slick/slick — pull request #847 by imcharsi (2014-06-06 11:27:01+00:00)
Title: The experimental patch to leave out parenthesis around the Union.

Hi.
At first, My english is not good.

Let's say a example query.
```
tabA.union(tabB).union(tabC)
```
this patch creates sql like below
```
select ... from tA union select ... from tB union select ... from tC
```
original implementation creates below sql.
```
select ... from (select ... from (select ... from tA union select ... from tB) union select ... from tC)
```
this patch is intended to solve the With Clause's parenthesis probelm
because there are some syntax restrictions like ORA-32042.
I want to say that this is not a workaround for something error or bug.

This patch doesn't use the `Union` in node permutation(fuse).
the `Union` is only used with `Query.union`,
and all union-ed nodes are converted and collected to `Comprehension`'s `unionNodes` as wrapper node `InternalUnionNode`.
so, there is no the `Union` in `JdbcStatementBuilderComponent` phase.

Problem
1:
With this patch, in the Derby DBMS, `NewQuerySemanticsTest.scala:350` is not executed.
Otherwise, there is no problem in another DBMSs.
so, I think that maybe this is the Derby's bug.
As workaround, I skipped the line of the test only when the Derby DBMS is used.
Please check this.

Thanks.