MASTERING SQL FOR DATA ENGINEERING: ADVANCED QUERIES, OPTIMIZATION AND DATA MODELLING BEST PRATICES

INTRODUCTION

SQL(Structured Query Language) is a very crucial tool for data engineering which enables to access databases, developing data pipeline, data transformation and intergrating analytics. SQL is important for daily operations across analytics, engineering and architectural data roles.

SELECT 
    department,
    COUNT(*) as employee_count,
    AVG(salary) as avg_salary
FROM employees e
JOIN departments d ON e.dept_id = d.id
WHERE hire_date >= '2023-01-01'
GROUP BY department
HAVING COUNT(*) > 5;

• SELECT: Specifies columns we want to choose
• JOIN: Connects employees and departments tables using dept_id and id
• WHERE: Filters only employees hired since 2023
• GROUP BY: Groups results by department
• HAVING: Filters groups with more than 5 employees

ADVANCED SQL TECHNIQUES

1. WINDOW FUNCTION
These are calculations/operations that you do across a set of tables rows that are related to the current row. They return results per rows unlike aggregate functions which returns one value.
SYNTAX FOR WINDOW FUNCTIONS
SELECT column1, column2, function() OVER(PARTITION BY Partition_expression ORDER BY sort_expression) AS result_name form table_name

SELECT 
    employee_name,
    salary,
    department,
    AVG(salary) OVER (PARTITION BY department) as dept_avg,
    salary - AVG(salary) OVER (PARTITION BY department) as diff_from_avg
FROM employees;

Explanation:

• PARTITION BY divides data into department/categories
• AVG(salary) OVER calculates average within each partition
• Compares individual salary to department average
• It is useful for trend analysis and comparisons

2. CTE(COMMON TABLE EXPRESSIONS)

CTE are temporary memory result set that can be referenced within a SQL statement. They last only within the execution of the query. They are used where there are many subqueries hence making the code more readable.

SYNTAX OF CTEs
WITH CTE_name AS(SQL query)
SELECT * FROM table;

WITH revenue_data AS (
    SELECT product_id,
           SUM(amount) as total_revenue
    FROM sales
    GROUP BY product_id
),
product_rankings AS (
    SELECT product_id,
           total_revenue,
           RANK() OVER (ORDER BY total_revenue DESC) as revenue_rank
    FROM revenue_data
)
SELECT * FROM product_rankings WHERE revenue_rank <= 10;

• First CTE revenue_data is created with keyword WITH to calculate total revenue per product
• Second CTE product_rankingsis created to rank products by revenue
• We selects top 10 products

3. STORED PROCEDURE
Stored procedure is a SQL statement/ block of queries that you can save and use in the future. They help to save time, make our complex queries more simplified during execution and enhance the database security.
SYNTAX OF STORED PROCEDURE

CREATE PROCEDURE procedure_name
AS
BEGIN
SQL STATEMENT
END;
EXEC procedure_name;

QUERY OPTIMIZATION AND PERFORMANCE TUNING

SQL performance tuning refers to the process of optimizing database queries and operations to ensure faster and more efficient data retrieval.It enhances query execution by addressing problems in storing, indexing, and accessing data. Implementing indexing, eliminating unneccesary operations, use of appropriate data types and reducing subqueries can help achieve faster query execution and better resource utilization.

Before optimization:

SELECT * 
FROM orders 
WHERE YEAR(order_date) = 2024;

After optimization:

SELECT order_id, customer_id, amount 
FROM orders 
WHERE order_date >= '2024-01-01' 
AND order_date < '2025-01-01';

• Replaced YEAR() function with date range (allows index usage)
• Specified needed columns instead of *
• Added proper date range conditions

CREATE MATERIALIZED VIEW monthly_sales AS
SELECT 
    DATE_TRUNC('month', order_date) as month,
    customer_id,
    SUM(amount) as total_amount,
    COUNT(*) as order_count
FROM orders
GROUP BY 1, 2
WITH DATA;

CREATE INDEX idx_monthly_sales_customer 
ON monthly_sales(customer_id);

DATA MODELLING

Data modelling is the creation of your data structure. This refers to how you will want your data to be organized/visual representation.
Types of data modelling

1.Conceptual data modelling

This represents the high level business overview of the data without going much into details. it is the simple the simplke representation of what we want from our data.

2. Lgical data modelling

This describe data elements in details and helps to create a visual understanding of the data.

3. Physical data modelling

This shows how data will be stored in a database for example, student_ID INT PRIMARY KEY

4. Entity-relational data model

This shows the relationship that are between different database objects/entities. There are different relationships that exists between different tables in a database namely: one to many, many to many, one to one, many to many relationship.
_
Data Normalization_

Normalization the process of organizing data to reduce redundancy and ensure data consistency. When you normalize a database, you break down large tables into smaller with a specific purpose.

levels of normalization

1. First Normal Form (1NF)

Each cell in database table must contain only a single value – no lists or multiple values are allowed. For example, instead of having multiple phone numbers in one cell, you'd create a separate phone numbers table.

*2. Second Normal Form (2NF) *

Ensures that all non-key attributes fully depend on the primary key.
**

1. Third Normal Form (3NF)**

Eliminates transitive dependencies, that is, data must be moved to a separate table.
Denormalization

This is creating a reference sheet to combine information from multiple sources into one place for easy access. It involves deliberately combining tables that were previously separated during normalization.