SQL Server group by/ order by

Question

I have the following query

select
    datepart(yy, orderDate) as 'year',
    datename(mm, OrderDate) as 'month',
    count(*) as 'Orders'
from orders     (yeah I know its missing the schema, its from a book)
group by
    datepart(yy, orderdate),
    datename(mm, orderdate)
order by
    datepart(yy, orderdate),
    datename(mm, orderdate);

which returns 3 columns but datename(mm, orderdate) returns a string and therefore ordering by it puts August before January etc.

The solution to this was the following:

select
    datepart(yy, orderDate) as 'year',
    datename(mm, OrderDate) as 'month',
    count(*) as 'Orders'
from orders  (yeah i know its missing the schema, its from a book)
group by
    datepart(yy, orderdate),
    datename(mm, orderdate),
    datepart(mm, orderdate)
order by
    datepart(yy, orderdate),
    datepart(mm, orderdate);

I'm still a little confused with the whole group by / order by sections and how it actually works.

As far as I have understood, group by is creating a work table with 4 columns (that might be wrong) datepart(yy, orderdate), datename(mm, orderdate), datepart(mm, orderdate), and a count column.

Each time it encounters an orderdate that it has in the work table it increases the count, otherwise it adds a new row?

Originally I thought I could remove the DateName(mm, orderdate) from within the group by section but the book said that's not possible.

If someone could step through what actually happens behind the scenes/point out a resource which explains how this works in a little more detail id appreciate it.

thanks for your help.

Answer 1

Any time you use an aggregate function (COUNT,SUM,MAX, etc.) you need to include all other columns in a GROUP BY clause. COUNT in your example is returning the number of records that have the same value for datepart(yy, orderdate), datename(mm, orderdate), datepart(mm, orderdate).

An example:

SELECT col1, col2, col3, MAX(col4)
FROM MyTable
GROUP BY col1, col2, col3

Say this returned:

1,2,3,9
1,2,5,9

If you changed your query to this:

SELECT col1, col2, MIN(col3), MAX(col4)
FROM MyTable
GROUP BY col1, col2

It would return:

1,2,3,9

Notice that I added an Aggregate function to col3 (MIN) so I was able to remove col3 from my GROUP BY clause.