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DBMS → Introduction to DBMS 34

$table->softDeletes() adds
deleted_at NULLABLE TIMESTAMP column
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$table->boolean('is_deleted') creates
TINYINT(1) MySQL boolean column
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$table->foreign('assignment_id')->references('id')->on('assignments') creates
Foreign key constraint referencing assignments.id
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$table->decimal('price',10,2) creates
DECIMAL(10,2) - 10 total digits 2 decimal places
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$table->json('metadata') creates
JSON column for native JSON storage
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$table->index(['assignment_id','q_level']) creates
Composite index on (assignment_id, q_level)
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hasMany(Comment::class) on Post means
Post HAS MANY Comments (FK post_id in comments table)
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belongsTo(Post::class) on Comment means
Comment belongs to ONE Post (stores FK post_id)
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Local scope scopeActive() on model allows
Reusable query constraint callable as Question::active()->get()
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paginate(15) returns
Paginator with 15 records per page plus metadata (total, pages, etc.)
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DB::table('questions')->pluck('question') returns
Flat Collection of values from question column only
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DB::table('questions')->skip(10)->take(5)->get() returns
Questions 11 through 15 (offset 10, limit 5)
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whereIn('id',[1,2,3]) generates
WHERE id IN (1,2,3)
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whereNull('q_level') generates
WHERE q_level IS NULL
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DB::table('questions')->max('id') executes
SELECT MAX(id) FROM questions
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Question::chunk(100, function($qs){}) processes
Records 100 at a time preventing memory exhaustion
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DB::table('questions')->insert([rows]) bulk inserts
All rows in single efficient INSERT statement
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DB::table('questions')->truncate() before seeding
Removes all existing rows quickly before inserting fresh data
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Which query finds employees earning more than their department average?
SELECT * FROM emp e WHERE salary > (SELECT AVG(salary) FROM emp WHERE dept_id=e.dept_id)
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Second highest salary in SQL
SELECT MAX(salary) FROM emp WHERE salary < (SELECT MAX(salary) FROM emp)
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NOT IN with NULL in subquery causes
No rows returned (NULL comparisons return UNKNOWN, NOT IN evaluates to UNKNOWN)
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Schedule with precedence graph T1→T2→T1 (cycle) is
NOT conflict serializable (cycle in precedence graph)
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In ARIES, pageLSN on each data page records
LSN of LAST log record that modified that page
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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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DBMS → Relational Model 1

R(A,B,C,D) with FDs: A→B, B→C, A→D. The candidate key is
A
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DBMS → Normalization 1

R(A,B,C) with AB→C and C→B. Is C→B a BCNF violation?
Yes - C is NOT a superkey (C+={C,B}≠{A,B,C}) so C→B violates BCNF
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DBMS → SQL Basics 1

SELECT RANK() OVER(PARTITION BY dept ORDER BY salary DESC) ranks employees
Within each department partition by salary descending
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DBMS → Joins 1

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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DBMS → Transactions 1

Which 2PL variant holds ALL locks until transaction commits?
Rigorous 2PL
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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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