Fix distinct alias when multiple databases used

[aidanharan]

Fix issue with the generated SQL for DISTINCT queries as reported in #1256. The generated SQL:

• Unnecessarily added a ALIAS AS projection.
• The column used of the ALIAS AS was being incorrectly quoted. This was caused by MySQL database also being used in multi database Rails application.

Post.includes(:comments).joins(:comments).first

Before:

SELECT DISTINCT [posts].[id] AS alias_0, [posts].[id] FROM [posts] INNER JOIN [comments] ON [comments].[post_id] = [posts].[id] ORDER BY [posts].[id] ASC OFFSET 0 ROWS FETCH NEXT @0 ROWS ONLY

After:

SELECT DISTINCT [posts].[id] FROM [posts] INNER JOIN [comments] ON [comments].[post_id] = [posts].[id] ORDER BY [posts].[id] ASC OFFSET 0 ROWS FETCH NEXT @0 ROWS ONLY