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Has someone a link to a tutorial or a manual that explain the functionalities of the 'SQL query composer' for QGIS?

I'm working with WFS vectors connected from the same server and I would like to download datas after querying that involve different layers to minimizing the downloading datas.

SQL-query-compozer

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Edited

The SQL query composer is an interface to create SQL queries, and the drop-down menus on the left-hand side give you an overview of the tables/columns/functions that are available.

A very basic query is to select the attributes that you want to load and subset the data based on an attribute.

In the example below I only select the region and sqkm attribute from the worldregions layer, and I subset the records by the region ('Antarctica'). example QGIS WFS Query Composer

More complicated SQL queries are also possible (based on joins between different tables, calculations, spatial queries), these require some more advanced SQL understanding.

Another possibility is to first add the layer, and then subset the layer with the Query Builder:

  • Is the query builder documented in those references, the same query builder as the SQL query composer asked about in the question ~ where you build a query on a WFS connection to be applied when fetching the data? Probably it would make sense if they were the same, but it isn't obvious. – nmtoken Apr 15 at 10:42
  • I modified the answer, does this give more info? – Lennert Apr 18 at 9:37

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