Host Variable - Program variable

Enhanced Definition

A host variable is a program variable declared within an application program (e.g., COBOL, PL/I, C/C++) that is used to exchange data with an embedded SQL statement. It acts as a placeholder for data that needs to be passed between the application program's logic and the database management system (DBMS), primarily DB2 for z/OS or IMS DB/DC.

Key Characteristics

- Declared in the host language's data declaration section, such as the WORKING-STORAGE SECTION or LINKAGE SECTION in COBOL.
- Prefixed with a colon (:) when referenced within an SQL statement to distinguish it from database column names or literals.
- Must have a data type that is compatible with the corresponding database column or expression it interacts with.
- Can be associated with an indicator variable to handle NULL values, data truncation, or other status information during data transfer.
- Processed by the DB2 precompiler (or equivalent) which converts embedded SQL statements into host language call statements, replacing host variable references with parameters.
- Typically declared within BEGIN DECLARE SECTION and END DECLARE SECTION blocks in COBOL or PL/I for explicit identification by the precompiler.

Use Cases

- Data Retrieval: Using a SELECT INTO statement to fetch a single row's column values from a DB2 table directly into corresponding host variables.
- Data Insertion: Providing values for new rows in an INSERT statement, where host variables supply the data for each column.
- Data Update: Supplying new values for columns in an UPDATE statement or providing values for the WHERE clause to identify rows to be updated.
- Data Deletion: Supplying values for the WHERE clause in a DELETE statement to specify which rows should be removed.
- Dynamic SQL: Used to hold input parameters or output results when executing dynamically prepared SQL statements.

Related Concepts

Host variables are fundamental to Embedded SQL, serving as the primary mechanism for application programs to interact with DB2 for z/OS. The DB2 precompiler plays a crucial role by translating SQL statements containing host variables into executable code. They are often used in conjunction with indicator variables to manage NULL values and data truncation, and the SQLCA (SQL Communication Area) provides status feedback on SQL statement execution, which might be influenced by host variable usage.

Best Practices:

Data Type Compatibility: Ensure that the data type and length of host variables precisely match or are compatible with the corresponding database columns to prevent data conversion errors, truncation, or performance overhead.
Initialize Variables: Always initialize host variables before using them in INSERT or UPDATE statements to avoid inserting unpredictable or garbage data into the database.
Use Indicator Variables: For any database column that can contain NULL values, always declare and use an associated indicator variable to correctly handle NULLs during retrieval and insertion/update.
Meaningful Naming: Use clear, descriptive names for host variables to enhance code readability and maintainability, especially in large COBOL programs.
Declare Sections: Enclose host variable declarations within EXEC SQL BEGIN DECLARE SECTION; and EXEC SQL END DECLARE SECTION; blocks in COBOL to clearly delineate them for the DB2 precompiler.