Views :Data about an application object is often distributed on several tables. By defining a :view, you can define an application-dependent view that combines this data. The structure of such a view is defined by specifying the tables and fields used in the view. Fields that are not required can be hidden, thereby minimizing interfaces. A view can be used in ABAP programs for data selection.

The data of a view is derived from one or more tables, but not stored physically. The simplest form of deriving data is to mask out one or more fields from a base table (projection) or to include only certain entries of a base table in the view (selection). More complicated views can comprise several base tables, the individual tables being linked with a relational join operation.

The base tables of the view must be selected in the first step of a view definition. In the second step, these tables must be linked by defining the join conditions. It is also possible to use the join condition from a foreign key defined between the tables .
In the third step, you must select the fields of the base tables to be used in the view. Selection conditions that restrict the records in the view can be formulated in the fourth step.
Four different view types are supported. These differ in the way in which the view is implemented and in the methods permitted for accessing the view data.

· Database views implemented with an equivalent view on the database.
· Projection views used to hide fields of a table (only projection).
· Help views used as selection method in search helps
· Maintenance views permit you to maintain the data distributed on several tables for one application object at one time.
Database views implement an inner join. The other view types implement an outer join

The join conditions for database views can be formulated using equality relationships between any base fields. The join conditions for the other view types must be obtained from existing foreign keys. Tables therefore can only be combined in a maintenance view or help view if they are linked to one another with foreign keys.
The maintenance status whether you can only read data with the view or whether you can also insert and change data with it.

Database Views :
Data about an application object is often distributed on several database tables. A database view provides an application-specific view on such distributed data.
Database views are defined in the ABAP Dictionary. A database view is automatically created in the underlying database when it is activated.
Application programs can access the data of a database view using the database interface. You can access the data in ABAP programs with both OPEN SQL and NATIVE SQL. However, the data is actually selected in the database. Since the join operation is executed in the database in this case, you can minimize the number of database accesses in this way. Database views implement an inner join (see Inner and Outer Join )

If the database view only contains a single table, the maintenance status can be used to determine if data records can also be inserted with the view. If the database view contains more than one table, you can only read the data.
Database views should be created if want to select logically connected data from different tables simultaneously. Selection with a database view is generally faster than access to individual tables. When selecting with views, you should also ensure that there are suitable indexes on the tables contained in the view.
Since a database view is implemented in the database, a database view may only contain transparent tables.
The technical settings of a database view control whether the view data should be buffered.
Projection Views :
Projection views are used to hide fields of a table. This can minimize interfaces; for example when you access the database, you only read and write the field contents actually needed.
A projection view contains exactly one table. You cannot define selection conditions for projection views.
There is no corresponding object in the database for a projection view. The R/3 System maps the access to a projection view to the corresponding access to its base table. You can also access pooled tables and cluster tables with a projection view.
The maintenance status of the view controls how the data of the table can be accessed with the projection view.

Help Views:
You have to create a help view if a view with outer join is needed as selection method of a search help
The selection method of a search help is either a table or a view. If you have to select data from several tables for the search help, you should generally use a database view as selection method. However, a database view always implements an inner join. If you need a view with outer join for the data selection, you have to use a help view as selection method.

All the tables included in a help view must be linked with foreign keys. Only foreign keys that have certain attributes can be used here. The first table to be inserted in the help view is called the primary table of the help view. The tables added to this primary table with foreign keys are called secondary tables.
The functionality of a help view has changed significantly between Release 3.0 and Release 4.0. In Release 3.0, a help view was automatically displayed for the input help (F4 help) for all the fields that were checked against the primary table of the help view. This is no longer the case in Release 4.0.
As of Release 4.0, you must explicitly create a search help that must be linked with the fields for which it is offered (see Linking Search Helps with Screen Fields ).
Existing help views are automatically migrated to search helps when you upgrade to a release higher than 4.0.
A help view implements an outer join, i.e. all the contents of the primary table of the help view are always displayed. You therefore should not formulate a selection condition for fields in one of the secondary tables of the help view. If records of these secondary tables cannot be read as a result of this selection condition, the contents of the corresponding fields of the secondary table are displayed with initial value.

Search Helps :
The input help (F4 help) is a standard function of the R/3 System. The user can display the list of all possible input values for a screen field with the input help. The possible input values can be enhanced with further information. This is meaningful especially when the field requires the input of a formal key.
Standard Input Help Process
A user calls an input help with the following steps (some steps can be omitted, depending on the definition of the input help):
The user starts the input help to display the possible input values for a field (search field) in a screen template.
The system offers the user a number of possible search paths. The user selects one of these search paths. Each search path offers a number of restrictions to limit the number of possible input values. These values are offered in a Dialog box for value restriction when the search path is selected.
The user enters restrictions if required and then starts the search.

The system determines the values that satisfy the entered restrictions (hits) and displays them as a list (hit list).

The user selects the most suitable line from the hit list by double-clicking. The value of the search field is returned to the screen template (possibly together with other values).
Steps 2 and 3 are omitted if there is only a single search path available. In this case the dialog box for the value selection is offered immediately. You can also output the hit list directly after starting the input help. Steps 2 to 4 are omitted in this case.
Function of a Search Help
This standard process can be completely defined by creating a search help in the ABAP Dictionary. This search help only has to be assigned to the screen fields in which they should be available.
There are two types of search help:
· Elementary search helps describe a search path. The elementary search help must define where the data of the hit list should be read from (selection method), how the exchange of values between the screen template and selection method is implemented (interface of the search help) and how the online input help should be defined (online behavior of the search help).
· Collective search helpscombine several elementary search helps. A collective search help thus can offer several alternative search paths.

Maintenance Views :
Maintenance views offer easy ways to maintain complex application objects.
Data distributed on several tables often forms a logical unit, for example an application object, for the user. You want to be able to display, modify and create the data of such an application object together. Normally the user is not interested in the technical implementation of the application object, that is in the distribution of the data on several tables.
A maintenance view permits you to maintain the data of an application object together. The data is automatically distributed in the underlying database tables. The maintenance status determines which accesses to the data of the underlying tables are possible with the maintenance view.
All the tables in a maintenance view must be linked with foreign keys, that is the join conditions for maintenance views are always derived from the foreign key
cannot directly enter the join conditions as for database views.
There are some restrictions for the attributes of the foreign keys with which the tables in a maintenance view can be linked .
A standardized table maintenance transaction is provided (SM30), permitting you to maintain the data from the base tables of a maintenance view together.
Maintenance mechanisms, like screens and processing programs, must be created from the view definition with the transaction Generate Table View (SE54). This makes it possible to create easy-to-use maintenance interfaces in a simple manner.

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