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DBMS → Introduction to DBMS 37

Eloquent create() requires
$fillable array set on model (mass assignment protection)
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$question->save() performs
INSERT for new model or UPDATE for existing model
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Soft delete uses
deleted_at TIMESTAMP column (NULL=active, timestamp=deleted)
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withTrashed() includes
All records including soft-deleted ones
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belongsToMany() is used for
Many-to-many relationship requiring pivot table
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chunk(100, callback) in Eloquent
Processes records 100 at a time preventing memory exhaustion
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DB::table('questions')->get() returns
Plain PHP stdClass objects (not Eloquent model instances)
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DB::table('questions')->where('is_deleted',0)->count() returns
Integer count of non-deleted questions
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php artisan migrate:fresh
Drops ALL tables then re-runs all migrations
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Schema::create() in migration
Creates a new database table
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$table->id() creates
AUTO_INCREMENT UNSIGNED BIGINT primary key column named id
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$table->string('question') creates
VARCHAR(255) column named question
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$table->text('solution') creates
TEXT column for large text content
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$table->timestamps() adds
created_at and updated_at TIMESTAMP NULL columns
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$table->softDeletes() adds
deleted_at NULLABLE TIMESTAMP column
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$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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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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