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DBMS → Introduction to DBMS 38

Union rule for FDs is derived from which two axioms
Augmentation and Transitivity
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Decomposition rule for FDs is derived from
Reflexivity only
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Canonical cover has no extraneous attributes and no redundant FDs. Which algorithm finds it
Standard algorithm: remove extraneous attributes (left then right side), then remove redundant FDs
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To check if FD A→B is redundant in set F, compute
A+ without the FD A→B; if B∈A+ then A→B is redundant
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R(A,B,C,D,E) with FDs: AB→C, C→D, D→E, E→A. What is the closure AB+?
ABCDE
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Which query finds employees in BOTH department 10 AND department 20 (transferred)
SELECT empid FROM emp WHERE dept=10 INTERSECT SELECT empid FROM emp WHERE dept=20
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Which SQL deletes duplicate rows keeping only lowest id
DELETE FROM emp WHERE id NOT IN (SELECT MIN(id) FROM emp GROUP BY name,salary)
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Which query efficiently finds all questions with no answer set (right_answer_id IS NULL)
SELECT * FROM questions WHERE right_answer_id IS NULL
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To update q_level for all questions in assignment_id=1 to level 3
UPDATE questions SET q_level=3 WHERE assignment_id=1
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Which SQL finds questions that have all 5 options filled (option_e is not NULL)
SELECT * FROM questions WHERE option_e IS NOT NULL
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To soft-delete all questions for assignment_id=2 in Laravel
Question::where('assignment_id',2)->delete() with SoftDeletes trait
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To permanently delete soft-deleted questions in Laravel
Question::onlyTrashed()->forceDelete()
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To restore soft-deleted questions in Laravel
Question::onlyTrashed()->restore()
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In Laravel, Question::where('assignment_id',1)->get() vs Question::where('assignment_id',1)->first()
get() returns Collection of all matches; first() returns only the first matching model or NULL
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DB::table('questions')->value('question') returns
First question's text as a plain value (not Collection/model)
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DB::table('questions')->exists() returns
Boolean: TRUE if at least one row matches the query, FALSE otherwise
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DB::table('questions')->doesntExist() returns
Boolean: TRUE if NO rows match the query
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Question::withCount('options') in Laravel Eloquent
Adds options_count attribute with count of related options
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DB::table('questions')->orderByRaw('FIELD(q_level,3,2,1)')->get() in MySQL
Orders by custom field order: 3 first, then 2, then 1
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DB::table('questions')->selectRaw('COUNT(*) as total, AVG(q_level) as avg_level')->first()
Returns a stdClass with total count and average q_level
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To get questions grouped by q_level with count in Laravel
DB::table('questions')->groupBy('q_level')->selectRaw('q_level,COUNT(*) as cnt')->get()
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Which Laravel method chains multiple OR conditions cleanly
->where(function($q){ $q->where('a',1)->orWhere('b',2) }) for grouped OR
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Which SQL finds the n-th highest salary (n=3rd highest)
SELECT salary FROM emp ORDER BY salary DESC LIMIT 2,1
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Which SQL lists all tables with their row counts
SELECT TABLE_NAME, TABLE_ROWS FROM information_schema.TABLES WHERE TABLE_SCHEMA='dbname'
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EXPLAIN output 'type=ALL' in MySQL means
Full table scan - no index used (worst case for large tables)
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EXPLAIN output 'type=ref' in MySQL means
Non-unique index lookup - finds rows matching an index condition
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EXPLAIN output 'type=const' in MySQL means
Single row found via unique index or PK - fastest possible lookup
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Which SQL creates a composite index for queries filtering by assignment_id and q_level
CREATE INDEX idx ON questions(assignment_id,q_level)
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SELECT DISTINCT question_type FROM questions returns
Unique question_type values (one row per distinct type)
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To add assignment_id=3 and copy all questions from assignment 1 in SQL
INSERT INTO questions(assignment_id,...) SELECT 3,...FROM questions WHERE assignment_id=1
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TRUNCATE vs DELETE: which can be rolled back (in most databases)
DELETE can be rolled back (DML); TRUNCATE typically cannot (DDL in most databases)
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Which SQL gets all questions where solution contains the word 'DBMS'
SELECT * FROM questions WHERE solution LIKE '%DBMS%'
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DB::table('questions')->where('question','LIKE','%DBMS%')->get() generates
SELECT * FROM questions WHERE question LIKE '%DBMS%'
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To get paginated questions (page 3, 10 per page) in Laravel
DB::table('questions')->skip(20)->take(10)->get()
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Laravel Eloquent selectRaw('option_a as correct_answer') WHERE right_answer_id = 'option_a' uses
SELECT with alias
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To find questions where right_answer_id is not one of the standard options
SELECT * FROM questions WHERE right_answer_id NOT IN ('option_a','option_b','option_c','option_d','option_e')
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Which Laravel method returns the count of questions per assignment efficiently
DB::table('questions')->select('assignment_id',DB::raw('COUNT(*) as cnt'))->groupBy('assignment_id')->get()
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Which SQL verifies data integrity after bulk insert of questions
SELECT * FROM questions WHERE option_a IS NULL OR option_b IS NULL OR option_c IS NULL OR option_d IS NULL OR right_answer_id IS NULL
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DBMS → Relational Model 2

R(A,B,C,D,E) with above FDs. How many candidate keys are there
4 (AB, CB, DB, EB)
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R(A,B,C) FDs: A→B, B→C, C→A. All candidate keys are
A, B, and C (each determines all others)
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