group by, orderby, multiple column

[group by]

Group By X means put all those with the same value for X in the one group.

Group By X, Y means put all those with the same values for both X and Y in the one group.

To illustrate using an example, let's say we have the following table, to do with who is attending what subject at a university:

Table: Subject_Selection

Subject   Semester   Attendee
---------------------------------
ITB001    1          John
ITB001    1          Bob
ITB001    1          Mickey
ITB001    2          Jenny
ITB001    2          James
MKB114    1          John
MKB114    1          Erica

When you use a group by on the subject column only; say:

select Subject, Count(*)
from Subject_Selection
group by Subject

You will get something like:

Subject    Count
------------------------------
ITB001     5
MKB114     2

...because there are 5 entries for ITB001, and 2 for MKB114

If we were to group by two columns:

select Subject, Semester, Count(*)
from Subject_Selection
group by Subject, Semester

we would get this:

Subject    Semester   Count
------------------------------
ITB001     1          3
ITB001     2          2
MKB114     1          2

This is because, when we group by two columns, it is saying "Group them so that all of those with the same Subject and Semester are in the same group, and then calculate all the aggregate functions (Count, Sum, Average, etc.) for each of those groups". In this example, this is demonstrated by the fact that, when we count them, there are three people doing ITB001 in semester 1, and two doing it in semester 2. Both of the people doing MKB114 are in semester 1, so there is no row for semester 2 (no data fits into the group "MKB114, Semester 2")

출처 : http://stackoverflow.com/questions/2421388/using-group-by-on-multiple-columns

[order by]

ORDER BY column1 DESC, column2

This sorts everything by column1 (descending) first, and then by column2 (ascending, which is the default) whenever the column1 fields for two rows are equal.

The other answers lack a concrete example, so here it goes:

Given the following People table:

 FirstName |  LastName   |  YearOfBirth
----------------------------------------
  Thomas   | Alva Edison |   1847
  Benjamin | Franklin    |   1706
  Thomas   | More        |   1478
  Thomas   | Jefferson   |   1826

If you execute the query below:

SELECT * FROM People ORDER BY FirstName DESC, YearOfBirth ASC

The result set will look like this:

 FirstName |  LastName   |  YearOfBirth
----------------------------------------
  Thomas   | More        |   1478
  Thomas   | Jefferson   |   1826
  Thomas   | Alva Edison |   1847
  Benjamin | Franklin    |   1706

출처 : http://stackoverflow.com/questions/2051162/sql-multiple-column-ordering