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Module 14:- Database Interface

COBOL – Database Interface

  • COBOL program can interface with DB2 database.
  • DB2 stands for ‘Data Base 2’ and it is IBM designed relational database management system.
  • Data in DB2 system is stored in form of table, that contains rows(tuples) and columns(attributes).
  • DB2 system is almost similar to SQL with some added features to support large-scale mainframe applications
  • Following topics will help you understand how COBOL program can interface with DB2 systems
    • Embedded SQL
      • Use of SQL statements within source code of your program is known as Embedded SQL.
      • COBOL allows you include Embedded SQL statements in your COBOL programs. Any language interfacing with DB2 is referred to as Host Language. In our case COBOL is Host Language.
      • This feature allows COBOL programmer to directly perform standard SQL operation on database from COBOL program
      • Embedded SQL is explained in detail here
    • DB2 Application Programming
      • Embedded SQL statementi is allowed to program in COBOL.
      • Embedded SQL statement must be enclosed between “EXEC SQL” and  “END-EXEC”
      • DB2 Application programming is explained in detail here
    • Host variables
      • Host Variables are used to store data that is accessed by both COBOL and DB2.
      • Host vatiable allows us to 
        • Collect data from DB2 table using FETCH, INTO, VALUES INTO etc. clause
        • Place data into Host variable and perform INSERT, UDPATE etc. operations
        • Use the data in the host variable when evaluating a WHERE or HAVING clause.
        • Use the host variable to indicate a null value
      • Host variables is explained in detail here
    • DCLGEN
      • DCLGEN stands for Declarations Generator.
      • It is a tool that is used to generate COBOL copybook for DB2 table. This copybook will contain COBOL host variables structure that is map to the columns of DB2 table.
      • DCLGEN is explained in detail here
    • SQLCA
      • SQLCA stands for SQL Communication Area and it is DB2 system provided copybook.
      • SQLCA has multiple Host variables defined to hold the information about last executed SQL statement
      • All host variables get updated after the execution of each SQL statement.
      • Thus, every COBOL program containing Embedded SQL statements must have SQLCA included in WORKING-STORAGE SECTION.
      • SQLCA is explained in detail here
    • SQL Queries
      • Popular SQL statements like SELECT, INSERT, UDPATE DELETE can be used in our COBOL-DB2 Program. It it explained with help of COBOL-DB2 program example here
    • DB2 Cursors
      • Cursor is used when more than one row of table is to be processed.
      • Cursor allows COBOL program to retrieve the set of rows (result set) and then process that returned data one row at a time.
      • Cursor can be thought of as data structure that holds all the resultant records of query.
      • By using cursor, program can retrieve each row sequentially from the result table until End of Data is reached
      • DB2 Cursors is explained in detail here

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