQuery

I need to count the number of distinct items from this table but the distinct is over two columns.

My query works fine but I was wondering if I can get the final result using just one query (without using a sub-query)

Answer 1

What is it about your existing query that you don't like? If you are concerned that DISTINCT across two columns does not return just the unique permutations why not try it?

It certainly works as you might expect in Oracle.

SQL> select distinct deptno, job from emp
  2  order by deptno, job
  3  /

    DEPTNO JOB
---------- ---------
        10 CLERK
        10 MANAGER
        10 PRESIDENT
        20 ANALYST
        20 CLERK
        20 MANAGER
        30 CLERK
        30 MANAGER
        30 SALESMAN

9 rows selected.


SQL> select count(*) from (
  2  select distinct deptno, job from emp
  3  )
  4  /

  COUNT(*)
----------
         9

SQL>

edit

I went down a blind alley with analytics but the answer was depressingly obvious...

SQL> select count(distinct concat(deptno,job)) from emp
  2  /

COUNT(DISTINCTCONCAT(DEPTNO,JOB))
---------------------------------
                                9

SQL>

edit 2

Given the following data the concatenating solution provided above will miscount:

col1  col2
----  ----
A     AA
AA    A

So we to include a separator...

select col1 + '*' + col2 from t23
/

Obviously the chosen separator must be a character, or set of characters, which can never appear in either column.

Answer 2

Many (most?) SQL databases can work with tuples like values so you can just do: SELECT COUNT(DISTINCT (DocumentId, DocumentSessionId)) FROM DocumentOutputItems; If your database doesn't support this, it can be simulated as per @oncel-umut-turer's suggestion of CHECKSUM or other scalar function providing good uniqueness e.g. COUNT(DISTINCT CONCAT(DocumentId, ':', DocumentSessionId)).

A related use of tuples is performing IN queries such as: SELECT * FROM DocumentOutputItems WHERE (DocumentId, DocumentSessionId) in (('a', '1'), ('b', '2'));

Answer 3

How about this,

Select DocumentId, DocumentSessionId, count(*) as c 
from DocumentOutputItems 
group by DocumentId, DocumentSessionId;

This will get us the count of all possible combinations of DocumentId, and DocumentSessionId

Answer 4

You can just use the Count Function Twice.

In this case, it would be:

SELECT COUNT (DISTINCT DocumentId), COUNT (DISTINCT DocumentSessionId) 
FROM DocumentOutputItems

Answer 5

Edit: Altered from the less-than-reliable checksum-only query I've discovered a way to do this (in SQL Server 2005) that works pretty well for me and I can use as many columns as I need (by adding them to the CHECKSUM() function). The REVERSE() function turns the ints into varchars to make the distinct more reliable

SELECT COUNT(DISTINCT (CHECKSUM(DocumentId,DocumentSessionId)) + CHECKSUM(REVERSE(DocumentId),REVERSE(DocumentSessionId)) )
FROM DocumentOutPutItems