To edit a relation,
select the line connecting the tables and right click:

The Edit Relation window will then appear:

The Left Table Name, Left Field Name, Right Table Name, Right Field Name cannot be edited in the dialog box. These contain the tables and fields you selected in the Database Diagram.
Check one of the Relation
Type radio buttons:
· Only include
rows where joined fields are equal - this option grabs records from both
tables only where there is a match between the id_dep
and iddep_emp table fields.
· Include
all records from the A table and only those
from the B table where joined fields are
equal - with this option, all employee records will be selected, and only
departments with an ID that matches the iddep_emp
field from the employees_emp table.
· Include
all records from the B table and only those
from the A table where joined fields are
equal - here, all department records are listed, and only those employees
whose iddep_emp matches one of the department
ID's.