Add logical functions chapter

src/data/courses/sql-mastery/chapters/scalar-functions/lessons/logical-functions.md

@@ -0,0 +1,184 @@
+---
+title: Logical Functions
+description: Learn how to use SQL logical functions for conditional operations
+order: 150
+type: lesson-challenge
+setup: |
+  ```sql
+  CREATE TABLE customer (
+      id INT PRIMARY KEY,
+      name VARCHAR(255),
+      phone VARCHAR(20),
+      email VARCHAR(255)
+  );
+
+  INSERT INTO customer (id, name, phone, email) VALUES
+  (1, 'John Doe', '123-456-7890', 'john.doe@example.com'),
+  (2, 'Jane Smith', '098-765-4321', 'jane.smith@example.com'),
+  (3, 'Alice Johnson', NULL, 'alice.johnson@example.com'),
+  (4, 'Bob Brown', NULL, NULL);
+
+  CREATE TABLE product (
+    id INT PRIMARY KEY,
+    name VARCHAR(255),
+    description VARCHAR(255),
+    price DECIMAL(10, 2)
+  );
+
+  INSERT INTO product (id, name, description, price) VALUES
+  (1, 'Product 1', 'Description 1', 10.00),
+  (2, 'Product 2', 'Description 2', 20.00),
+  (3, 'Product 3', '', 30.00);
+
+  CREATE TABLE sale (
+    id INT PRIMARY KEY,
+    customer_id INT,
+    product_id INT,
+    status VARCHAR(1),
+    days_pending INT,
+    amount DECIMAL(10, 2),
+    customer_type VARCHAR(10)
+  );
+
+  INSERT INTO sale (id, customer_id, product_id, status, days_pending, amount, customer_type) VALUES
+  (1, 1, 1, 'P', 3, 1200.00, 'VIP'),
+  (2, 2, 2, 'C', 1, 800.00, 'Regular'),
+  (3, 3, 3, 'X', 0, 1500.00, 'Regular'),
+  (4, 4, 1, 'P', 7, 2000.00, 'VIP');
+
+  CREATE TABLE product (
+    id INT PRIMARY KEY,
+    name VARCHAR(255),
+    description VARCHAR(255),
+    price DECIMAL(10, 2),
+    in_stock BOOLEAN
+  );
+
+  INSERT INTO product (id, name, description, price, in_stock) VALUES
+  (1, 'Product 1', 'Description 1', 10.00, true),
+  (2, 'Product 2', 'Description 2', 20.00, false),
+  (3, 'Product 3', '', 300.00, true);
+  ```
+---
+
+Logical functions help you handle conditional logic and `NULL` values in SQL. These functions are essential for data transformation, handling missing values, and implementing business rules in your queries.
+
+## COALESCE Function
+
+The `COALESCE` function returns the first non-NULL value in a list of expressions. This is particularly useful for handling `NULL` values and providing default values:
+
+```sql
+-- Basic COALESCE examples
+SELECT COALESCE(NULL, 1, 2);          -- Returns: 1
+SELECT COALESCE(NULL, NULL, 3);       -- Returns: 3
+SELECT COALESCE(NULL, NULL, NULL);    -- Returns: NULL
+
+-- e.g. when fetching customer data
+SELECT
+    name,
+    COALESCE(phone, email, 'No Contact') as contact_info
+FROM customer;
+```
+
+## NULLIF Function
+
+The `NULLIF` function compares two expressions and returns `NULL` if they're equal, otherwise returns the first expression. This is useful for handling special values or converting empty strings to `NULL`:
+
+```sql
+-- Basic NULLIF examples
+SELECT NULLIF(10, 10);     -- Returns: NULL
+SELECT NULLIF(10, 20);     -- Returns: 10
+
+-- e.g. to be consistent with empty strings
+SELECT
+    id,
+    NULLIF(TRIM(description), '') as clean_description
+FROM product;
+
+-- e.g. default values for missing data
+SELECT
+    name,
+    COALESCE(description, 'No description available') as description,
+    COALESCE(price, 0.00) as price,
+    COALESCE(in_stock, false) as is_available
+FROM product;
+```
+
+## CASE Expression
+
+The `CASE` expression provides if-then-else logic in SQL. There are two forms: simple CASE and searched CASE.
+
+### Simple CASE
+
+Used when comparing a value against multiple possible matches:
+
+```sql
+-- Map status codes to descriptions
+SELECT
+    id,
+    CASE status
+        WHEN 'P' THEN 'Pending'
+        WHEN 'C' THEN 'Completed'
+        WHEN 'X' THEN 'Cancelled'
+        ELSE 'Unknown'
+    END as status_description
+FROM sale;
+```
+
+Here is another example using aggregate functions:
+
+```sql
+SELECT
+    count(CASE WHEN status = 'P' THEN 1 END) as pending_orders,
+    count(CASE WHEN status = 'C' THEN 1 END) as completed_orders,
+    count(CASE WHEN status = 'X' THEN 1 END) as cancelled_orders
+FROM sale;
+```
+
+### Searched CASE
+
+Used for more complex conditions:
+
+```sql
+-- Assign shipping priority
+SELECT
+    id,
+    CASE
+        WHEN customer_type = 'VIP' AND amount > 1000 THEN 'Express'
+        WHEN customer_type = 'VIP' OR amount > 1500 THEN 'Priority'
+        ELSE 'Standard'
+    END as shipping_method
+FROM sale;
+```
+
+Another example to categorize products by price:
+
+```sql
+SELECT
+    name,
+    price,
+    CASE
+        WHEN price >= 100 THEN 'Premium'
+        WHEN price >= 50 THEN 'Standard'
+        ELSE 'Budget'
+    END as price_category
+FROM product;
+```
+
+Example to calculate discounts based on multiple conditions:
+
+```sql
+SELECT
+    id,
+    amount,
+    customer_type,
+    CASE
+        WHEN amount >= 1000 AND customer_type = 'VIP' THEN amount * 0.8
+        WHEN amount >= 1000 THEN amount * 0.9
+        WHEN customer_type = 'VIP' THEN amount * 0.95
+        ELSE amount
+    END as discounted_amount
+FROM sale;
+```
+
+Let's look at some exercises to practice these functions and move on to the next chapter about subqueries.