Utilities > Dissolve (Merge Parts by Field) (select from Database Table Menu)
Merge geometries of records that share a common field value.
Dissolve merges geometries of records that share a common value in a specified field (viz. the Dissolving Field).
The user can choose between polygon (Merge Polygon Geometry) or polyline (Merge Polyline Geometry) entities to be merged. One can also specify all records or Only Selected Records to be merged and a Rank Field can be applied. The latter, indicates which record is the primary record in the merging process - the properties of this primary record will be transferred to the final merged entity's properties. The geometric properties (such as area, perimeter, length, etc.) will, however, automatically be recalculated for the final merged entity.