What are the differences between GROUP BY and ORDER BY... The manual is not clear on this

GROUP BY is a way to sub-total your results, or perform some other "aggregate" function on them.

Example :

SELECT department, sum(salary)
FROM tblEmployee
GROUP BY department;

will give you salary totals by department, whereas the sum statement by itself would just give you the grand total of all salaries in tblEmployee.

ORDER BY is simply a way to sort your results - it does not affect what shows up in your resultset, only what order it is displayed.

Example :

SELECT *
FROM tblEmployee
ORDER BY lastname;

will give you all tblEmployee data, in order of last name.  If you want the results in descending (reversed) order, simply add DESC to the end of the clause :
ORDER BY lastname DESC;

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