The purpose of the foreign key is to validate the data that is being entered into a table by checking entries in a check table. Foreign keys are checked by the front end user interface only and it is not checked if you issue a direct a SQL statement to update the database.
Follow the steps given below to create a foreign key in SAP table.
Step 1: Open the table in Data Dictionary (SE11) for which you want to create a foreign key. Select the field for which you want to create the foreign key and press Foreign Keys button.
Step 2: In the popup window enter the check table name and press Generate proposal button.
Step 3: The system proposes the foreign key relation based on the domain. Check that the foreign key relationship proposed by the system is correct and press copy.
Foreign key is created, now save and activate the table. To check the foreign key go to menu path Utilities->Table Contents->Create Entries.
Try to create an entry in “Zemployee” table with “Department ID” that is not there in the “Department” table. Enter the values and try to save the entry.
Observe the error message in the status bar. The user interface does the foreign key validation before creating the entries.