Practice Question - Corelated Subquery

Duration: 7 min

This video lesson is available to enrolled students.

Return to /learn/RSSB-BASIC-COMPUTER-INSTRUCTOR/paper-2-database-management-system/sql/nested-correlated-subqueries/asset-practice-question-corelated-subquery after enrolling

Inside: a video lesson and guided study material.

Module outline

  1. About the Course: About the Course
  2. Paper-1 & 2 | Basic Numeracy: Ratio and Proportion (Ratios), Divisibility Rules, Mixture and Alligation, Permutation and Combination, Logarithm, Number System, HCF LCM, Sequence and Series (Series), Speed Time and Distance, Percentage, Average, Mensuration and Geometry, Age Problems, Arithmetic, Statistics, Probability, Profit and Loss, Simple Interest and Compound Interest, Partnership, Speed Mathematics and Basic Mathematics, Algebra, Powers and Exponents (Surds and Indices), Time and Work, Pipe and Cistern, RSSB Specific Videos - Basic Numeracy
  3. Paper-1 & 2 | Data Interpretation: Data Interpretation
  4. Paper-1 & 2 | General Mental Ability: Mental Ability, RSSB Specific Videos - Mental Ability
  5. Paper-1 & 2 | Logical Reasoning and Analytical Ability: Series (Number and Letter Series) (Numerical Relations and Reasoning), Coding Decoding, Data Sufficiency, Non Verbal Reasoning (Spatial Aptitude) (Spatial Reasoning) (Visual Reasoning), Syllogisms, Seating Arrangements, Blood Relations, Directions (Direction Test), Analytical Reasoning (Counting Figures Reasoning), Puzzle Solving (Puzzles), Calendar, Clock, Cubes & Dices, Venn Diagram, Analogy, Ranking, Order and Sequence, Deductive and Inductive Reasoning (Logical Deduction and Induction) (Prepositional Reasoning), Data Arrangement, Logical Sequence Of Words, Crypt Arithmetic
  6. Paper-1 & 2 | Decision Making and Problem Solving: Selection Decision Table (Decision Making)
  7. Paper-1 | General Science: Physics, Biology, Chemistry, RSSB Specific Videos
  8. Paper-1 | General Awareness: State Wise GK, GK- Current Affairs
  9. Paper - 2 | Digital Logic Systems: Digital Systems & Boolean Basics, Logic Gates & Hardware, Combinational Circuit, Sequential Circuits
  10. Paper-2 | Database Management System: Basics of DBMS, ER Diagram, Relational Model & Functional Dependencies, Keys & Integrity Constraints, Normalization (1NF - BCNF), Decomposition Properties & 4NF, Relational Calculus, Relational Algebra, SQL, NoSQL
  11. Paper-2 | Data Structures & Algorithms: Introduction to DS, Array, Stack, Queue, Linked List, Tree, Graphs, Algorithm Analysis, Sorting Algorithms, Intro to Compilers, Hashing
  12. Paper 2 | Data Communication and Network Security: Introduction to CN, Data Communication, Data Link Layer - Ethernet, DLL: Error Control, Net Layer: IPv4 & Proto, Net Layer: IP Addressing, Application Layer, Hardware basics, Network Security, Ethical Hacking
  13. Paper-2 | Operating Systems: Introduction to OS, Process Management, Memory Management, File Management, Deadlock
  14. Paper-2 | Computer Organization and Architecture: Basics of COA, Floating Point Rep, Cache Memory Organization, Input Output Organisation, Instr Formats & Modes, Control Unit Design, Register Transfer and Microoperations, Programming the Basic Computer, Number System, Number Representation
  15. Paper 2 | System Analysis and Design: Fundamentals of Systems and Information Systems, Fundamentals of Software Engineering, SDLC and Development Approaches, Software Documentation and Software Requirements Specification (SRS), SDD and Quality Assurance, Security, Risk Management & Maintenance, Software Testing
  16. Paper-2 | C Programming: C Fundamentals, Control Flow, Functions, Arrays & Pointers, Storage Classes, Structures & Enums, DMA, Macros, Scoping & File Handling
  17. Paper 2 | OOPS , C++ , JAVA: Language Design, OOPS with C++, Java, .Net Programming, Java Fundamentals
  18. Paper 2 | Python: Introduction to Python, Tokens, Variables, Data Types, I/O, Operators & Expressions, Control Flow & Looping, Collections & Strings, Functions & Modules, File Handling & Data Analysis
  19. Emerging Trends and AI: Emerging Tech Trends, Artificial Intelligence
  20. Paper-2 | Data Processing (Ms-Office): MS Office Applications
  21. Paper-2 | Internet and Web Technologies: HTML Basics, CSS, XML, JavaScript-Basics, Internet Fundamentals, Protocols & Services, Web Publishing & Hosting, Server Side & Security, Emerging Technologies, Terms & Abbreviations, Cyber Threats and Malware Attacks, Role of Internet and Web Services, Security in Mobile and Wireless Computing
  22. Paper 2 | Pedagogy: Child Development, Theories, Concept of Pedagogy, Inclusive Education, Experiential Pedagogy, Assessment & Evaluation, Psychology, Curriculum Framework & Reforms, National Education Policy (NEP) 2020
  23. Live Classes: RSSB Live Sessions
  24. Paper 1 - Full Mock Tests:
  25. Paper 2 - Full Mock Tests:
  26. BCI Previous Year Papers:
AI summary & chapters

AI Summary

An AI-generated summary of this video lecture.

This educational video analyzes a SQL practice problem centered on correlated subqueries and ranking logic. The core task involves determining which books are selected by a query that filters based on the count of other books with higher prices. The instructor systematically breaks down the problem statement, which defines a relation 'book' containing titles and prices. The primary SQL query selects the title from table B where a subquery counts rows in table T such that T.price is greater than B.price, with the condition that this count must be less than 5. The video emphasizes interpreting this logic as a ranking mechanism, effectively identifying books that are among the top five most expensive. Through manual tracing using sample data, the instructor demonstrates how correlated subqueries execute row-by-row, comparing each book's price against all others to calculate the count of more expensive items.

Chapters

  1. 0:00 – 2:00 00:00-02:00

    The video begins by presenting the SQL problem statement on screen, defining a relation 'book (title, price)' and asking what specific query lists. The instructor displays the SQL code: 'select title from book as B where (select count(*) from book as T where T.price > B.price) < 5'. Key visible events include the display of multiple-choice options A through D, with Option A suggesting 'Titles of the four most expensive books'. The instructor highlights the critical components of the query, specifically focusing on the inner subquery condition 'T.price > B.price' and the outer threshold '< 5'. This section establishes the foundational concept of using a correlated subquery to perform ranking without explicit ORDER BY clauses, setting the stage for logical analysis.

  2. 2:00 – 5:00 02:00-05:00

    The instructor transitions to manual verification by drawing a sample table with eight books labeled A through H and assigning specific prices (e.g., A=100, B=200, C=180). He traces the execution logic for each row in table B against all rows in table T. For a specific book like A with price 100, he counts how many other books have a higher price. The visual evidence shows handwritten tables labeled 'B' and 'T', where the instructor compares values to demonstrate that a book with price 100 has seven other books more expensive, failing the '< 5' condition. This step-by-step evaluation clarifies how the subquery correlates with the outer query's current row, ensuring students understand that 'count(*)' reflects the number of strictly more expensive items relative to the current book being evaluated.

  3. 5:00 – 6:48 05:00-06:48

    In the final segment, the instructor synthesizes the manual tracing results to identify the correct answer. He determines that only books with fewer than five other books more expensive satisfy the condition, which corresponds to the top five most expensive items. The on-screen text confirms the query structure 'where T.price > B.price' and the threshold '< 5'. By sorting the sample data (G=250, E=210, B=200, F=195, C=180), he shows that the fifth most expensive book (C) has exactly four books more expensive than it, satisfying 'count < 5'. Conversely, the sixth most expensive book (H=170) has five books more expensive, failing the condition. The video concludes by reinforcing that this logic selects the titles of the five most expensive books, distinguishing it from options suggesting 'four' or 'fifth most inexpensive'.

The video effectively teaches the mechanics of correlated subqueries by moving from abstract syntax to concrete execution. The key takeaway is that 'count(*) < 5' in a subquery comparing prices acts as a ranking filter, selecting items where fewer than five others exceed the current value. This logic inherently captures the top N most expensive records without sorting. The instructor's use of sample data with explicit price values (100, 200, etc.) provides a verifiable method for students to trace query behavior manually. This approach demystifies the 'correlated' nature of the subquery, showing that it re-evaluates for every row in the outer table. The distinction between 'four most expensive' and 'five most expensive' is clarified through the strict inequality '< 5', which includes exactly five items (those with counts 0, 1, 2, 3, and 4).

Loading lesson…