What is a foreign key in SAP table definitions?

A foreign key in the ABAP Data Dictionary links a field in one table to a field in another (the check table), ensuring that any value entered must already exist in the referenced table — the same referential integrity concept found in any relational database, but configured through SAP's Data Dictionary transaction (SE11) rather than raw SQL DDL.

//  conceptually, in table VBAK (sales order header):
//  field KUNNR has a foreign key relationship to table KNA1 (customer master)
//  ensuring you can't create a sales order for a customer that doesn't exist

Beyond enforcing valid references, a defined foreign key also drives automatic input validation on data entry screens — when a user types a customer number into a field with a foreign key relationship, SAP can automatically check it against the check table and offer a value-help dropdown listing valid entries.

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