Creating a parametric view

A parametric SQL view is based on a SQL query containing the named parameters with values provided dynamically in WMS and WFS requests.

How to do it…

1. We will now create a new view by extending the code of the previous step. What if you want to have a dynamic view that works for each continent? You need to start the same way as we did previously: select Add a new resource from the Layer menu.
2. Select the data store pointing to PostGIS or your preferred RDBMS. Instead of selecting a table from the list, select the Configure new SQL view… link:
3. In the form, insert ContinentView as View Name and the following code as the SQL statement:

   SELECT 
     ADMIN,
     GEOM
   FROM 
     COUNTRIES
   WHERE 
     CONTINENT = '%continent%'

4. Go to the SQL view parameters section and click on the Guess parameters from SQL link. Insert Africa as the default value.
   
5. At the bottom of the page, select the Guess Geometry type and srid checkbox and click on Refresh. The 4326 EPSG code is properly detected, but you have to manually select MultiPolygon instead of the detect value of Polygon.
6. Click on Save, and you'll be taken to the publish the layer form. Click on the button to calculate the native data extent, and then click on Publish. Move to Layer Preview and select the ContinentView layer. Your map now looks like this one:
   
7. The preview map shows you the result of filtering with default values. Add &viewparams=continent:Europe to your request URL and reload the map. You should now see a different map:
   

How it works…

A parametric view lets you define one or more filtering parameters at request time. In this recipe, instead of building five different views (one for each continent), we have a single view, and the calling app can define which continent is relevant in the request.

Building a parametric view is not different from a standard SQL view. You can start creating the code without any parameters to check that you have no syntax or logical errors.

Once you are done with the code, you can select which field should become a parameter. To let GeoServer recognize the parameters, you simply enclose them within % characters, as shown in the following code:

WHERE CONTINENT = '%continent%'

GeoServer recognizes the parameters and lets you set an optional default value. Please note that if you don't set a default value, you always need to set a value for parameters when sending a request to GeoServer.

At run time, you set values for each parameter, adding the viewparams option to your request. As per the value of the option, you insert a set of params-value couples.