当我们在SQL查询中使用“ group by 1”时，这是什么意思

Question

I have come across a query where it is specified by

select concat(21*floor(diff/21), '-', 21*floor(diff/21) + 20) as `range`,     count(*) as 
`number of users` from new_table group by 1 order by diff;

here what exactly does group by 1 mean?

Answer 1

Assuming you have a Select:

SELECT name FROM employee GROUP BY 1;

No matter what, it will always group by the first column given in the select. In this case, the column 'name' is grouped.

So if we alternate the above statement to:

SELECT department FROM employee GROUP BY 1;

We now group the department, without having to change the '1' in the group by.

EDIT: (as requested by Stewart)

If we have the following Data in table 'employe':

-- name --
Walt
Walt
Brian
Barney

A simple select would deliver all rows above, whereas the 'group by 1' would result in one Walt-row:

output with group by:

-- name --
Walt
Brian
Barney

Answer 2

+1 to @FabianBigler for answering first, but I'll add this:

http://dev.mysql.com/doc/refman/5.6/en/select.html says:

Columns selected for output can be referred to in ORDER BY and GROUP BY clauses using column names, column aliases, or column positions. Column positions are integers and begin with 1.

For what it's worth, this is non-standard SQL, so don't expect it to work on other brands of SQL database.