In SQL, both `CAST` and `TRY_CAST` are used to convert data from one type to another, but they serve slightly different purposes, especially when handling invalid conversions.

### `CAST`:

The `CAST` function is used to explicitly convert an expression from one data type to another. It raises an error if the conversion is not possible. 

**Syntax:**

```sql
CAST(expression AS target_data_type)
```

### Example:
```sql
SELECT CAST('123' AS INT) AS number;  -- Converts '123' to an integer.
```

If you try to cast a string that cannot be converted to an integer (e.g., `'abc'`), it will throw an error:
```sql
SELECT CAST('abc' AS INT);  -- Error: Conversion failed
```

### `TRY_CAST`:

`TRY_CAST` is similar to `CAST`, but it provides a safer way to attempt the conversion. Instead of raising an error when the conversion fails, `TRY_CAST` returns `NULL`. This is useful when you want to handle invalid data gracefully without causing the entire query to fail.

**Syntax:**

```sql
TRY_CAST(expression AS target_data_type)
```

### Example:

```sql
SELECT TRY_CAST('123' AS INT) AS number;  -- Converts '123' to an integer (returns 123)
SELECT TRY_CAST('abc' AS INT) AS number;  -- Conversion fails, returns NULL instead of error
```

### Key Differences:

1. **Error Handling**:
   - **`CAST`**: Throws an error if the conversion fails.
   - **`TRY_CAST`**: Returns `NULL` if the conversion fails.

2. **Use Case**:
   - **`CAST`**: Use when you're confident the conversion will succeed or you want an error when it doesn’t.
   - **`TRY_CAST`**: Use when working with potentially invalid data and you want the query to continue without failure.

### Example Use Cases:

1. **Safely Converting Data**:
   You have a column in your table with mixed data types (numbers and text), and you want to convert only the valid numeric data to integers.

   ```sql
   SELECT TRY_CAST(column_name AS INT) AS converted_value
   FROM your_table;
   ```

2. **Handling Non-Numeric Data Gracefully**:
   You want to handle non-numeric values without throwing an error, so you use `TRY_CAST` to return `NULL` for those rows.

   ```sql
   SELECT name, TRY_CAST(salary AS DECIMAL(10, 2)) AS salary
   FROM employees;
   ```

### Summary:
- **`CAST`** converts data between types and raises an error if the conversion fails.
- **`TRY_CAST`** attempts the conversion and returns NULL on failure, making it safer to use in scenarios where data might not always be valid for conversion.



The `CASE WHEN` statement in SQL is used to introduce conditional logic within a query. It is similar to an `if-else` structure in programming languages. You can use it to return different values based on conditions for each row in a result set. It’s particularly useful when you want to categorize, transform, or manipulate data on the fly.

### Basic Syntax:
```sql
SELECT 
    CASE 
        WHEN condition1 THEN result1
        WHEN condition2 THEN result2
        ELSE default_result
    END AS column_name
FROM table_name;
```

- **`WHEN condition`**: This specifies the condition to check.
- **`THEN result`**: This is the result that will be returned if the condition is true.
- **`ELSE default_result`**: Optional. It specifies the result returned if none of the conditions are met. If omitted, it will return `NULL`.
- **`END`**: Closes the `CASE` statement.

### Example Use Cases:

#### 1. **Categorize Data Based on Conditions**

Suppose you have a table called `employees` with columns `employee_id`, `name`, and `salary`, and you want to categorize employees into salary ranges (Low, Medium, High) based on their salary:

```sql
SELECT 
    name,
    salary,
    CASE 
        WHEN salary < 3000 THEN 'Low'
        WHEN salary >= 3000 AND salary <= 6000 THEN 'Medium'
        ELSE 'High'
    END AS salary_category
FROM employees;
```

Here, employees are classified into different categories (`Low`, `Medium`, or `High`) based on their salary values.

#### 2. **Conditional Calculation**

You can also perform calculations conditionally. Let’s say you want to apply a discount to purchases based on the total purchase amount from the `orders` table:

```sql
SELECT 
    order_id,
    total_amount,
    CASE 
        WHEN total_amount > 500 THEN total_amount * 0.90  -- 10% discount for orders > 500
        WHEN total_amount BETWEEN 100 AND 500 THEN total_amount * 0.95  -- 5% discount for orders between 100 and 500
        ELSE total_amount  -- No discount for orders below 100
    END AS discounted_amount
FROM orders;
```

In this case, different discount percentages are applied based on the `total_amount` in each row.

#### 3. **Handling NULL values**

You can use `CASE WHEN` to handle `NULL` values. For example, if you want to display "Unknown" for any `NULL` values in a `department` column:

```sql
SELECT 
    employee_id,
    name,
    CASE 
        WHEN department IS NULL THEN 'Unknown'
        ELSE department
    END AS department_name
FROM employees;
```

This will replace any `NULL` values in the `department` column with the word "Unknown."

#### 4. **Use in `ORDER BY`**

`CASE WHEN` can also be used in the `ORDER BY` clause for custom sorting logic. For example, suppose you want to sort employees by their salary categories (Low, Medium, High) in a specific order:

```sql
SELECT 
    name, 
    salary
FROM employees
ORDER BY 
    CASE 
        WHEN salary < 3000 THEN 1  -- Low
        WHEN salary >= 3000 AND salary <= 6000 THEN 2  -- Medium
        ELSE 3  -- High
    END;
```

This will sort the employees in the order of `Low`, `Medium`, and `High` salary categories.

### Summary:
- The `CASE WHEN` statement allows conditional logic in SQL queries.
- You can use it in `SELECT`, `WHERE`, `ORDER BY`, and other parts of a query.
- It’s useful for data transformation, categorization, handling null values, and more.
