Introduction to CA IDMS Data Manipulation Language › Operating Environments › Accessing the Database

Accessing the Database

Your program can access a CA IDMS/DB database by using either navigational or LRF (logical record) DML statements:

• Navigational statements access database records and sets one record at a time.
• LRF statements access predefined groups of database records using the Logical Record Facility (LRF).

Navigational and LRF DML statements are discussed separately below.

Navigating the Database

Navigational DML statements access database records and sets one record at a time, checking and maintaining currency in order to assure correct results. Navigational DML statements provide:

• Control over error checking—You can check the result of each navigational statement
• Flexibility in choosing how you want to access the database—For example, your program can access the database either sequentially (performing an area sweep), by using a symbolic key value (CALC), or by using a database key value (DIRECT)

To use navigational DML statements, you must have a thorough knowledge of the database structure. The database structure is illustrated in a data structure diagram. For an example of a data structure diagram, see the EMPLOYEE Data Structure Diagram.

The following figure illustrates a database structure that contains two owner records (EMPLOYEE and JOB) that share one member record (EMPOSITION). To obtain EMPLOYEE and JOB information, the program must retrieve an EMPLOYEE record, the first EMPOSITION record in the EMP-EMPOSITION set, and the owner record in the JOB-EMPOSITION set.

Navigational DML statements are grouped into four categories:

• Control statements initiate and terminate processing, effect recovery, prevent concurrent updates, and evaluate set conditions
• Retrieval statements locate data in the database and make it available to the application program
• Modification statements update the database
• Accept statements pass database keys, storage address information, and statistics to the program

Accessing the Database Through LRF

LRF DML statements use the Logical Record Facility (LRF) to access database records. LRF accesses fields from multiple database records as if they were data fields in a single record. LRF DML statements allow your program to specify selection criteria (by using the WHERE clause) that enable your program to access only the logical records you need.

Note: For more information, see the Logical Record Facility Guide.

LRF DML statements provide:

• Easy access to database records—You need not be familiar with database structure, and your programs need not include database navigation logic.
• Data flexibility—You do not usually have to modify or recompile your LRF program when the database is changed.
• Runtime efficiency—LRF minimizes communication between the program and the database management system (DBMS).

The following figure illustrates how to use LRF DML statements to access the EMPJOBLR record. The EMPJOBLR record is a logical record that contains the EMPLOYEE record, the EMPOSITION record, the OFFICE record, and the JOB record. The EMPJOBLR logical record contains information from the EMPLOYEE, EMPOSITION, and JOB records.

The LRF DML statements are :

• @ERASE deletes a logical record from the database.
• @MODIFY updates a logical record.
• @OBTAIN retrieves a logical record.
• @STORE adds a new logical record to the database.