Duplicate Key

Enhanced Definition

A duplicate key refers to a record identifier or index value that is not unique within a dataset, file, or database table. In mainframe systems, it signifies that multiple records share the same value for a designated key field, which may be allowed or disallowed depending on the data structure's definition and integrity requirements. A duplicate key refers to a situation where an index or a key field within a dataset or database table contains the same value for multiple records or rows. In the mainframe context, this typically applies to indexed files like VSAM Key -Sequenced Data Sets (KSDS) or database management systems such as DB2 and IMS. It signifies that the key value, which is used for record identification and retrieval, is not unique across all entries.

Key Characteristics

- Non-Uniqueness: The defining characteristic is that the key value does not uniquely identify a single record; multiple records possess the identical key value.
- Index Definition: Its allowance or disallowance is determined by the index or file definition (e.g., UNIQUE vs. NON-UNIQUE index in DB2, WITH DUPLICATES clause in COBOL SELECT statements for VSAM files, or NONUNIQUEKEY attribute for VSAM Alternate Indexes).
- Data Integrity Impact: When duplicate keys are *not* intended (e.g., for a primary key), their presence indicates a data integrity violation, potentially leading to incorrect data retrieval or application errors.
- Access Method Handling: Different access methods and database systems handle duplicate keys distinctly. For instance, VSAM KSDS primary keys *must* be unique, while Alternate Indexes can permit duplicates.
- Performance Considerations: While non-unique indexes with duplicate keys can improve query performance for certain lookups, they can also increase storage requirements and update overhead compared to unique indexes.

Use Cases

- Non-Unique Indexing: Creating a non-unique index on a field like CUSTOMER_CITY in a CUSTOMER table to efficiently retrieve all customers residing in a specific city, where many customers will share the same city.
- COBOL File Processing: Processing a sequential or indexed file where records might legitimately share a key value, such as a transaction file where multiple transactions can occur for the same ACCOUNT_NUMBER on a given day.
- Data Loading and Validation: Identifying and handling duplicate records during bulk data loading into a database or file system, where business rules might dictate whether to reject, update, or log duplicate entries.
- VSAM Alternate Indexes: Defining a VSAM Alternate Index (AIX) over a KSDS or ESDS to allow access to records by a secondary key that is not unique, such as a DEPARTMENT_CODE in an EMPLOYEE