Getting maximum values of attribute of multiple Shapefiles merged into new shapefile in QGIS?

Question

I have thousands of points in multiple shapefiles. I want to get the maximum value of an attribute of each shapefile. Then I want to merge all the maximum values to a new layer.

Is there a way to extract max values as a batch process from multiple layers?

EDIT:

This is the sample attribute table. The max values I need are from "DTM50austri" from each Shapefile.

This is the query I've tried. "Zusammengeführt" is the merged shapefile.

Answer 1

This can be achieved doing the following steps:

1. Merge all your shapefiles using Vector --> Data Management Tools --> Merge Vector Layers

2. Create a new virtual layer Layer --> Add Layer --> Add/Edit Virtual Layer

3. Enter the following Query: Select max("id") as "maxid", "geometry" from "Merged" Group by "path" (See explanation below)
4. Make sure to select geometry as Geometry column and Point as Type, hit the Add button and set virtual layer's CRS to original/merged shapefiles CRS if needed (Right click layer --> Properties --> Source --> Geometry and Coordinate Reference System)
5. You can now save the virtual layer as shapefile or any other supported format (Right click layer --> Export --> Save Features As...)

Explantation for the query:

• id is the fieldname of the values you want the maximum's of (see hint below)
• maxid is the new fieldname for your maximum values in the virtual layer
• geometry selects the geometry of your points
• Merged is the vector layer of the merged shapefiles
• path is a field that will be generated by merging the shapefiles inside Merged vector layer and contains paths to your shapefiles as well as their names; so it is unique for each file --> this allows you to select the maximum id of each shapefile by grouping them by path
• max() command selects the maximum value of your id field

My Example:

In my example I select the maximum id's.

Additional notes:

Of course this will only work when the maximum values you want to select all have the same column/attribute name in their original shapefiles! If not, you need to generate a new field in the merged shapefile and copy the values over to a new column via field calculator.