Skills/mcd-mld-designer/references/merise-rules.md

Merise Methodology & Transformation Reference

This reference covers the formal Merise rules for constructing Conceptual Data Models (MCD) and translating them into Logical Data Models (MLD).

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1. Merise Concepts

1.1 Entité (Entity)

An object, concept, or person with an independent existence in the business domain.

• Has a unique identifier (Identifiant / Primary Key candidate).
• Contains atomic properties (Propriétés / Attributes).
• In MLD: Becomes a relational table prefixed with <DB_TRIGRAM>_.

1.2 Association (Relationship)

A semantic link between two or more entities.

• May be binary (between 2 entities), ternary (3 entities), or reflexive (an entity linked to itself).
• May carry properties (e.g. QUANTITY in a CONTAINS association).

1.3 Cardinalités (Cardinalities)

Defined as a pair (min, max) for each entity participating in an association:

• min: Minimum number of times an entity instance can participate in the association (0 or 1).
• max: Maximum number of times an entity instance can participate in the association (1 or N).

Common combinations:

• (0,1): Optional, at most one
• (1,1): Mandatory, exactly one
• (0,N): Optional, zero to many
• (1,N): Mandatory, at least one

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2. MCD to MLD Transformation Rules

Rule 1: One-to-Many Association ((X,1) ↔ (X,N))

When one side has a maximum cardinality of 1 and the other side has a maximum cardinality of N:

1. No new table is created for the association.
2. The primary key of the (X,N) entity migrates into the table of the (X,1) entity as a Foreign Key (FK).
3. Any attributes on the association migrate into the (X,1) table as well.
4. Naming convention: In the host table (HOST), the foreign key referencing TARGET is named: <HOST_TRIGRAM>_<TARGET_TRIGRAM>_<COLUMN_NAME> Example: ECO_ORDER (ORD) has FK ORD_CUS_ID referencing ECO_CUSTOMER (CUS).

Rule 2: Many-to-Many Association ((X,N) ↔ (X,N))

When both sides have a maximum cardinality of N:

1. A new table is created for the association.
2. The table name uses a meaningful functional name (e.g. ECO_ORDER_ITEM) or a concatenation of entity names (e.g. ECO_ORDER_PRODUCT).
3. The Primary Key is composite, consisting of the foreign keys referencing both participating entities:
   • (#ITE_ORD_ID, #ITE_PRO_ID)
4. Any attributes of the association become columns in this junction table (e.g. ITE_QUANTITY, ITE_UNIT_PRICE).

Rule 3: One-to-One Association ((X,1) ↔ (X,1))

When both sides have a maximum cardinality of 1:

• Case A: (1,1) ↔ (0,1): The primary key of the (0,1) entity migrates into the table of the (1,1) entity as an FK with a UNIQUE constraint.
• Case B: (0,1) ↔ (0,1): The primary key migrates into either table (whichever entity is created second or is functionally secondary) with a UNIQUE constraint.
• Case C: (1,1) ↔ (1,1): Usually indicates the two entities should be merged into a single table, unless there is a strong architectural reason for separation.

Rule 4: Reflexive Association (Self-Referencing)

An association between an entity and itself (e.g. Employee has a Manager who is an Employee, or Category has a Parent Category):

1. Reflexive (0,1) / (1,1) to (0,N):
   • A foreign key column is added to the same table.
   • Named: <TBL_TRIGRAM>_<TBL_TRIGRAM>_<ROLE_NAME>_<COL>
   • Example: In ECO_CATEGORY (CAT), parent category FK: CAT_CAT_PARENT_ID.
   • Example: In COR_EMPLOYEE (EMP), manager FK: EMP_EMP_MANAGER_ID.
2. Reflexive (0,N) to (0,N) (e.g. Product is composed of other Products - Bill of Materials):
   • A junction table is created (e.g. ECO_PRODUCT_COMPONENT).
   • Trigram COM: COM_PRO_PARENT_ID and COM_PRO_CHILD_ID forming the composite PK.

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3. Normalization Rules (1NF, 2NF, 3NF)

Every relational model produced by this skill must satisfy Third Normal Form (3NF):

First Normal Form (1NF)

• Every table has a primary key.
• Every column contains only atomic (indivisible) scalar values (no CSV strings, no JSON objects as substitute for relational tables).
• No repeating groups or duplicate columns (e.g. PHONE_1, PHONE_2 is invalid; use a separate phone table if multiple phones are supported).

Second Normal Form (2NF)

• Must be in 1NF.
• Every non-key column must depend on the entire primary key (applies to composite primary keys in junction tables).
• Example Violation: In junction table ECO_ORDER_ITEM with PK (ITE_ORD_ID, ITE_PRO_ID), having ITE_PRODUCT_NAME is invalid because the product name depends only on PRO_ID, not on ORD_ID.

Third Normal Form (3NF)

• Must be in 2NF.
• No non-key column may depend on another non-key column (no transitive dependency).
• Example Violation: In ECO_CUSTOMER, having CUS_CITY_NAME and CUS_CITY_POSTAL_CODE where postal code determines city name. If postal code determines city, normalize into a separate ECO_CITY table or ensure direct dependency on customer address.