Tables are joined on the basis of a primary key – a field in both tables that share the same values and field type. Field names for the key need not be the same, but field types must be identical. For example, tables could not be joined on a key that in one table is the field type “short integer” and in the other “text”, even if the field values are otherwise identical. In the example below the primary key is UNIT_ABBREV and LABEL. Both fields are type “text”. To see the field types of a table in ArcMap, examine the “Fields” tab in layer Properties. The same can be done in ArcCatalog. Right-click on the table to access Properties.

If the join will be done on a "text" primary key field, the key attributes in both tables must not only be "text", but of the same case; the text in one table's key field can't be upper case and the other lower case (at least for tables exported from Excel).

To Join the lower table with upper table in ArcMap, open the Layer Properties window of the upper table’s layer (in this case a polygon shapefile). Bring up the “Joins & Relates” tab and click the “Add” button in the Joins half (left half) of the dialog window to bring up the dialog window shown below:

Fill in the dialog boxes:

• Item 1 requests the primary key. In the example above you would select from the drop down menu “UNIT_ABBREV”.
• Item 2 requests the name and location of the table to join to – browse to that table. It need not be loaded in ArcMap, but it must be a table in dBase or text format; Excel or other tables must first be converted to one of these formats (see the tip on doing so).
• Item 3 requests the field to join on. In the example above it is “LABEL”.

The “Advanced…" button provides an additional options for how to handle records in the joined table for which there is nothing to join. Examine the options. The default is usually OK.

The result of a table Join is a modified attribute table for the layer to which the join is made. The new table shows the appended fields from the joined table and new field headings that reflect the join. The table cannot be edited, though the source tables can be, and is updated as changes to the source tables are made. The join only exists in a virtual state – no permanent changes are made to either table – and persists only within the map document. To establish the relationship more permanently, you can either store the join as part of the layer’s properties by creating a Layer file, or you can export the data to a new feature class or shapefile, which creates a new file with an attribute table that contains the joined fields. In the example above the resulting join will look like the table below, with the exception of the field names, which will also incorporate the table (file) names.