Call-Level Interfaces

The final technique for sending SQL statements to the DBMS is through a call-level interface (CLI). A call-level interface provides a library of DBMS functions that can be called by the application program. Thus, instead of trying to blend SQL with another programming language, a call-level interface is similar to the routine libraries most programmers are accustomed to using, such as the string, I/O, or math libraries in C. Note that DBMSs that support embedded SQL already have a call-level interface, the calls to which are generated by the precompiler. However, these calls are undocumented and subject to change without notice.

Call-level interfaces are commonly used in client/server architectures, in which the application program (the client) resides on one computer and the DBMS (the server) resides on a different computer. The application calls CLI functions on the local system, and those calls are sent across the network to the DBMS for processing.

A call-level interface is similar to dynamic SQL, in that SQL statements are passed to the DBMS for processing at runtime, but it differs from embedded SQL as a whole in that there are no embedded SQL statements and no precompiler is required.

Using a call-level interface typically involves the following steps:

1 The application calls a CLI function to connect to the DBMS.

2 The application builds an SQL statement and places it in a buffer. It then calls one or more CLI functions to send the statement to the DBMS for preparation and execution.

3 If the statement is a SELECT statement, the application calls a CLI function to return the results in application buffers. Typically, this function returns one row or one column of data at a time.

4 The application calls a CLI function to disconnect from the DBMS.