A foreign key creates a link between two tables T1 and T2. Every primary key field from T1 (check table) is assigned a field from table T2 (foreign key field). The fields from T2 assigned to primary key fields are marked as foreign key fields.
The most important function of the foreign key is the support of data integrity. The foreign key fields can only accept values which appear in the primary key of the check table. During input the values of the foreign key fields can thus be checked against the entries of the assigned key fields of the check table.
Foreign keys are also the foundation for defining lock objects, maintenance views and help views. These objects contain fields from several tables. All the tables used in such an object must be linked with foreign keys.
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How can I make a field which is not a primary key as a foreign key is that possible?
The answer is not possible.
The check table must have a key field which has the same domain as the check field of the foreign key table.
The table u selected cannot be used as a check table because it does not have a key field with the same domain as the check field.
The solution is :Select a table as check table which satisfies the above condition. The possible candidates can be displayed via the F4 help for the input field 'check table' in the foreign key maintenance.