Data Modeling Skill

Purpose

Create clear data models that document data structures, relationships, and business rules for system design and development.

When to Use

• Designing new database structures
• Documenting existing data for analysis
• Defining data requirements in FRS
• Integration design between systems
• Data migration planning

Data Modeling Concepts

Entity Relationship Diagram (ERD)

Entities: Objects/things we store data about (e.g., Customer, Order, Product) Attributes: Properties of entities (e.g., customer_name, order_date) Relationships: How entities relate to each other

Relationship Types

One-to-One (1:1) Example: User ↔ UserProfile

• One user has exactly one profile
• One profile belongs to exactly one user

One-to-Many (1:N) Example: Customer → Orders

• One customer can have many orders
• One order belongs to one customer

Many-to-Many (M:N) Example: Products ↔ Categories

• One product can be in many categories
• One category can have many products
• Requires junction/bridge table

Cardinality Notation (Crow's Foot)

||──────|| : One and only one (1:1)
||──────<  : One-to-Many (1:N)
>──────<   : Many-to-Many (M:N)
o|──────<  : Zero or one to Many
||──────o< : One to Zero or Many

ERD Examples

E-commerce ERD (Mermaid)

erDiagram
    CUSTOMER ||--o{ ORDER : places
    CUSTOMER {
        uuid customer_id PK
        string email UK
        string first_name
        string last_name
        string phone
        datetime created_at
    }
    ORDER ||--|{ ORDER_ITEM : contains
    ORDER {
        uuid order_id PK
        uuid customer_id FK
        string order_number UK
        decimal subtotal
        decimal tax
        decimal shipping
        decimal total
        string status
        datetime order_date
    }
    ORDER_ITEM {
        uuid item_id PK
        uuid order_id FK
        uuid product_id FK
        int quantity
        decimal unit_price
        decimal total_price
    }
    PRODUCT ||--o{ ORDER_ITEM : "included in"
    PRODUCT {
        uuid product_id PK
        string sku UK
        string name
        text description
        decimal price
        int stock_quantity
        string status
    }
    PRODUCT }|--|| CATEGORY : "belongs to"
    CATEGORY {
        uuid category_id PK
        string name
        uuid parent_id FK
    }

CRM ERD

erDiagram
    ACCOUNT ||--o{ CONTACT : has
    ACCOUNT ||--o{ OPPORTUNITY : has
    ACCOUNT {
        uuid account_id PK
        string name
        string industry
        string website
        int employee_count
        decimal annual_revenue
        uuid owner_id FK
    }
    CONTACT {
        uuid contact_id PK
        uuid account_id FK
        string first_name
        string last_name
        string email UK
        string phone
        string title
    }
    OPPORTUNITY }o--|| CONTACT : "primary contact"
    OPPORTUNITY {
        uuid opportunity_id PK
        uuid account_id FK
        uuid contact_id FK
        string name
        string stage
        decimal amount
        date close_date
        int probability
        uuid owner_id FK
    }
    LEAD {
        uuid lead_id PK
        string first_name
        string last_name
        string email
        string company
        string source
        int score
        string status
    }

Data Dictionary

Template

Attribute	Data Type	Size	Required	Default	Description	Validation
customer_id	UUID	-	Yes	Auto-gen	Unique identifier	UUID format
email	VARCHAR	255	Yes	-	Customer email	Valid email
status	ENUM	-	Yes	'active'	Account status	active, inactive, suspended

Example: Order Entity

Attribute	Type	Size	Required	Default	Description	Rules
order_id	UUID	-	Yes	Auto	Primary key	Unique
order_number	VARCHAR	20	Yes	Generated	Display number	Format: ORD-YYYYMMDD-XXXX
customer_id	UUID	-	Yes	-	FK to Customer	Must exist
order_date	DATETIME	-	Yes	NOW()	When order placed	Cannot be future
status	ENUM	-	Yes	'pending'	Order status	pending, processing, shipped, delivered, cancelled
subtotal	DECIMAL	10,2	Yes	0.00	Sum of items	>= 0
tax	DECIMAL	10,2	Yes	0.00	Calculated tax	>= 0
shipping_cost	DECIMAL	10,2	Yes	0.00	Shipping fee	>= 0
total	DECIMAL	10,2	Yes	-	Final total	= subtotal + tax + shipping
shipping_address	JSON	-	Yes	-	Delivery address	Valid address
billing_address	JSON	-	Yes	-	Billing address	Valid address
notes	TEXT	-	No	NULL	Order notes	Max 2000 chars
created_at	DATETIME	-	Yes	NOW()	Record created	Immutable
updated_at	DATETIME	-	Yes	NOW()	Last modified	Auto-update

Normalization

First Normal Form (1NF)

• Eliminate repeating groups
• Each cell contains single value
• Each record is unique

❌ Bad: customer_phones = "123-456, 789-012" ✅ Good: Separate phone table with customer_id FK

Second Normal Form (2NF)

• Meet 1NF
• No partial dependencies (all non-key attributes depend on entire primary key)

Third Normal Form (3NF)

• Meet 2NF
• No transitive dependencies (non-key attributes don't depend on other non-key attributes)

❌ Bad: Order has customer_email (depends on customer_id, not order) ✅ Good: Get customer_email via Customer table join

Domain-Specific Data Patterns

E-commerce

• Products with variants (SKU per variant)
• Hierarchical categories
• Shopping cart → Order transition
• Address normalization
• Price history tracking

ERP

• Chart of Accounts structure
• Multi-company data isolation
• Master data (customer, vendor, product)
• Transaction tables with journals
• Audit trails

CRM

• Lead → Contact → Account conversion
• Activity logging (calls, emails, meetings)
• Opportunity → Quote → Order pipeline
• Campaign → Member → Response tracking

CDP

• Customer identity resolution
• Event/behavioral data (time-series)
• Profile attributes (unified)
• Segment membership
• Consent tracking

Best Practices

✅ Do:

• Use consistent naming conventions (snake_case)
• Include audit fields (created_at, updated_at, created_by)
• Define primary keys explicitly
• Document foreign key relationships
• Include data types and constraints
• Consider soft deletes vs. hard deletes
• Plan for data growth

❌ Don't:

• Store calculated values (unless for performance)
• Use ambiguous names
• Skip documentation
• Ignore data validation rules
• Forget about NULL handling

Tools

• Figma: Visual ERD design
• Mermaid: Code-based diagrams in docs
• dbdiagram.io: Quick ERD creation
• Lucidchart: Professional diagrams

Next Steps

After data modeling:

1. Review with technical team
2. Include in FRS documentation
3. Create migration scripts
4. Plan data validation rules
5. Design API contracts based on data model

References

• Database Normalization (1NF, 2NF, 3NF)
• Entity Relationship Modeling
• Data Dictionary Standards