Lesson 28 Logical Data Bases

LOGICAL DATA BASES:

In general, the system reads data that will appear in a list from the database.You can use OPEN SQL or NATIVE SQL statements to read data from the database.The use of a logical database provides you with an alternative to having to program database accesses individually. Logical databases retrieve data records and make them available to ABAP programs.

The same logical database can be the data source for several Quick Views, queries, and programs. In the Quick View, the LDB can be specified directly as a data source. A query works with the logical database when the functional area that generated the query is defined with a logical database. In the case of type 1 programs, the LDB is entered in the attributes or called using function module LDB_PROCESS. See appendix for information on how to use the function module.

Logical databases offer several advantages:

The system generates a selection screen. The use of selection screen versions or variants provides the required flexibility.The user does not have to know the exact structure of the tables involved (especially the foreign key dependencies); the data is made available in the correct order at GET events.

Performance improvements within logical databases directly affect all programs linked to the logical database, without having to change the programs themselves.

Maintenance can be performed at a central location.

Authorization checks can also be performed centrally.

A logical database is an ABAP program that reads predefined data from the database and makes it available to other programs.

A hierarchical structure determines the order in which the data is supplied to the programs. A logical database also provides a selection screen that checks user entries and conducts error dialogs. These can be extended in programs.

SAP provides some 200 logical databases in Release 4.6. The names of logical databases have been extended to 20 places in Release 4.0 (namespace prefix max. 10 characters).

In the case of executable programs, you can enter a logical database in the attributes.

Use the NODES statement to specify the nodes of the logical database that You want to use in the program. NODES allocates the appropriate storage space for the node - that is, a work area or a table area depending on the node type.

The logical database makes the data records available for the corresponding GET events.
The sequence in which these events are processed is determined by the structure of the logical database.

Logical databases are made up of several sub-objects. The structure determines the hierarchy, and thus the read sequence of the data records.

Node names can contain up to 14 characters. There are four different node types.

Table (type T): The node name is the name of a transparent table (this type corresponds to the concept prior to Release 4.0A). The table name must be identical to the node name. Deep types (complex) are not allowed.

DDIC type (type S): Any node name is possible. It is assigned a structure or a table type from the Dictionary. The node name can differ from the type name. Deep structures are possible.

Type groups (type C): The node type is defined in a type group. The name of the type group must be maintained in the "Type group" field. You should generally prefer DDIC types, as the other applications that use the logical database (such as SAP Query) can access them (short texts, and so on).

Dynamic nodes (type A): These nodes do not have a fixed type; they are not classified until the program runtime. Which types are generally allowed is determined when the structure is created.

Nodes are declared using language element NODES.

Processing blocks are always allocated to an event. A processing block is closed by the next event key word, the start of form routines, or by the end of the program.

The START-OF-SELECTION event is triggered before control is given to the read routine of the logical database. The END-OF-SELECTION event is triggered after all GET events have been processed - that is, all data records have been read and processed.

The GET event is triggered whenever the logical database supplies data for this node. This means that GET events are processed several times, and that data has already been read from the database for these events. The sequence in which the GET events are processed is determined by the structure of the logical database.

The GET LATE event is triggered when all subordinate nodes of node have been processed, before the data is read for the next ; that is, whenever a hierarchy level has been completed.

At the start of the event, the system automatically adds a line feed and configures the default formats (for example, INTENSIFIED ON).

CHECK statements end the current processing block.

STOP statements end program processing. However, in contrast to the EXIT statement, the processing block END-OF-SELECTION is processed first (if it exists).

If there is a STOP statement within the END-OF-SELECTION processing block, program processing ends immediately and a list is displayed.

The EXIT statement exits the program and displays the list.

You can also use the REJECT statement. The data record is not processed further. Processing continues on the same hierarchy level when the next data record is read. REJECT, unlike the CHECK statement, can also be used within a subroutine.

Use the selection include dbsel to define selection screens for logical databases. The addition FOR NODE assigns selections to individual logical nodes. The appearance of a selection screen thus directly depends on the NODES statement contained within your program.

A field selection can be defined for the individual nodes. To do this, you have to specify the addition FIELD SELECTION FOR NODE in the SELECTION-SCREEN statement. You can then use GET FIELDS to restrict the amount of data returned.

You can designate individual nodes for dynamic selection using the addition DYNAMIC SELECTIONS FOR NODE. The Dynamic selection pushbutton then appears on your selection screen. You can determine which selection fields can be set by choosing a particular selection view yourself (type: CUS) or by using the selection view delivered by SAP (type: SAP).

With large logical databases you can define several selection screen versions. Each selection screen version contains a subset of your selection criteria (language element: EXCLUDE). Specify the name of a selection screen version in the program attributes.

When you enter a logical database in the attributes of your type 1 program, the system processes the selection screen of the logical database. The concrete characteristics of the selection screen depend upon the node specified in the NODES statement. If you specify a node of type T (table), you can also declare the table work area with the TABLES statement.

If you address only subordinate nodes (in the hierarchy) of the logical database in the program (for example sflight), the selection screen criteria for the superior node in the hierarchy (spfli) also appear. You can thus restrict the dataset to be read so that it meets your specific requirements.

Note: A logical database always reads in accordance with its structure. This means that if you only need data from a node deep in the hierarchy, you will achieve better performance by programming the access yourself. This avoids unnecessary reading of the database.

If the logical database supports dynamic selections, the pushbutton for Dynamic selections appears on the selection screen. When the user presses this button, a second selection screen is displayed.
This screen allows the user to select additional database fields. The system transfers the selections directly to the logical database program and therefore to the database (dynamic selections).

he selection view determines which fields are displayed on the selection screen. Create your own view with type CUS, and have it override the view with type SAP.

Database program sapdb for logical database is a collection of subroutines, each of which is performed for specific events. For example, subroutine is processed once at the start of the database program. This program can be used to define default values for the selection screen of the LDB.

Other subroutines also exist that are processed during events PBO (Process Before Output) and PAI (Process After Input) of the selection screen. Checks, such as authorization checks (AUTHORITY-CHECK), are usually performed during event PAI.

The database accesses (SELECT statements) are programmed in the put_ subroutines. These subroutines may be processed several times, depending on which selection criteria the user specifies. The sequence in which these subroutines are processed is determined by the structure of the logical database.

Database access (SELECT statements) should be programmed with optimal performance in mind. When creating a logical database you generate the corresponding database program after first having determined its structure and selection attributes. You can find performance tips in the comment lines.

When a program that has been assigned a logical database is started, control is initially passed to the database program of the logical database. Each event has a corresponding subroutine in the database program - for example, subroutine init for event INITIALIZATION. During the interaction between the LDB and the associated program, the subroutine is always processed first, followed by the event (if there is one in the report).

Logical database programs read data from a database according to the structure declared for the logical database. They begin with the root node and then process the individual "branches" consecutively from top to bottom.

The logical database reads the data in the put_ subroutines. During event PUT, control is passed from the database program to the GET event of the associated report.

The data is made available in the corresponding work areas in the report. The processing block defined for the GET event is performed. Control then returns to the logical database.
PUT activates the next form subroutine found in the structure. This flow is continued until the report has collected all the available data.

The depth of data read in the structure depends upon a program's GET events. A logical database reads to the lowest GET event contained within the structure attributes. Only those GET events for which processing is supposed to take place are written into the report program. Logical databases read all data records found on the direct access path.

If you specify a logical database and declare additional selections in the program attributes that refer to the fields of a node not designated for dynamic selection, you must use the CHECK statement to see if the current data record fulfills the selection criteria.

If the data record does not fulfill these selection criteria, current event block processing ends.