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SQL

SQL

SQL interview questions, practical queries and real-world transaction case studies.

14 topics248 questions & lessons

SQL Fundamentals

10 questions · Open → 02

Queries and Aggregation

13 questions · Open → 03

NULL Values and Joins

10 questions · Open → 04

Subqueries and Salary Ranking

7 questions · Open → 05

Window Functions

10 questions · Open → 06 INTERVIEW SET

5-Year Interview: Ranking, Joins & Aggregation

14 sections · Open → 07

Banking: Concurrent Withdrawals

23 sections · Open → 08

Account A to B Transfer

23 sections · Open → 09

University Fees: 5,000 Students

23 sections · Open → 10

University Payroll: 2,000 Employees

23 sections · Open → 11

Corporate Salary: 10,000 Employees

23 sections · Open → 12

Failure, Rollback, Retry & Reconciliation

23 sections · Open → 13

Transaction Schema, SQL & Laravel

23 sections · Open → 14 INTERVIEW SET

Transaction Interview Case Studies

23 sections · Open →

Topics

Optional study guide

See the big picture · SQL

Follow the rows through a query

Start with the diagram, compare the concepts, then apply them to the lesson.

FROM / JOINChoose and combine source rows.
WHEREFilter individual rows.
GROUP BY / HAVINGGroup rows and filter the groups when needed.
SELECT / ORDER BYChoose output expressions and sort the result.

At a glance
Clause	Works on	Use it to
WHERE	Individual rows	Keep salary > 50000
GROUP BY	Rows with shared values	Create one group per department
HAVING	Groups	Keep groups with COUNT(*) > 5
ORDER BY	Result rows	Display salaries from highest to lowest

Watch for this: This is a simplified logical model, not a claim about the database execution plan. SQL syntax differs between database products.

Read the PostgreSQL table expressions