# 📘 Week 1 · Day 3 — SQL Exercises

This document describes **all exercises for Week 1 Day 3**, focused on **SQL conditionals, filtering, and algorithmic thinking** using **PostgreSQL**.

The goal of today’s exercises is to translate the control-flow logic you used in **C (Day 1)** and **Python (Day 2)** into **declarative SQL queries**, while keeping the same performance and security mindset.

---

## 🎯 Learning Objectives

By completing today’s exercises, you practice:

* Replacing loops with SQL filtering (`WHERE`)
* Using conditions to select and classify data
* Understanding how databases execute algorithms internally
* Reasoning about runtime complexity in a data-driven context
* Applying validation logic at the database layer (security best practice)

---

## 🛠 Database Setup (Reference)

All exercises assume the following PostgreSQL table:

inti_db.sql

```sql
CREATE TABLE users (
    id SERIAL PRIMARY KEY,
    username TEXT
);
```

Sample data:

```sql
INSERT INTO users (username) VALUES
('admin'),
('root1'),
('John_Doe'),
('Alice'),
('Bob42'),
('charlie'),
('eve99'),
('Mallory');
```

---

## 🧪 Exercises

### Exercise 1 — Length Filtering

**Task:**
Select all usernames that are **longer than 5 characters**.

**Concepts practiced:**

* `WHERE` clause
* String length evaluation

---

### Exercise 2 — Letters-Only Usernames

**Task:**
Select usernames that:

* Contain **only letters (A–Z, a–z)**
* Contain **no digits**
* Contain **no underscores or symbols**

**Concepts practiced:**

* Regular expressions in PostgreSQL
* Character validation at the database level
* Replacing character-by-character loops with pattern matching

---

### Exercise 3 — Accepted vs Refused Classification

**Task:**
Return a result set containing:

* `username`
* `status` column with values:

  * `Accepted`
  * `Refused`

**Rules:**

* Username length ≤ 20
* Username contains letters only

**Concepts practiced:**

* `CASE WHEN` expressions
* Conditional logic inside SQL queries
* Translating `if / else` into declarative form

---

### Exercise 4 — Count Valid Usernames

**Task:**
Return **only the number** of usernames that are valid according to the same rules:

* Length ≤ 20
* Letters only

**Concepts practiced:**

* Aggregate functions (`COUNT`)
* Combining filtering and aggregation
* Thinking about algorithm cost without explicit loops

---

### Exercise 5 — Algorithm Analysis (Written)

**Task:**
Answer the following questions:

1. If the `users` table **doubles in size**, what happens to the execution time?
2. Write the runtime formula using:

   * `n` = number of rows
   * `m` = average username length

**Expected reasoning:**

* SQL engines still scan rows internally
* Regex checks still evaluate characters

**Expected formula:**

```
T(n, m) = n × m
```

Explain this **in plain language**, from a sysadmin or security perspective.

---

## 🔐 Security Perspective

These exercises demonstrate why **validating data in SQL** is powerful:

* Reduces reliance on frontend-only checks
* Prevents bad data from entering critical systems
* Improves consistency across applications
* Aligns with real-world authentication, logging, and SIEM pipelines

---

## ✅ Completion Criteria

Day 3 is considered complete when:

* All four SQL queries execute correctly
* The algorithm explanation clearly explains scaling behavior
* The student can explain how SQL replaces procedural loops

---

**Next Step:** Week 1 Day 4 — deeper control flow and scripting (Bash or Rust, depending on track).