SQL Learning Roadmap: From Fundamentals to Window Functions

1. Why SQL is Mandatory for Placements

Technical recruiters in campus rounds and off-campus tests love SQL because it tests your logical relational reasoning without language-specific syntax boilerplate.

Whether you interview for backend development, data engineering, or quantitative analytics, you will be expected to write performant queries under time constraints.

2. Tier 1: SQL Logical Query Processing Order

To write bug-free SQL, you must know the exact order database engines evaluate query clauses:

1. FROM & JOIN
2. WHERE (Filters individual row records)
3. GROUP BY (Groups rows into aggregated sets)
4. HAVING (Filters aggregated groups)
5. SELECT (Evaluates expressions and aliases)
6. DISTINCT
7. ORDER BY
8. LIMIT / OFFSET

Key Interview Insight: You cannot use a SELECT alias in a WHERE clause because the WHERE step executes before the SELECT step!

3. Tier 3: Window Functions (The Gold Standard of Placement Rounds)

Window functions calculate values across a set of table rows that are related to the current row, without collapsing the individual rows into a single summary like GROUP BY does.

Finding the Nth Highest Salary per Department:

Notice why DENSE_RANK() is used instead of RANK(): if two employees tie for #1, DENSE_RANK() will assign #2 to the next salary, whereas RANK() would skip to #3!