[Haskelldb-users] Request for code review: Patch removing "distinct" option, adds "unique" function

[Justin B. <jgb...@gm...>]

All,

Attached you'll find a patch that removes the mandatory "DISTINCT"
clause from select statements. Instead, if a query has the "unqiue"
function applied, a 'group by' statement will appear in the select,
grouping by all non-aggregate columns. This works properly with
optimization so unused columns do not appear in the group-by. Note
this patch replaces my previous one - I think this approach is a lot
better and works in more situations.

One change I'm unsure of is that "unique" is NOT default behavior
anymore. I think a select statement with a "distinct" on all columns
is very surprising so I like the change. Others may disagree.

A summary of significant changes:

    M ./src/Database/HaskellDB/PrimQuery.hs -3 +7
* Added "Group" value to PrimQuery. This value holds an association
list and a query. The association list is the columns/expressions that
will be grouped; the query is the query to which Group was applied. In
SQL, grouping can happen on expressions, which is why an Assoc list is
used instead of a simple list of column names.


    M ./src/Database/HaskellDB/Query.hs -9 +30
* Added the 'unique' function, which causes the Group value to be
added to the query. All currently projected columns are examined and
non-aggregates are added to the Group value. This allows those columns
to be later generated in a "group by" statement.

    M ./src/Database/HaskellDB/Sql/Generate.hs +3
* Added 'sqlGroup' field to SqlGenerator value. Used to process Group values.

    M ./src/Database/HaskellDB/Sql.hs -12 +16
* Changed the 'groupby' field from a list of SqlExpr to a list of
(SqlColumn, SqlExpr) pairs. Not really needed right now, but will
allow 'GROUP BY' to be as expressive as the 'column list' available to
SELECT.

    M ./src/Database/HaskellDB/Sql/Default.hs -8 +12
* Added 'defaultSqlGroup' converts the Assoc list from the Group value
to a list of (SqlColumn, SqlExpr) pairs and stores them in the groupby
field.
* Note that defaultSqlSelect did NOT have to change - it already
handled grouping for non-aggregate expressions properly.

    M ./src/Database/HaskellDB/Sql/Print.hs -3 +9
    M ./src/Database/HaskellDB.hs -1 +1
    M ./src/Database/HaskellDB/Optimize.hs -7 +16
* Various small changes to support new values and changed structures.

I welcome feedback on this patch. I'm happy to clarify anything or
provide examples.

Justin