The domain describes the value range of a field by specifying its data type and field length. If only a limited set of values is allowed, they can be defined as fixed values.Specifying fixed values causes the value range of the domain to be restricted by these values. Fixed values are immediately used as check values in screen entries. There is also an F4 help.Fixed values are only checked in screens. No check is made when data records are inserted in a table by an ABAP program.Fixed values can either be listed individually or defined as an interval.The value range of a field can also be defined by specifying a value table in the domain.

In contrast to fixed values, however, simply specifying a value table does not cause the input to be checked. There is no F4 help either.If you enter a value table, the system can make a proposal for the foreign key definition.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.

In the ABAP Dictionary, such relationships between two tables are called foreign keys and they must be defined explicitly for the fields.Foreign keys are used to ensure that the data is consistent. Data that has been entered is checked against existing data to ensure that it is consistent.In order to define the foreign key, these three fields are assigned to fields of the foreign key table (foreign key fields) with which the input to be checked is entered on the screen. In table SBOOK these are the fields: MANDT, CARRID, COUNTER. The entry is accepted if it represents a valid counter; otherwise the system will reject it.

The foreign key is defined for field SBOOK-COUNTER (check field), which means that the entry in this field is checked. Field COUNTER is therefore called the check field for this foreign key.A combination of fields of a table is called a foreign key if this field combination is the primary key of another table.A foreign key links two tables.The check table is the table whose key fields are checked. This table is also called the referenced table .

An entry is to be written in the foreign key table. This entry must be consistent with the key fields of the check table.The field of the foreign key table to be checked is called the check field.Foreign keys can only be used in screens. Data records can be written to the table without being checked using an ABAP program.

In the ABAP Dictionary, the same domain is required for the check field and referenced key field of the check table so that you do not compare fields with different data types and field lengths. Domain equality is essential. Different data elements can be used, but they must refer to the same domain.The requirement for domain equality is only valid for the check field. For all other foreign key fields, it is sufficient if the data type and the field length are equal. You nevertheless should strive for domain equality. In this case the foreign key will remain consistent if the field length is changed because the corresponding fields are both changed. If the domains are different, the foreign key will be inconsistent if for example the field length is changed.

If the domain of the check field has a value table, you can have the system make a proposal with the value table as check table. In this case a proposal is created for the field assignment in the foreign key.The constellation that a domain that itself has table SAIRPORT as value table is used following field SAIRPORT-Airport is correct !! However, a foreign key is never defined on this field (avoiding a loop).

The cardinality describes the foreign key relationship with regard to how many records of the check table are assigned to records of the foreign key table. The cardinality is always defined from the point of view of the check table.The type of the foreign key field defines whether or not the foreign key field identifies a table entry.This means that the foreign key fields are either key fields or they are not key fields or they are a special case, namely the key fields of a text table.There are the following kinds of foreign key fields:

not specified: No information about the kind of foreign key field can be given

no key fields/candidates: The foreign key fields are neither primary key fields of the foreign key table nor do they uniquely identify a record of the foreign key table (key candidates).

The foreign key fields therefore do not (partially) identify the foreign key table.

Key fields/candidates:

The foreign key fields are either primary key fields of the foreign key table or they uniquely identify a record of the foreign key table (key candidates). The foreign key fields therefore (partially) identify the foreign key table.

Key fields of a text table :

The foreign key table is a text table of the check table, i.e. the key of the foreign key table only differs from the key of the check table in an additional language key field. This is a special case of the category Key fields / candidates.

Only one text table can be created for a table.



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