[Qgis-user] New Labeling: conditional labels

[Mayeul Kauffmann]

Martin wrote:
>I would suggest to implement the CASE statement 
Yes that would be great!!
CASE also supports another syntax which is even more flexible:

       CASE WHEN a=1 THEN 'one'
            WHEN a=2 THEN 'two'
            ELSE 'other'
       END

instead of:

CASE a WHEN 1 THEN 'one'
              WHEN 2 THEN 'two'
              ELSE 'other'
       END

(example from [2]).

The first syntax allows to create conditions based on several fiels, like in the rule based renderer, for instance:
       CASE WHEN a=1 AND b=2 THEN 'one'
            WHEN c=3 THEN 'two'
            ELSE 'other'
       END

Cheers,
Mayeul


Le vendredi 06 janvier 2012 à 13:25 +0100, Martin Dobias a écrit :
> Hi
> 
> On Thu, Dec 29, 2011 at 11:33 AM, Nathan Woodrow <madmanwoo at gmail.com> wrote:
> > It would be handy to add condition statements to QgsExpression so we can
> > support this kind of thing, Martin would be the guy to talk to about adding
> > this kind of thing.  He knows the most about QgsExpression.
> 
> SQL recognizes CASE statement with following alternate syntax:
> 
> 1. "if" equivalent:
> CASE
>   WHEN x > 10 THEN 'big' ELSE 'small'
> END
> 
> 2. "switch" equivalent:
> CASE x
>   WHEN 5 THEN 'five'
>   WHEN 6 THEN 'six'
>   ELSE 'something else'
> END
> 
> The "else" is not required, if there is no matching "when" then NULL
> is returned.
> 
> Some SQL engines add further functions for convenience:
> - MySQL [1]: IF(), IFNULL(), NULLIF()
> - PostgreSQL [2]: COALESCE(), NULLIF()
> 
> I would suggest to implement the CASE statement to stay compliant with
> SQL syntax (although I find It a bit verbose).
> 
> Regards
> Martin
> 
> [1] http://dev.mysql.com/doc/refman/5.0/en/control-flow-functions.html
> [2] http://www.postgresql.org/docs/8.1/static/functions-conditional.html
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