Step 1: SQL Mastery for Data Engineering

In Data Engineering, SQL is used for everything from simple data extraction to complex business logic. You must master Window Functions and CTEs.

🛠️ Code Example: Window Functions

This query calculates the running total of sales and the 7-day moving average.

WITH DailySales AS (
    SELECT 
        order_date,
        SUM(total_amount) as daily_total
    FROM orders
    GROUP BY order_date
)
SELECT 
    order_date,
    daily_total,
    -- 1. Running Total
    SUM(daily_total) OVER (ORDER BY order_date) as running_total,
    -- 2. 7-Day Moving Average
    AVG(daily_total) OVER (
        ORDER BY order_date 
        ROWS BETWEEN 6 PRECEDING AND CURRENT ROW
    ) as moving_avg
FROM DailySales
ORDER BY order_date;

🎯 Key Concepts

CTEs (WITH): Use these to break complex queries into readable, logical steps.
Window Functions (OVER): Perform calculations across a set of rows related to the current row without grouping.

🥅 Your Goal

Practice the RANK() function to find the top 3 customers per month.
Learn the difference between INNER, LEFT, and FULL OUTER joins.