QGIS » QGIS Application

"Merge: This geoprocess can work with many input layers. It merges all of
these creating a new result layer which has a feature for each feature
of all input layers. Because each input layer could have its own
alphanumeric schema, user must select which layer will preserve its
schema. The rest of input layers will only preserve those attributes
whose name and type would be the same that an attribute of the input
layer whose schema will be preserved."

As described in the above definition of "merge" it is necessary to select a "base" layer (the one that will preserve its schema) and how many other layer to merge with it. If too complicated it would be enough to have the chance to select the "base" layer and just another one.

This for instance would be the approach used by gvSIG. Another option but would be
the Arcgis approach: when selecting the layers (two or more) to be merged, the interface creates a list of all columns available and leave the user select what columns to keep and what columns not to use.

If there are columns with the same name, then they are not replicated (so the values of columns of different layers will be merged into the same column) and if they are of different type, than the type of the column of the first selected layer is used.

This approach is more flexible but has one disvantage: in some cases, when there are columns with the same name but different type, the operation fails. Example: when it cannot copy a text value into a numeric type column.

If the latter approach will be used then this detail should be considered.