Add conversion functions

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

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+---
+title: Conversion Functions
+description: Learn how to convert data between different types in SQL
+order: 140
+type: lesson-challenge
+setup: |
+  CREATE TABLE raw_data (
+      value VARCHAR(255)
+  );
+
+  INSERT INTO raw_data (value) 
+  VALUES ('123'), 
+      ('456.78'), 
+      ('abc'), 
+      ('123.45'),
+      ('true'),
+      ('false'),
+      ('1'),
+      ('0');
+
+  CREATE TABLE imported_sales_data (
+      id_string VARCHAR(10),
+      product_name VARCHAR(100),
+      amount_string VARCHAR(20),
+      sale_date VARCHAR(20)
+  );
+
+  INSERT INTO imported_sales_data (id_string, product_name, amount_string, sale_date) VALUES
+    ('1', 'Laptop', '999.99', '2024-01-15'),
+    ('2', 'Mouse', '24.99', '2024-01-16'),
+    ('', 'Keyboard', '59.99', '2024-01-17'),
+    ('4', 'Monitor', '', '2024-01-18'),
+    ('5', 'Headphones', '79.99', ''),
+    ('abc', 'USB Drive', '29.99', '2024-01-20'),
+    ('7', 'Webcam', 'N/A', '2024-01-21'),
+    ('', '', '', ''),
+    ('9', 'Printer', '299.99', '2024-01-23'),
+    ('10', 'Scanner', '199.99', '2024-01-24');
+---
+
+Conversion functions allow you to transform data from one type to another. These functions are essential when you need to ensure data compatibility or format data for specific operations.
+
+## Using CAST
+
+The `CAST` function is the standard SQL way to convert between data types. It's widely supported across all major databases:
+
+```sql
+-- Convert string to integer
+SELECT CAST('123' AS INTEGER);       -- Returns: 123
+
+-- Convert string to decimal
+SELECT CAST('123.45' AS DECIMAL(10,2));  -- Returns: 123.45
+
+-- Convert number to string
+SELECT CAST(123.45 AS VARCHAR);      -- Returns: '123.45'
+
+-- Convert string to date
+SELECT CAST('2024-03-15' AS DATE);   -- Returns: 2024-03-15
+
+-- Convert string to timestamp
+SELECT CAST('2024-03-15T12:00:00' AS TIMESTAMP);
+```
+
+## Regex Checking
+
+When converting data, it's important to handle potential errors.
+
+In the following example, we're using a regex pattern to check if the value is an integer. If it is, we'll cast it to an integer. If it's not, we'll return `NULL`.
+
+```sql
+-- Using CASE to handle invalid conversions
+SELECT
+    CASE
+        -- Checking value against the regex pattern for integers
+        WHEN value ~ '^[0-9]+$' THEN CAST(value AS INTEGER)
+        ELSE NULL
+    END AS safe_integer
+FROM raw_data;
+```
+
+> **Note**: The `~` operator is used to check if the value matches the regex pattern for integers. This is a PostgreSQL-specific operator.
+
+## Numeric Conversions
+
+When working with numbers in different formats:
+
+```sql
+-- Convert percentage string to decimal
+SELECT CAST(REPLACE('15%', '%', '') AS DECIMAL(5,2)) / 100.0;
+-- Returns: 0.15
+```
+
+## Date Conversions
+
+Converting between different date formats:
+
+```sql
+-- Convert string dates to date type
+SELECT
+    CAST('2024-03-15' AS DATE) AS standard_date,
+    CAST('03/15/2024' AS DATE) AS us_date,
+    CAST('15-Mar-2024' AS DATE) AS custom_date;
+
+-- Convert date to different string format
+SELECT TO_CHAR(
+    CAST('2024-03-15' AS DATE),
+    'Month DD, YYYY'
+) AS formatted_date;
+```
+
+## Boolean Conversions
+
+Converting various values to boolean:
+
+```sql
+-- Convert strings/numbers to boolean
+SELECT
+    CAST('true' AS BOOLEAN),     -- Returns: true
+    CAST('1' AS BOOLEAN),        -- Returns: true
+    CAST('false' AS BOOLEAN),    -- Returns: false
+    CAST('0' AS BOOLEAN);        -- Returns: false
+```
+
+> **Note**: While `CAST` is standard SQL, some databases offer alternative syntax. For example, PostgreSQL supports the `::` operator (`'123'::integer`), and SQL Server supports the `CONVERT` function. The handling of invalid conversions also varies between databases. Always check your database's documentation for specific behavior and best practices.
+
+In the next lesson, we'll explore logical functions for handling conditional operations.