Re: [gnome-db] GdaSqlBuiler: JOIN AS

[Vivien Malerba <vmalerba gmail com>]

On 17 May 2010 14:53, Murray Cumming <murrayc murrayc com> wrote:
> On Mon, 2010-05-17 at 14:43 +0200, Vivien Malerba wrote:
>> On 17 May 2010 14:39, Murray Cumming <murrayc murrayc com> wrote:
>> > On Mon, 2010-05-17 at 14:20 +0200, Vivien Malerba wrote:
>> >> On 17 May 2010 10:07, Murray Cumming <murrayc murrayc com> wrote:
>> >> > In Glom, I use the AS keyword with JOINS, to allow multiple joins with
>> >> > the same table. A simple case looks like this, though it's unnecessary
>> >> > in this particular example:
>> >> >
>> >> > SELECT "albums"."album_id", "albums"."name", "albums"."artist_id",
>> >> > "relationship_artist"."name",
>> >> > FROM "albums"
>> >> > LEFT JOIN "artists"
>> >> >  AS "relationship_artist"
>> >> >  ON ("albums"."artist_id" = "relationship_artist"."artist_id")
>> >> > WHERE "albums"."album_id" = 123
>> >> >
>> >> > (That might be an "alias", but I don't know if that's the right
>> >> > terminology.)
>> >> >
>> >> > But I don't see how to do that with
>> >> > gda_sql_builder_select_join_targets()
>> >> > http://library.gnome.org/devel/libgda/unstable/GdaSqlBuilder.html#gda-sql-builder-select-join-targets
>> >> >
>> >>
>> >> To do this you need to specify an alias when using the
>> >> gda_sql_builder_select_add_target*() methods.
>> >
>> > But that's for adding table targts, right? How would it know which join
>> > the alias name is for?
>> >
>>
>> The alias is for a target (table or sub select), not for a join: you
>> don't need to name joins as they are never referenced anywhere else in
>> the SQL statement, you can name targets if necessary.
>
> In my example, the join name is mentioned in the list of fields. For
> instance, relationship_artist.name.
>
> Here's a silly example that shows the need for the name, because using
> the table name would be ambiguous:
>
> SELECT "albums"."album_id", "albums"."name",
> "albums"."artist_singer_id", "relationship_artist_singer"."name",
> "albums"."artist_drummer_id", "relationship_artist_drummer"."name",
>  FROM "albums"
>  LEFT JOIN "artists"
>    AS "relationship_artist_singer"
>    ON ("albums"."artist_singer_id" =
> "relationship_artist_singer"."artist_id")
>  LEFT JOIN "artists"
>    AS "relationship_artist_drummer"
>    ON ("albums"."artist_drummer_id" =
> "relationship_artist_drummer"."artist_id")
> WHERE "albums"."album_id" = 123
>

This example illustrates why you need to be able to set targets'
aliases. In this example you have the following targets:
A: "albums"
B: "artists" with the "relationship_artist_singer" alias
C: "artists" with the "relationship_artist_drummer" alias

and you reference the targets using their alias in the join condition
and in the list of fields to be selected.

and 2 joins (joinning conditions not mentionned here):
A <--> B
A <--> C

the joins themselves are never referenced anywhere in the statement,
so they are not named.

Vivien