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書名： QGIS Python Programming Cookbook
作者名： Joel Lawhead
本章字?jǐn)?shù)： 284字
更新時(shí)間： 2021-07-23 19:48:53

Loading a vector layer from a spatial database

The PostGIS geodatabase is based on the open source Postgres database. The geodatabase provides powerful geospatial data management and operations. PyQGIS fully supports PostGIS as a data source. In this recipe, we'll add a layer from a PostGIS database.

Getting ready

Installing and configuring PostGIS is beyond the scope of this book, so we'll use a sample geospatial database interface from the excellent service www.QGISCloud.com. www.QGISCloud.com has its own Python plugin called QGIS Cloud. You can sign up for free and create your own geodatabase online by following the site's instructions, or you can use the example used in the recipe.

How to do it...

Perform the following steps to load a PostGIS layer into a QGIS map:

First, create a new DataSourceURI instance:
```
uri = QgsDataSourceURI()
```

Next, create the database connection string:

uri.setConnection("spacialdb.com", "9999", "lzmjzm_hwpqlf", "lzmjzm_hwpqlf", "0e9fcc39")

Now, describe the data source:

uri.setDataSource("public", "islands", "wkb_geometry", "")

Then, create the layer:

layer = QgsVectorLayer(uri.uri(), "Islands", "postgres")

Just to be safe, make sure everything works:

if not layer.isValid():
 print "Layer %s did not load" % layer.name()

Finally, add the layer to the map if everything is okay:
```
QgsMapLayerRegistry.instance().addMapLayers([layer])
```

You can see the islands layer in the map, as shown in the following screenshot:

How it works...

PyQGIS provides an object in the API to create a PostGIS data source in QgsDataSourceURI(). The connection string parameters in the second line of code are the database server, port, database name, user, and password. In the example, the database, username, and password are randomly generated unique names. The data source parameters are the schema name, table name, geometry column, and an optional SQL WHERE to subset the layer as needed.

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QGIS Python Programming Cookbook

Loading a vector layer from a spatial database

Getting ready

How to do it...

How it works...