**NOT mySQL**, but for other people finding this question and using SQL, another way to resolve the [tag:greatest-n-per-group] problem is using [`Cross Apply`][cross apply] in MS SQL


    WITH DocIds AS (SELECT DISTINCT id FROM docs)
    
    SELECT d2.id, d2.rev, d2.content
    FROM DocIds d1
    CROSS APPLY (
      SELECT Top 1 * FROM docs d
      WHERE d.id = d1.id
      ORDER BY rev DESC
    ) d2

###[Here's an example in SqlFiddle][sql fiddle]

[cross apply]: http://technet.microsoft.com/en-us/library/ms175156(v=sql.105).aspx
[sql fiddle]: http://sqlfiddle.com/#!18/25b5c/1

I have this table for documents (simplified version here):


| id   | rev   | content                              |
|------|-------|--------------------------------------|
| 1    | 1     | ...                                  |
| 2    | 1     | ...                                  |
| 1    | 2     | ...                                  |
| 1    | 3     | ...                                  |


How do I select one row per id and only the greatest rev?  
With the above data, the result should contain two rows: `[1, 3, ...]` and `[2, 1, ..]`. I'm using ***MySQL***.

Currently I use checks in the `while` loop to detect and over-write old revs from the resultset. But is this the only method to achieve the result? Isn't there a **SQL** solution?



SQL select only rows with max value on a column

just exclude those group ID's that have a record where HasData = 0.

    select distinct a.groupID
    from table1 a 
    where not exists(select * from table1 b where b.HasData = 0 and b.groupID = a.groupID)



I'd like to do a query for every GroupID (which always come in pairs) in which both entries have a value of 1 for HasData.

	|GroupID | HasData |
	|--------|---------|
	|  1     |  1      |
	|  1     |  1      |
	|  2     |  0      |
	|  2     |  1      |
	|  3     |  0      |
	|  3     |  0      |
	|  4     |  1      |
	|  4     |  1      |

So the result would be:

    1
    4

here's what I'm trying, but I can't seem to get it right.  Whenever I do a `GROUP BY` on the GroupID then I only have access to that in the selector



    SELECT GroupID
    FROM Table
    GROUP BY GroupID, HasData
    HAVING SUM(HasData) = 2 

But I get the following error message because HasData is acutally a bit:  

    Operand data type bit is invalid for sum operator.

Can I do a count of two where both records are true?