Foreign keys are used to establish relationships between tables in an ABAP dictionary. They allow value checks to validate fields in one table against fields in another table. The table containing the foreign key is called the foreign key table, while the other table containing the valid fields is the value table. Compound foreign keys use two or more fields for validation. Generic foreign keys allow selective validation while constant foreign keys validate against a fixed value. Cardinality specifies the number of rows allowed in the foreign key table for each value in the check table.

2. FOREIGN KEY
A PRIMARY KEY IS A FIELD OR COMBINATION OF FIELD THAT UNIQUELY IDENTIFY A
RECORD IN A TABLE.
• FOREIGN KEYS ARE USED TO ESTABLISH THE RELATIONSHIP BETWEEN THE DIFFERENT
TABLES PRESENT IN THE ABAP DICTIONARY.
• WE CAN CREATE VALUE CHECKS FOR INPUT FIELDS USING THE FOREIGN KEYS. VALUE
CHECKS ARE REQUIRED TO VALIDATE THE VALUES OF SOME FIELDS IN A TABLE WITH
THE VALUES OF OTHER FIELDS OF DIFFERENT TABLES.
• THE TABLE THAT CONTAINS THE FOREIGN KEY IS CALLED A FOREIGN KEY TABLE AND
ANOTHER TABLE THAT CONTAINS VALID FIELDS, KNOWN AS THE VALUE TABLE.

4. WHY DO WE NEED TO ESTABLISH
RELATIONSHIP BETWEEN TABLES ?
1.DATA REDUNDANCIES WILL BE REMOVED.
2.CRASHING DATABASE VERY OFTEN WILL BE PREVENTED IN OTHER WORDS,
EFFICIENCY CAN BE IMPROVED BY SPLITTING AND ESTABLISHING
RELATIONSHIP AMONG TABLES.
3.SEARCHING AND FILTERING DATA WILL BE IMPROVED USING QUERY OF
RELATED TABLE INSTEAD OF USING A VERY LARGE TABLE.
4.DATA MANAGEMENT UPDATE, DELETE ETC. WILL BE IMPROVED. RELATIONSHIP
PROVIDES FLEXIBILITY ABOUT SUCH MANAGEMENT.

5. COMPONENT OF FOREIGN KEY
• BELOW ARE SOME OTHER CONCEPTS RELATED TO THE FOREIGN KEY:
• COMPOUND FOREIGN KEYS
• GENERIC AND CONSTANT FOREIGN KEYS
• THE CARDINALITY OF FOREIGN KEY

6. COMPOUND FOREIGN KEYS:
• A FOREIGN KEY CONSISTING OF TWO OR MORE FIELDS IS CALLED
A COMPOUND FOREIGN KEY.
• TO APPLY CHECKS FOR COMPOUND FOREIGN KEYS, THE SYSTEM COMPARES
TWO FIELDS IN THE FOREIGN KEY TABLE AGAINST THE TWO FIELDS IN THE
CHECK TABLE.

7. GENERIC AND CONSTANT
FOREIGN KEYS
• AS WE KNOW, TO CREATE A FOREIGN KEY, ALL THE PRIMARY KEYS OF THE
CHECK TABLE MUST BE INCLUDED IN A FOREIGN KEY RELATIONSHIP.
• BUT IN SOME CASES, WE JUST WANT TO PERFORM VALIDATION AGAINST
SELECTED FIELDS ONLY, AND OTHER FIELDS CAN BE EXCLUDED, SO FOR SUCH
CASES, WE CAN USE THE GENERIC FOREIGN KEYS.
• WE CAN ALSO ASSIGN A CONSTANT VALUE TO THE KEY FIELD OF THE CHECK
TABLE, WHICH WILL ALLOW US TO CHECK THE VALUES AGAINST A CONSTANT
VALUE OF THE CHECK TABLE.

8. THE CARDINALITY OF FOREIGN
KEY
• THE CARDINALITY OF THE FOREIGN KEY IS USED TO SPECIFY THE ALLOWED
ROWS IN THE FOREIGN KEY TABLE FOR THE CORRESPONDING VALUE IN THE
CHECK TABLE.
• IT IS DEFINED WHILE CREATING THE FOREIGN KEY RELATIONSHIP.
• IT CAN BE EXPRESSED AS X: Y RELATIONSHIP, WHERE X IS USED FOR THE
CHECK TABLE, AND Y IS FOR THE FOREIGN KEY TABLE.
• EXAMPLE: IF THE CARDINALITY IS GIVEN AS 1:N, IT MEANS THERE IS EXACTLY
ONE RECORD THAT EXISTS FOR EACH RECORD OF THE FOREIGN KEY TABLE.

10. ABAP TEXT TABLES
• A TEXT TABLE IN SAP ABAP IS A DATABASE TABLE USED TO STORE THE
DESCRIPTIONS OF A FIELD IN DIFFERENT LANGUAGES.
• FOR EXAMPLE, IN ENGLISH WE STORE A DESCRIPTION AS “COMPUTER” AND IF
YOU WANT TO STORE THIS IN GERMAN LANGUAGE ALSO WE CAN ALSO DO THE
SAME.

13. CURRENCY AND QUANTITY FIELD
• A CURRENCY FIELD IS A COMPONENT OF A STRUCTURE, DATABASE TABLE, OR VIEW
DEFINED IN ABAP DICTIONARY OF TYPE CURR AND USED TO RECORD AN AMOUNT
IN A SPECIFIC CURRENCY. THE AMOUNT IS SPECIFIED AS A WHOLE NUMBER IN THE
SMALLEST UNIT OF THE CURRENCY.
• A CURRENCY FIELD MUST BE ASSOCIATED WITH A CURRENCY KEY OF A
STRUCTURE, DATABASE TABLE, OR VIEW THAT SPECIFIES THE CURRENCY AND THE
NUMBER OF DECIMAL PLACES. A CURRENCY KEY IS A COMPONENT OF A
STRUCTURE, DATABASE TABLE, OR VIEW DEFINED IN ABAP DICTIONARY, HAS THE
TYPE CUKY, AND CAN CONTAIN A CURRENCY ID FROM THE DATABASE
TABLE TCURC.