Because relational databases handle I/O at the column (field) level, and COBOL handles I/O at the record level, some mapping is necessary to associate records with their fields. One function of Database Connectors is to map COBOL records into database fields, and to map the database fields back into records. Database Connectors does this by consulting XFDs generated by the Compiler. The detailed structure of XFDs is discussed in the section XFD Files in the chapter XFDs.
Database Connectors builds its own database queries dynamically whenever an input or output request is received. You just compile your program, and Database Connectors does the rest.