Spatial Databases - Desktop GIS: Mapping the Planet with Open Source Tools - page 105

Figure 7.6: PostGIS layers created with the query builder
Note that now there is something in the “Sql” column next to the gnis
layer. This is just the contents of the query box but serves to remind
us what we are adding in the event that we set up queries and add
more than one layer at a time. With the gnis layer selected, we click
the Add button to add it to the map. In Figure 7.6 , you can see that
in addition to the school layer, we added layers for airports and mines.
QGIS doesn't provide a very pleasing name in the legend when adding
layers in this way, so we took the liberty of renaming each of the GNIS
layers to something sane. So, now we have a map with three separate
layers, all derived from the gnis layer in our database.
Now maybe you are asking yourself, why not just add the gnis layer
and symbolize it based on type? We could do that, and it might work
assuming the following:
• Our data is not too dense.
• We want to see all types, not just schools, airports, and mines.
• Our layer isn't so large that it causes performance problems.