Foreign Key Relationships

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Objectives• The participants will be able to:

– Define and Use Foreign Keys– Understand the Prerequisites for Constructing

Foreign Key Relationships– Interpret Cardinality and Foreign Key Field Types– Create Foreign Key Relation

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Definition of Foreign Keys

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Relationships between tables are represented in the ABAP Dictionary by foreign keys.

A foreign key is a field (or combination of fields) that represents the primary key of another table.

Uses of Foreign Keys• Maintain data integrity• Provide help texts• Create aggregate dictionary objects

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Prerequisites for Constructing Foreign Key Relationships

• The foreign key field and the primary key of the check table must share the same domain.

• A value table must exist for that domain.

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Foreign Keys: Key Terminology

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Check TableCheck Table

Value TableValue Table

Foreign Key TableForeign Key Table

The table containing the set of allowed values attached to a domain

The table that is referenced by a foreign key

The table containing fields that are the primary key of the other table

Foreign Key Terminology: Check Table = Value Table

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Check Table

Value

Table

In many cases, the value table and the check table will be the same

Foreign Key Terminology: Check Table Value Table

Check Table

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Value

Table

Sometimes check table is another table containing a subset of the records in a value table.

Establishing a Foreign Key Relationship in the ABAP Dictionary

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DataDictionary

During defining Foreign Key Relationship, system suggests a Foreign Key Relationship for you.

Cardinality

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n n : : m mn n : : m m1

C

1

C

N

CN

When creating foreign key relationships, you should always specify the cardinality of that relationship

Creating a Foreign Key Relationship

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Foreign key push-button

Maintaining a Foreign Key Relationship’s Attributes

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Enter a description (short text).

Maintain foreignkey type

Check table

Maintain cardinality n : m

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If the primary key of a check table has multiple fields (i.e. it has a composite primary key), some type of assignment must be made for each field when creating a foreign key relationship.

Foreign Keys with Multiple Fields

Field-By-Field Assignment

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Check TableCheck TableForeign Key TableForeign Key Table

When performing a field-by-field assignment, all key fields in the check table are used to determine valid entries in the foreign key table.

Partial Foreign Keys

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When creating a partial foreign key, certain fields are flagged as generic.

These fields are ignored by the system when validating records that are inserted into the foreign key table.

Constant Foreign Keys

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“BIO”

When creating a constant foreign key, certain fields are assigned a literal value

Only records in the check table with that value in the corresponding field are used to determine whether records in the foreign key table are valid.

Combined use of Partial and Constant foreign keys

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Generic foreign key

Constant foreign keyConstant foreign key

Demonstration

• Creation of a new transparent table for holding the payroll data for employees in an organization and establish a foreign key relationship with the employee table created for the previous chapter.

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Practice

• Creation of a new transparent table for holding the payroll data for employees in an organization and establish a foreign key relationship with the employee table created for the previous chapter.

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Summary

• Relationships between tables are represented in the ABAP Dictionary by foreign keys. A foreign key is a field (or combination of fields) that represents the primary key of another table.

• Foreign keys maintain data integrity & provide help texts.

• In order to establish a foreign key relationship in SAP, the fields involved must share the same domain, and that domain must have a value table specified.

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Questions

• What is a foreign key ?• What is the need to define a foreign key

relationship ?• What is meant by cardinality in a foreign key

relationship ?

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