sql group by versus distinct

Question

Why would someone use a group by versus distinct when there are no aggregations done in the query?

Also, does someone know the group by versus distinct performance c

Answer 1

A little (VERY little) empirical data from MS SQL Server, on a couple of random tables from our DB.

For the pattern:

SELECT col1, col2 FROM table GROUP BY col1, col2

and

SELECT DISTINCT col1, col2 FROM table 

When there's no covering index for the query, both ways produced the following query plan:

|--Sort(DISTINCT ORDER BY:([table].[col1] ASC, [table].[col2] ASC))
   |--Clustered Index Scan(OBJECT:([db].[dbo].[table].[IX_some_index]))

and when there was a covering index, both produced:

|--Stream Aggregate(GROUP BY:([table].[col1], [table].[col2]))
   |--Index Scan(OBJECT:([db].[dbo].[table].[IX_some_index]), ORDERED FORWARD)

so from that very small sample SQL Server certainly treats both the same.