In order to extract raster values in QGIS I want to use the grass command v.what.rast.points in the toolbox. But the process won't start - it claims: "Where conditions of SQL statement without 'where' keyword". What am I supposed to fill in?
Another problem: I created an extra attribute column for the extracted values to be stored in (type: string; length: 7) , but it won't show in the selection tab of the v.what.rast.pints tool. Only the ID field can be chosen. 
The where parameter lets you specify which field and values you want to use in the calculation.
For example, if you have an id field and you only want to retrive the values for those points with an id less than 50, then you would use:
id < 50
If you want to use all values in a field (i.e. all features), you can just type in the field name:
id
In terms of the fields, it is mentioned in the GRASS v.what.rast help page that the field type must be numeric:
So you will need to create a numeric field, not string, in order to store the results.
