How do I group by only one column in a SELECT statement?

Question

Hi everyone,
Here is my novice question:
I would like to run a query where I select (display) several columns from my table, yet group (categorize) by one of these columns.
For example:
SELECT A, B, COUNT(A)
FROM TableX
GROUP BY A
I realize now that this is not valid. My first question is why? Doesn't it make sense that you would want to group by only one of the columns? To me, it seems natural that you would most often want to categorize by one attribute, which is why it seems odd that this is not valid. I realize that I can remove column B, but I do want to display that info. I could also change the last statement to GROUP BY A, B to avoid errors, but this will also not display what I am looking for.
In addition to understanding why Oracle is set up this way, I guess I also need to know how I can legally accomplish this query, grouping by A but also displaying the values of B.
Thank you very much for your help!
Holly

Answer 1

Hi everyone,
Thank you for your responses... very interesting and useful. Since I came across inline views in my reading, I tried the approach recommended by g feng, although I am eager to try the other approaches too.
I ended up using an inner join simply to get more descriptive info. on the column that was being counted. With Y representing this second table, Y.B being the column containing the descriptive info., and A being the common key between X and Y, here is the simple code I came up with.
SELECT Y.B, V.*
FROM Y,
SELECT A AV, COUNT (*)
FROM X
GROUP BY A
) V
WHERE V.AV = Y.A
Does this seem correct... anything I need to watch out for?
Thanks again to everyone for your help!!
Holly