🟢 SQL Basics (Data Engineering Focus)

SQL is the core language of data engineering used for pipelines, analytics, and data warehouses.

The goal is not just to write queries but to understand how data flows.

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🧠 Mental Model

Think of SQL as a pipeline:

Data → Filter → Transform → Output

Before writing any query, ask:

• What data do I need?
• How do I filter it?
• How should it be sorted or grouped?

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📌 SELECT

Used to fetch data from a table.

SELECT name, salary
FROM employees;

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📌 WHERE

Used to filter rows.

SELECT *
FROM employees
WHERE salary > 50000;

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📌 ORDER BY

Used to sort results.

SELECT *
FROM employees
ORDER BY salary DESC;

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📌 LIMIT

Used to restrict output rows.

SELECT *
FROM employees
LIMIT 10;

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📌 DISTINCT

Used to remove duplicates.

SELECT DISTINCT department
FROM employees;

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📌 NULL HANDLING

NULL means missing or unknown value.

Correct:

SELECT *
FROM employees
WHERE manager_id IS NULL;

Incorrect:

manager_id = NULL

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📌 ALIAS

Used to rename columns.

SELECT name AS employee_name
FROM employees;

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📌 EXECUTION ORDER (IMPORTANT)

SQL runs in this order:

1. FROM
2. WHERE
3. GROUP BY
4. HAVING
5. SELECT
6. ORDER BY
7. LIMIT

This is different from how we write SQL.

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🔥 REAL EXAMPLE

SELECT name, salary
FROM employees
WHERE department = 'Engineering'
ORDER BY salary DESC
LIMIT 5;

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⚠️ COMMON MISTAKES

• Using = NULL instead of IS NULL
• Using SELECT * unnecessarily
• Confusing WHERE and HAVING
• Ignoring execution order

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🎯 INTERVIEW QUESTIONS

• What is SQL execution order?
• Difference between WHERE and HAVING?
• Why is NULL special in SQL?
• Why is SELECT * discouraged?

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🚀 SUMMARY

SQL is about data flow thinking.

• WHERE filters rows
• ORDER BY sorts results
• NULL must be handled carefully
• Execution order matters more than syntax

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