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GK One-Line Question & Answer

15541+ short questions with short answers, covering every category and sub-category on the site — no long articles to scroll through. Good for a fast recap before an exam, or a few minutes of daily practice.

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DBMS → Joins 2

Hash join should hash which table when R has 1000 rows and S has 100 rows?
S (smaller) into memory hash table then probe with R
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Which join algorithm is best when inner table has index on join attribute?
Index nested-loop join (uses index to find matching rows rapidly)
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DBMS → Transactions 2

Which 2PL variant holds ALL locks until transaction commits?
Rigorous 2PL
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In ARIES, winners (committed transactions at crash) require which recovery action?
REDO - their changes must be reapplied to ensure durability
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DBMS → Introduction to DBMS 34

Which SQL finds department with most employees?
SELECT dept_id FROM emp GROUP BY dept_id ORDER BY COUNT(*) DESC LIMIT 1
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EXPLAIN/EXPLAIN ANALYZE in SQL is used to
Show query execution plan (indexes used, join types, row estimates)
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Given R(A,B,C,D,E) with FD AB→CDE. AB+ equals
ABCDE
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π(A)(R) where R has 10 rows, A has 4 distinct values returns
4 rows
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Which creates table same structure as questions but no data?
CREATE TABLE questions_bak AS SELECT * FROM questions WHERE 1=0
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Which window function computes running total of salaries ordered by hire_date?
SELECT SUM(salary) OVER(ORDER BY hire_date) FROM emp
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DB::table('questions')->where('q_level',3)->orWhere('q_level',2)->get() generates
WHERE q_level=3 OR q_level=2
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To retrieve only active non-deleted questions in Laravel
DB::table('questions')->where('is_deleted',0)->get()
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right_answer_id storing 'option_a','option_b' etc. is which design?
Storing column name reference indicating which option column has correct answer
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question_type=1 in questions table likely represents
MCQ (Multiple Choice Question) type
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is_deleted=0 in questions table represents
Record is ACTIVE (not deleted) - soft delete pattern
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To count questions per assignment_id and q_level in one query
SELECT assignment_id,q_level,COUNT(*) as cnt FROM questions GROUP BY assignment_id,q_level
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$table->foreignId('assignment_id')->constrained() creates
UNSIGNED BIGINT column PLUS FK constraint referencing assignments.id
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MERGE statement (SQL:2003) performs
INSERT UPDATE or DELETE based on match condition (UPSERT)
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To add composite index on (assignment_id, q_level) in Laravel migration
$table->index(['assignment_id','q_level'])
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Which SQL finds duplicate right_answer_id values in questions?
SELECT right_answer_id,COUNT(*) FROM questions GROUP BY right_answer_id HAVING COUNT(*)>1
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To get correct answer text when right_answer_id='option_b' for question id=1
SELECT option_b FROM questions WHERE id=1
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Eloquent query to get hard questions for assignment 1 that are not deleted
Question::where('q_level',3)->where('assignment_id',1)->where('is_deleted',0)->get()
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solution column in questions table contains
Full explanation/solution text for the question
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$table->enum('right_answer_id',['option_a','option_b','option_c','option_d','option_e']) creates
ENUM allowing only specified values
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option_e=NULL in questions table means
Question has only 4 options (no 5th option needed)
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created_at and updated_at in questions table are managed by
Eloquent automatic timestamp management ($timestamps=true by default)
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Which command adds CHECK constraint on q_level after table creation?
ALTER TABLE questions ADD CONSTRAINT chk_level CHECK (q_level BETWEEN 1 AND 3)
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To bulk insert 2000 questions efficiently without memory issues
DB::table()->insert(array_of_2000_arrays) - single bulk INSERT statement
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Which SQL gets questions for multiple assignments (1, 2, 3)?
DB::table('questions')->whereIn('assignment_id',[1,2,3])->get()
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Database design process order is
ER Model (conceptual) → Normalize (logical) → Migrate (physical implementation)
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To retrieve the right answer option text dynamically in SQL for any question
SELECT CASE right_answer_id WHEN 'option_a' THEN option_a WHEN 'option_b' THEN option_b WHEN 'option_c' THEN option_c WHEN 'option_d' THEN option_d ELSE option_e END as answer FROM questions WHERE id=1
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Which SQL removes all questions for a specific assignment and resets auto-increment?
UPDATE questions SET is_deleted=1 WHERE assignment_id=1 (soft delete preferred)
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In ARIES, losers (uncommitted at crash) require which recovery action?
UNDO - their incomplete changes must be reversed
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B+tree with height h requires how many I/Os for point query?
h+1 I/Os (h for tree traversal + 1 to fetch actual record)
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SQL injection is prevented by
Using parameterized queries (prepared statements) and input validation
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Principle of Least Privilege means
Grant ONLY minimum permissions needed for each user's tasks
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TDE (Transparent Data Encryption) protects
Data at rest by encrypting database files on disk
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RBAC means
Access based on assigned roles - permissions granted to roles, users assigned roles
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DBMS → Functional Dependency 1

Armstrong axiom: from A→BC, we can derive?
A→B and A→C (decomposition rule)
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DBMS → Normalization 1

Normalization and Laravel migrations are related because
Migrations implement the normalized schema design in version-controlled PHP code
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