Data Analytics on Telegram: “🚀 Data Analyst Roadmap — Part 13 🗄️ SQL

🚀 Data Analyst Roadmap — Part 13

🗄️ SQL — Level 3: CASE WHEN, NULL Handling & Conditional Logic

In the previous part, you learned how to summarize data using GROUP BY and aggregate functions.

Now we're going to make SQL more powerful by learning how to create categories, handle missing data, and apply business rules.

These skills are extremely important because real-world datasets are rarely perfect.

You may need to answer questions like:

Which orders are High, Medium, or Low value?

How many customers have missing information?

What should we display when a value is NULL?

How many employees are above their target?

This is where CASE WHEN and NULL-handling functions become essential.

1️⃣ What Is CASE WHEN?

CASE WHEN allows SQL to make decisions.

Think of it as the SQL equivalent of Excel's:

IF()

For example:

CASE
    WHEN Sales >= 100000 THEN 'High'
    WHEN Sales >= 50000 THEN 'Medium'
    ELSE 'Low'
END

SQL evaluates the conditions and returns the appropriate category.

2️⃣ Basic CASE WHEN

Suppose you have:

Order_ID | Sales

1001 | 120,000

1002 | 75,000

1003 | 30,000

You want to classify orders.

SELECT
    Order_ID,
    Sales,
    CASE
        WHEN Sales >= 100000 THEN 'High'
        WHEN Sales >= 50000 THEN 'Medium'
        ELSE 'Low'
    END AS Sales_Category
FROM Orders;

CASE
    WHEN Sales >= 100000 THEN 'High'
    WHEN Sales >= 50000 THEN 'Medium'
    ELSE 'Low'
END

If Sales = 120000:

Is it ≥ 100000? ✅

Return High

Stop evaluating the remaining conditions.

That's why the order of conditions matters.

4️⃣ CASE WHEN with Categories

Suppose employees have salaries.

You want:

₹100,000+ → Senior

₹60,000–99,999 → Mid-Level

Below ₹60,000 → Junior

SELECT
    Name,
    Salary,
    CASE
        WHEN Salary >= 100000 THEN 'Senior'
        WHEN Salary >= 60000 THEN 'Mid-Level'
        ELSE 'Junior'
    END AS Salary_Level
FROM Employees;

This is a common data transformation technique.

5️⃣ CASE WHEN with Text Conditions

You can also evaluate text.

Suppose:

Department

IT

HR

Finance

Sales

You want to categorize IT and Finance as:

Business-Critical

and everything else as:

Other

SELECT
    Name,
    Department,
    CASE
        WHEN Department IN ('IT', 'Finance')
            THEN 'Business-Critical'
        ELSE 'Other'
    END AS Department_Type
FROM Employees;

6️⃣ CASE WHEN with AND

You can combine multiple conditions.

Suppose an employee qualifies for a bonus if:

Department = IT

Salary > ₹80,000

SELECT
    Name,
    Department,
    Salary,
    CASE
        WHEN Department = 'IT'
             AND Salary > 80000
        THEN 'Bonus Eligible'
        ELSE 'Not Eligible'
    END AS Bonus_Status
FROM Employees;

Both conditions must be true.

7️⃣ CASE WHEN with OR

Suppose employees from IT or Finance should receive a particular classification.

SELECT
    Name,
    Department,
    CASE
        WHEN Department = 'IT'
             OR Department = 'Finance'
        THEN 'Priority'
        ELSE 'Standard'
    END AS Employee_Type
FROM Employees;