What does SQL clause “GROUP BY 1” mean?

Someone sent me a SQL query where the GROUP BY clause consisted of the statement: 

GROUP BY 1.

This must be a typo right? No column is given the alias 1. What could this mean? 

Am I right to assume that this must be a typo?

 Answers

It means to group by the first column regardless of what it's called. You can do the same 

with ORDER BY.

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In addition to grouping by the field name, you may also group by ordinal, or position the

 field is.

This is generally unadvised if you're grouping on something specific, since the table/view 

structure may change; additionally, it is more difficult to read (credit: Yuck). However if you

 are returning a unique set of something, then it is okay.

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It will group by the column position you put after the group by clause.

for example if you run 'SELECT SALESMAN_NAME, SUM(SALES) FROM SALES GROUP BY 1' 

it will group by SALESMAN_NAME.

One risk on doing that is if you run 'Select *' and for some reason you recreate the table 

with columns on a different order, it will give you a different result than you would expect.