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DBMS → Concurrency Control 1

Thomas Write Rule modification to timestamp ordering
Silently ignores an obsolete write when a newer transaction already wrote the item
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DBMS → Transactions 1

2PC blocking problem: coordinator fails between PREPARE and COMMIT causes
Participants that voted COMMIT block indefinitely waiting for coordinator decision
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DBMS → Introduction to DBMS 31

3PC (Three-Phase Commit) adds which phase to resolve 2PC blocking
PRE-COMMIT phase between PREPARE and COMMIT making protocol non-blocking
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RAID 0 provides
Striping only (improved performance, NO fault tolerance)
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RAID 6 provides
Double parity - can survive TWO simultaneous disk failures
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SSD vs HDD in databases: key advantage of SSD
Much faster random I/O: near-zero seek time and rotational latency
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Buffer pool hit ratio measures
Percentage of page requests served from memory vs disk (higher=better)
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Query rewrite: pushing projection down the operator tree
Reduces tuple width early limiting data carried through subsequent operations
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Which is a complete set of minimal Armstrong's axioms
Reflexivity, Augmentation, Transitivity
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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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DBMS → Joins 2

Which join algorithm is best when inner relation has no index and doesn't fit in memory
Block nested-loop join (reads outer in blocks to reduce I/O)
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Cost of index-based nested-loop join (outer b_r blocks, inner has index)
b_r + n_r × (cost of index lookup)
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DBMS → Normalization 3

Which normal form specifically addresses redundancy due to functional dependencies
BCNF
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If R(A,B,C) is in BCNF is it also in 3NF
Yes - every BCNF relation is also in 3NF
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A relation in 4NF is also in
BCNF (since 4NF ⊂ BCNF ⊂ 3NF ⊂ 2NF ⊂ 1NF)
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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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