abap dictionary table learning

The most important data classes are master data,transaction data,organizational data and system data.

Master data is data that is only seldomly modified.An example of master data is the data of an address file,for example the

name, address and telephone number.

Transaction data is data that is frequently modified.An example is the material stock of a warehouse,which can change after

each purchase order.

Organizational data is data that is defined during customizing when the system is installed and that is only seldomly omodified

thereafter.The country keys are an example.

System data is data that the SAP System itself needs.The program sources are an example.

Further data classes,called customer data classes(USER,USER1),are provided for customers.These should be used for customer

developments.Special storage areas must be allocated in the database.





When creating indexes,please note:
-- An index can only be used up to the last specified field in the selection! The fields that are specified in the WHERE

clause for a large number of selections should be in the first position.

-- Only those fields whose values siginificantly restrict the amount of data are meaningful in an index.

-- When you change a data record of a table,you must adjust the index sorting.Tables whose contents are frquently changed

should not have too many indexes.

-- Make sure that the indexes on  a table are as disjunct as possible.



A value table only becomes a check table when a foreign key is defined.If you refer to a domain with a value table in a

field,but no foreign key was defined at field level,there is no check.