Order by and Group by syntax?

I tend to use them both together

SELECT CategoryID
FROM InterestingTable
GROUP BY CategoryID
ORDER BY CategoryID

This will give me, for example,
CategoryID
1
2
4
5
7

Without the ORDER BY, I will get the same data, just not sorted.

Without the GROUP BY, I will get all the rows- sorted- but I might get duplicates wherever more than one row shared the same CategoryID

The trick comes in when you want to include other columns.  You will need to specify WHICH of the values for that given column you will want to see for that particular CategoryID.  You will use aggregate functions such as MAX or MIN or AVERAGE, etc.

Sample tables to go with the above comment:

BookID       CategoryID     Cost      Description
10                1                   15         Book 11
20                2                   11         Book 12
30                4                   19         Book 13
40                7                   25         Book 31
50                5                   5           Book 11
60                5                   35         Book 21
70                2                   35         Book 21