theammarngp-makes · theammarngp-makes · Jul 5, 2026 · Jul 5, 2026
diff --git a/10_Schema/DATABASE_SCHEMA.md b/10_Schema/DATABASE_SCHEMA.md
@@ -2,107 +2,165 @@
 
 ## Overview
 
-This handbook uses a custom employee management database designed for learning SQL concepts ranging from fundamentals to advanced analytics.
+Every query in the SQL Engineering Handbook runs against a single, consistent
+employee management database. Using one schema across all 65+ files — instead
+of a different toy table per lesson — mirrors how you'd actually work against
+a real company database, and lets concepts compound: a JOIN you learn in
+Module 3 uses the exact same tables a Window Function uses in Module 7.
 
 The schema contains three related tables:
 
 ```text
-employes
+locations
+    ▲
     │
-    ▼
 departments
+    ▲
     │
-    ▼
-locations
+employes
 ```
 
+Read the diagram bottom-up: many **employes** roll up into a **department**,
+and many **departments** roll up into a **location**.
+
 ---
 
-## Table: employes
+## Table: `employes`
+
+Stores individual employee records, including a self-referencing manager
+relationship.
+
+| Column       | Data Type    | Nullable | Description                              |
+|--------------|--------------|----------|-------------------------------------------|
+| `emp_id`     | `INT`        | No (PK)  | Unique employee identifier                |
+| `emp_name`   | `VARCHAR(50)`| Yes      | Employee's full name                      |
+| `dept_id`    | `INT`        | Yes (FK) | References `departments.dept_id`          |
+| `manager_id` | `INT`        | Yes (FK) | References `employes.emp_id` (self-join)  |
+| `hire_date`  | `DATE`       | **No**   | Date the employee joined the company      |
 
-Stores employee information.
+> `manager_id` is `NULL` for top-level managers who report to no one in this
+> dataset (e.g. `emp_id = 3`, `Sahil`).
 
-| Column | Data Type | Description |
-|----------|----------|----------|
-| emp_id | INT | Employee ID |
-| emp_name | VARCHAR(50) | Employee Name |
-| dept_id | INT | Department ID |
-| manager_id | INT | Reporting Manager |
+**Sample rows:**
+
+| emp_id | emp_name | dept_id | manager_id | hire_date  |
+|--------|----------|---------|------------|------------|
+| 1      | Ammar    | 1       | 11         | 2023-01-15 |
+| 3      | Sahil    | 1       | NULL       | 2022-11-10 |
+| 11     | Rohit    | 1       | NULL       | 2020-04-11 |
 
 ---
 
-## Table: departments
+## Table: `departments`
+
+Stores department metadata.
 
-Stores department information.
+| Column        | Data Type    | Nullable | Description                        |
+|---------------|--------------|----------|-------------------------------------|
+| `dept_id`     | `INT`        | No (PK)  | Unique department identifier        |
+| `dept_name`   | `VARCHAR(50)`| Yes      | Department name                     |
+| `location_id` | `INT`        | Yes (FK) | References `locations.location_id`  |
 
-| Column | Data Type | Description |
-|----------|----------|----------|
-| dept_id | INT | Department ID |
-| dept_name | VARCHAR(50) | Department Name |
-| location_id | INT | Location ID |
+**Sample rows:**
+
+| dept_id | dept_name        | location_id |
+|---------|------------------|-------------|
+| 1       | Data Analytics   | 1           |
+| 2       | Engineering      | 2           |
 
 ---
 
-## Table: locations
+## Table: `locations`
+
+Stores city/country metadata for each department's base of operations.
 
-Stores location information.
+| Column         | Data Type    | Nullable | Description       |
+|----------------|--------------|----------|--------------------|
+| `location_id`  | `INT`        | No (PK)  | Unique location ID |
+| `city`         | `VARCHAR(50)`| Yes      | City name          |
+| `country`      | `VARCHAR(50)`| Yes      | Country name       |
 
-| Column | Data Type | Description |
-|----------|----------|----------|
-| location_id | INT | Location ID |
-| city | VARCHAR(50) | City Name |
-| country | VARCHAR(50) | Country Name |
+**Sample rows:**
+
+| location_id | city      | country |
+|-------------|-----------|---------|
+| 1           | Nagpur    | India   |
+| 3           | Mumbai    | India   |
 
 ---
 
 ## Relationships
 
-### Employee → Department
-
-Many employees belong to one department.
+### `employes` → `departments` (many-to-one)
 
 ```text
-employes.dept_id
-      ↓
-departments.dept_id
+employes.dept_id  →  departments.dept_id
 ```
 
-### Department → Location
+Many employees belong to one department.
 
-Many departments can belong to one location.
+### `departments` → `locations` (many-to-one)
 
 ```text
-departments.location_id
-        ↓
-locations.location_id
+departments.location_id  →  locations.location_id
 ```
 
-### Employee → Manager
+Many departments can be based in one city.
 
-Self-referencing relationship.
+### `employes` → `employes` (self-referencing, many-to-one)
 
 ```text
-employes.manager_id
-      ↓
-employes.emp_id
+employes.manager_id  →  employes.emp_id
 ```
 
+An employee reports to at most one manager, who is themself a row in the
+same `employes` table. This relationship is what makes `03_Joins/04_SELF_JOIN.md`
+and hierarchical query examples possible.
+
+---
+
+## Dataset Size
+
+| Table         | Row Count |
+|---------------|-----------|
+| `locations`   | 5         |
+| `departments` | 10        |
+| `employes`    | 50        |
+
+Source of truth for both structure and data:
+- Structure: [`01_CREATE_TABLES.sql`](./01_CREATE_TABLES.sql)
+- Data: [`02_INSERT_DATA.sql`](./02_INSERT_DATA.sql)
+
 ---
 
-## Concepts Demonstrated
+## Concepts This Schema Is Designed to Teach
 
-- Primary Keys
-- Foreign Keys
-- Self Join Relationships
-- Multi-table Joins
-- Hierarchical Data
+- Primary keys and foreign keys
+- One-to-many relationships
+- Self-joins and hierarchical (manager → report) data
+- Multi-table joins (3+ tables)
+- NULL handling on optional foreign keys
 
 ---
 
-## Business Use Cases
+## Business Use Cases Modeled by This Schema
+
+- Workforce and headcount analytics
+- Department-level performance reporting
+- Manager span-of-control analysis
+- Location-based organizational reporting
+- Tenure and hiring trend analysis (via `hire_date`)
+
+---
+
+## Related Documentation
+
+- [ERD.md](./ERD.md) — visual entity-relationship diagram
+- [01_CREATE_TABLES.sql](./01_CREATE_TABLES.sql) — DDL
+- [02_INSERT_DATA.sql](./02_INSERT_DATA.sql) — seed data
+
+---
 
-- Workforce Analytics
-- Department Reporting
-- Manager Analysis
-- Location Analysis
-- Organizational Structure Reporting
+<p align="center">
+  <i>Part of the <a href="../">SQL Engineering Handbook</a></i>
+</p>