In a mainframe batch environment, how can you send data from a COBOL file to a DB2 table in a controlled and reusable way?

Introduction / Context:
Moving data from flat files into relational tables is a common task on IBM mainframe systems. In COBOL-DB2 environments, interviewers often ask how you would send data from a COBOL file to a DB2 table. The answer reveals whether you understand embedded SQL, batch processing patterns, and alternative utilities such as LOAD and IMPORT. Here we focus on the controlled, programmatic approach using a COBOL-DB2 application.

Given Data / Assumptions:

• We have a sequential COBOL file or VSAM file containing records with fields that match or map to columns in a DB2 table.
• We are running batch jobs under JCL on z/OS.
• We want a reusable, controllable method for populating or updating a DB2 table from this file.

Concept / Approach:
The standard approach is to write a COBOL-DB2 batch program that uses file I/O to read each record and embedded SQL to insert or update rows in the DB2 table. The program declares a host structure that matches the DB2 table columns, moves fields from the input record into the host variables, and then executes an INSERT or UPDATE statement. This allows you to perform validation, transformation, error handling, and logging as you load data, making the process robust and maintainable.

Step-by-Step Solution:
Step 1: Design or identify the DB2 table structure that will hold the data from the COBOL file, ensuring compatible data types and lengths. Step 2: In the COBOL-DB2 program, define an FD for the input file and a working storage structure that corresponds to the DB2 table columns as host variables. Step 3: Use EXEC SQL INCLUDE statements for the table copybook or define explicit host variables for each column. Step 4: In the PROCEDURE DIVISION, open the input file, read each record sequentially, and move the record fields into the host variables, applying any necessary validation or transformation. Step 5: For each record, issue an embedded SQL INSERT INTO table-name VALUES (:host-var-list) statement inside an EXEC SQL block, handle SQLCODE values, and commit at appropriate intervals to balance performance and recovery.

Verification / Alternative check:
You can verify this process by running the batch program and then querying the DB2 table to confirm that the expected number of rows has been inserted and that fields match the source data. Log files and SQLCODE checks help identify any errors or constraint violations. Although DB2 utilities such as LOAD or IMPORT can also move data from files to tables, they are typically controlled by DB2 utility jobs rather than by COBOL-DB2 application logic, so the embedded SQL approach is the most direct answer for a programming interview question.

Why Other Options Are Wrong:
Option B is incorrect because simply renaming a file does not cause DB2 to import its contents; DB2 requires utilities or programs to read and interpret the data. Option C is wrong because copying a flat file into DB2 catalog datasets with IEBCOPY would corrupt internal structures; DB2 tables are stored in managed table spaces, not as simple editable files. Option D, while technically possible on a very small scale, is impractical, error prone, and not considered a real solution in professional environments.

Common Pitfalls:
Common pitfalls include failing to handle duplicate keys or constraint violations gracefully, omitting COMMIT statements leading to long running units of work, and mismatching data types between COBOL fields and DB2 columns. Another issue is not using parameterised SQL properly, which can hurt performance. Developers must also consider performance tuning when dealing with large volumes, such as batching commits, using arrays or multi row inserts where supported, and considering utility based approaches for bulk loads. Properly designed COBOL-DB2 programs provide flexibility and control for ongoing file to table data movement.

Final Answer:
A standard solution is to write a COBOL-DB2 batch program that reads records from the COBOL file and uses embedded SQL INSERT (or UPDATE) statements with host variables to send the data into the DB2 table under program control.